agrometeorological lecture notes for observers 2
TRANSCRIPT
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AGROMETEOROLOGICAL LECTURE NOTES FOR OBSERVERS
BY
ALMAZ DEMESSIE
AGROMETEOROLOGIST
NATIONAL METEOROLOGICAL SERVICES AGENCY
P.O. BOX 1090, ADDIS ABABA, ETHIOPIA
February 1999
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INTRODUCTION
The importance of weather and climate for agricultural productionAgrometeorology is the application of meteorological knowledge, information and
data to weather sensitive problems in agriculture. Amongst other things this includes problems dealing with the effect of weather, climate and their variability.
The meteorological services of many countries includes units specially devoted to agrometeorology and similar work is undertaken in universities, colleges and research institutes wide range of expertise is involved. Much of the work has concentrated on studding the influences of weather and climate on local agriculture, often for commercial or environmental purposes.
An agrometeorological service is of great importance for countries with a primary agricultural economy. It supplies forecasts of frost, heavy rainfall or drought and information, which is important for sowing, application of pesticides, and for harvesting. Agrometeorological advisories for general public play an increasingly important role in modern society.
Agrometeorological advisory service was started in Ethiopia long years back. The main objectives are to facilitate wider range of services to agriculture, to conduct applied agrometeorological research and to increase the quality and quantity of agrometeorological output.
Scope of Agricultural Meteorology
Definition
Agricultural meteorology deals with the interaction between meteorological and
hydrological factors, on the one hand, and agriculture in the widest sense, including
horticulture, animal husbandry and forestry, on the other. Its objective is to discover
and define such effects, and then to apply knowledge of weather to practical agricultural
use. Its field of interest extends from the soil layer of deepest plant and tree roots,
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through the air layer near the ground in which crops grow and animals live, to the
higher levels of the atmosphere of interest in aerobiology, the latter with particular
reference to the effective transport of seeds, spores, pollen and insects.
In addition to natural climate and its local variations, agricultural meteorology is
also concerned with modifications in the environment.
The collaboration between meteorology and agriculture
The objectives of agricultural meteorology can be fully achieved only if there is
close collaboration between agricultural and meteorological interests.
The services which meteorologists can provide to agriculturists may be
grouped broadly as follows: -
a. To co-operate with and seek advice from agricultural services in all matter
of common interest;
b. To supply as far as practicable, any available meteorological data required
by agricultural scientist in their research experimental and advisory work;
c. To advice on the best utilization of weather and climate data in attaining
such objective as improving agricultural production introducing new species
of plants and animals and increasing the area in efficient farming use;
d. To assist agricultural and allied interests in combating unfavorable weather
and climate.
eg. - artificial modification of climate
- wind breaks, shelter-belts, irrigation and glass house
- aforestation (by planting trees)
e. To assist in the fight against agricultural pests and diseases by considering
both the environmental factors during their life histories and the
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meteorological factors which may influence the effectiveness of the
protective measures taken;
f. To advise on the protection of agricultural products, in storage and in
transit, against damage.
The services which agriculturists can provide to meteorologists may be
grouped broadly as follows:-
a) To co-operate with and seek advice from Meteorological Services in all matters
of common interest;
b) To supply meteorologists with such ecological data on the life-histories of
plants and animals, and of the pests, as may serve a guide for preparing
corresponding weather forcasts and for formulating climatological advice on
such matters as the introduction of new plants and animals;
c) To encorage the implementation of continuous and comprehensive national
surveys of important agricultural pests, so that their relationships with weather
factors in different regions may be studied further;
d) To supply, as far as practicable, statistical data on crop yields, etc, adequate
for investigatios of reliable crop-weather relationships;
e) To co-operate with Meteorological Services in establishing standard
agrometeorlogical stations and in collecting other relevant data.
Joint sevices by meteorologists and agriculturalists
Meteorologists and agriculturists should co-operate towards making
meteorological and climatological information “an operational tool” in every farmer’s
day to day activities and in his weekly, seasonal and long-range planning. Theis jointly
developed and furnished services should include:
a) Operationally useful forecasts of meteorological variables that are important to
current farming operations together with an agricultural interpretation of such
forecasts;
b) Education programmes for farmers to demonstrate the usefulness of weather
information for agricultural planning and operations;
c) Joint research projects on agriculture-wether relationships and their applicationsto
farming practices.
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The six main points governing relationship between weather and
agriculture
1. Soils
a. weather is important factor in determining the nature of soil
b. Climate and weather affect
i. Chemical physical and mechanical properties of the soil
ii. The organism it contains.
iii. Its capacity for retaining and giving up heat.
c. Rainfall adds chemical contents to soil and washes soil nutrients
d. State of soil is affecting cultivation, pest control and harvesting.
2. Plants
The plant is affected at every stages of growth (phenological phases)
by environmental conditions and before planting and after harvesting
(storage, transport etc) Post harvest operation such as drying of grains and
other crops are affected by seasonal weather/storage quality of fruit,
vegetable and other farm products, defence against forest and grass fires.
3. Farm animals
Weather affects animals in the following ways:-
a. direct
b. crops on which they feed
c. soil on which they are kept
d. their geographical distribution
e. yield and quality of animal product
f. preparing these products and their capacity for storage and transport
4. Diseases and pests of animals
a. weather influences susceptibility of plants and animals to attack by
pests and diseases
b. nature, number and activity of pests and diseases
c. has impact upon the effectiveness of control measures
d. on the amount and toxicity of spray on harvested crops
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5. Farm buildings and equipment
a. weather conditions should be considered while planning farm
buildings particularly designing animal housing and storage space for
agricultural products
b. It also influences the choice, up-keep and best use of farm machinery.
6. Artificial modification of meteorological and hydrological regimes
Irrigation, mulching, wind breaks and shelter belts.
Agrometeorological observations
1.1 Meteorological and agricultural observations
Meteorological and climatological data are now a days used in planning various human
activities, one of the most important being agriculture. For many years, the only data
used for agricultural purposes were of rainfall and temperature but in some domains,
additional meteorological data were soon required. With the advances made in the
physical sciences, new meteorological observation were gradually introduced - air
humidity, wind, radiation, soil temperature, sunshine duration, cloudiness, water surface
evaporation, atmospheric pressure etc. The data for all this meteorological element
were and will continue to be, very important for agriculture. However, that data from
the meteorological elements alone had limited application and were not sufficient to
satisfy the growing demands of agriculture. The effect of weather on agriculture can not
be determined by observing only the weather. In addition to meteorological
observation therefore, agrometeorologists began observing plants, soils domestic
animals, the occurrence of pests and disease etc. Modern agrometeorology needs and
uses data from both observations of the weather and of agriculture.
Agrometeorological observations
The agrometeorological observations are far more than meteorological but it must
be borne in mind that nearly all elements of agricultural production depend on, or are
some how related to, the weather and climate. Some of the agrometeorological
observations are:-
- Plant development (phenology)
- State of the plant
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- Plants yield
- Plants height
- Damage from adverse meteorological phenomenon
- Extent of weeds
- Soil moisture in the field of crops
- State of soil surface
- Health of domestic animals
- Productivity of domestic animals
- Occurrence of diseases etc.
Depending upon their applicability and use, the agrometeorological observations
can be divided into two main groups:-
a. Observations made for specific research projects.
b. Observations used as permanent (routine) in a network of agrometrological
stations
Some aspects of agrometeorological observations
Observations to be carried out at agricultural meteorological stations.
I Observations of the physical environment
An agricultural meteorological stations should include observations of some or all of the following elements characterizing the physical environment:
a) Temprature and humidity of the air; The temprature and humidity of the air should be measured in representative places, at
different levels in the layer adjacent to the soil. They should be made at principal agricultural meteorological stations frome gruond level up to about 10 m above the upper limit of the prevailing vegetation.
Humidity- the best method for measuring humidity distribution in the layers near the ground is by using thermo-electric equipment. Ventlated psychrometers may be used for levels at least 50 cm above bare soil or dense vegetation. Hair hygrometers and hair hygrographs may give acceptable values only if great care is taken in their use and maintenance.
b) Wind;
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The direction and speed of the wind should be measured or observed regularly at agricultural meteorological stations. To facilitate comparison with other stations, the measurement at 10m is taken as reference level.
Wind is a two dimentional vecto quantity expressed by its speed and its direction. In this sense it is termed as wind velocity. Wind speed is expressed as the average velocity in m/sec or as a total wind run in km/day. Conversion from one unit to the other:
1m/sec =86.4 Km/day Wind speed is measured with anemometers. Wind direction is given in degrees, and refers
to the direction from which the wind is blowing.Anemometers are placed at a chosen standard height in an open field location. The standard hieght in meteorology is 10 m. Anemometers are located in the field at a point at least 10 times the hieght of the nearest obstraction.
Since wind speed at a given location varies with time, it is necessary to express it as an average over a given time interval. The total wind run is found by subtracting the initial reading from the final reading. In this case, to convert it into m/sec use the following relation:
Wind speed mm/sec = (difference in wind run Km/day)* 10/864
Or
Wind speed m/sec = .0116*(difference in wind run Km/day)
c) Sunshine and radiation;
The duration of sunshine should be recorded at all agricultural meteorological stations, and this information should be supplemented wherever possible by data obtained from radiation instruments. Principal stations should make detailed observations of radiation including global solar and net radiation.
d) Clouds, hydrometeors and other water-balance factors(including hail, dew, fog, soil and water evaporation, plant transpiration, runoff and water table);
At an agricultural meteorological station, observations may be made at regular intervals of the total amount of cloud. In addition, cloud type and height of cloud base are required for studies of the radiation balance. Detailed observations of hydrometeors as given below are useful for many agricultural purposes: rain and drizzle(including intensity); snow(including thickness and density of snow cover, and water equivalent); hail(including water equivalent and siz of hailstornes); dew(amount and duration), frost, fog, etc.The amount of precipitation should be measured in the morning and evening as at synoptic stations. Additional measurements are desirable and the intensity of precipitation could be obtained by means of a recording raingauge.
e) Evaporation and water-balance measurements
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Measurement of evaporation from free water surfaces and from the soil and of transpiration from vegetation is of great importance in agricultural meateorology. Potentialevapotranspiration is defined as the amount of water which evaporates from the soil-air interface and from plants, when the soil is at field capacity. Actual evapotranspiration is defined as the evaporation at the soil-air interface, plus the transpiration of plants, in the existing conditions of soil moisture.
f) Soil temprature.
Agricultural meteorological station should also include soil-temperature measurements. The levels at which soil temperatures are observed should include the following depths: 5, 10, 20, 50 and 100 cm. At the deeper levels(50 and 100 cm.), where temperature changes are slow, daily readings are generally sufficient. At shallower depths the observations may comprise, in order of preference, either continuous values, daily maximum minimum temperatures, or readings at fixed hours ( the observations being preferably not more than six hours apart).
g) Soil moisture
Soil moisture should be measured at all principal stations and, wherever possible, at other agricultural meteorological stations. Although rigid standardizatio is neither necessary nor perhaps, even desirable, these measurments should, wherever possible, be madefrom the surface to depths of 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 cm. In deep soils, with a high rate of infiltiration, measurments should be extended to greater depths. Often levels will be selected in relation to the effective rooting depths of the plants Until it is possible to make reliable continous recordings at some of these levels, it is recommended that observations be made at regular intervals of about ten days; for the shallower depths, shorter intervals(seven or five days) will be necessary.
The following additional observations contribute towards better understanding of soil-mosture conditions:
- Field capacity of the soil;- Permanent wilting point; and- Depth of the groundwater
II. Phenological Observation
In the developmental process from the germination of seeds, plants show several
visible external changes, which are a result of the environmental conditions. These
external changes are called phenological phases (stages) of plant development and the
observations are called phenological observations.
The word "phenology" is of Greek origin "phaino" means to show, to appear and
"logos" means science. It is a branch of agrometeorological science dealing with the
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relationship between weather (or climate) and periodic biological phenomena such as
development phases of plants, migration of birds ... etc.
In a given location, the meteorological factors vary from year to year and,
therefore the dates of the beginning and end of the phenological phases of a given crop
vary accordingly.
Phenological data are used not only for research work but also in the operational
agrometeorological forecasts including expected yield. The mean dates of the
appearance of the phenological phases in an area form the so-called "crop calendars"
A. Definitions and methods of observation of individual phenological phase
Sprouting-This is defined as when seeds have developed shoots and the first leaves
have unfolded in different spots on the plots, the sprouting stage has
begun. Uniformly of emergence is described qualitatively by the following
criteria.
Good - over 90% of the entire field is uniformly emerged.
Fair - below 90% but above 75% of the entire fields is Uniformly emerged.
Poor - below 75% of the entire field is uniformly emerged.
Third leaf phase - When the third leaf unfolds the 3rd leaf stage begin.
Tillering - when the ends of the first leaves on the lateral shoots project from the
sheath of the leaves of the main stem, tellering phase begins. Cereals begin to branch
out several days after the appearance of the third leaf. The lateral shoots forming in the
leaf axils in underground stem nodes lie directly one on the other and constitute the so-
called "tillering node".
Stem extension (shooting) - This phase is defined as the beginning of the growth of
the stem.
Earing - This is defined as the forming of ears of cereals. This phases begins for rice
when half of the ear projects from the sheath of the upper leaf and for corn
(maize) when of the upper part of the tassel appears.
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Flowering - When 10% of the plants in the plots show open flowers it is noted as
the "first flowering" stage. When >50% of the plants in the plots have one
or more open flowers, it is "full flowering".
Milky Ripeness Phase - The following are the criteria for the determination of the
beginning of the milky ripeness phase. The grain must be almost as long as the
completely formed grain. It most be green and must burst and emit its content when it
is pressed between the fingers.
Waxy Ripeness Phase - The principle indication that the waxy ripeness stage has
begun is that the grain turns yellow and becomes waxy to the touch. Also the grains
loose their elasticity. A finger nail pressed against the grain leaves a permanent mark.
In corn (maize) this phase is marked by the fact that the husk looses its green
color, the grain on the middle of the cob take on a waxy consistency and are easily cut
with a knife, and when slit across do not eject a milky fluid.
Full ripeness - This phase is recognized by the fact that the grain become hard and
split when we hit with a knife.
Selection of fields for phenological observation
Phenological observations should not be made on crops involved in agricultural
experiments, such as testing fertilizers irrigation experiments etc. The phenological
observations however can be performed in the control plants of such experiments,
where all conditions are the same as in the surrounding area. The field work carried out
should also be typical of the area. it should be stated though, that if irrigation is the
general practice of the area, the phenological observations should also be performed in
an irrigated field.
Due to crop rotation over the years, a given agricultural crop is sown in different
fields and therefore, the phenological observations on this crop also move from one field
to another. In order for the phenological data obtained during the different years to be
comparable, the fields should have similar properties: types of soil, aspects, relief...etc
only in this way can one be sure that the development of the plant is influenced solely
by meteorological factors, and not by different soil factors.
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The fields selected from observations should be about one hectare (10,000m2),
whenever possible. If bigger an observational area of about one hectare should be
delineated. If fields of a sufficient size are not to be found in the area of a given
agrometeorological stations the observations could be carried out in small fields.
However, it is not advisable to make phenological observations in fields smaller than 0.2
ha.
Times of phenological observation
The phenological observations should be performed three times a week. If one of
the selected days is an official holiday, the observations should be performed a day
earlier or later.
Method of phenological observation
The method of performing phenological observations depends on the way a crop
is cultivated. For this reason all crops described here can be divided into three groups:-
i. Row grown crops
These are annual crops that are sown in rows of different width: maize, cotton, tobacco,
sunflower, etc.The phenological observations of these crops are carried out on 40 fixed
plants during the entire growing period. For this purpose after the emergence of the
plants at four places (replications, in the field usually at a distance of between 50 and
>50 m apart from each other (if the field is one hectare) ten normally developed plants
typical of the sowing area, are counted off and marked with rings made of string or wire.
The ten plants should be selected from the two neighboring rows, five plants form each
row.
It should be emphasized that each replication has its permanent number for the given
year. The plant selected for observations should be at least several rows inside the
field. If the field is smaller than one hectare, the distance between the replications will
also be shorter. It is not advisable however, to have them closer than fifteen to twenty
meters.
100m 50m
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100m 40m
maximum size of the field for minimum size of the field for
phenological observation phenological observation
ii. Crop with a continuous (merged) surface
These are annual and perennial crops whose canopy forms a continuous (merged)
surface. Although most of these crops are sown in rows the distance between two rows
small and soon after emergency the rows are hardly visible, the canopy looks like a
continuous surface. Crops grown this way are wheat, barley, oat, rice, etc...
The phenological observations are again carried out on 40 plants - ten plants are
observed at four places in the field. After the emergency of the plants the observer
should mark with sticks four replications in the field. Throughout the growing season,
ten plants should observed around each stick.
iii. Perennial trees and bushes
These are fruit trees, coffee, cocoa etc perennial plants have a more uniform
response to environmental factors and therefore, the phenological observation can be
carried out on fewer plants. For each crop observations are carried out only on ten fixed
plants. They should be of the same variety and of more or less the same age. They
should also be normally developed and typical of the whole plantation. The selected
trees or bushes are marked with labels or painted with dye. Some times, there are less
than ten plants of the same variety of crop in the area of an agrometeorological station.
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In such cases the phenological observations can be carried out on a smaller number of
plants. It is not advisable, however, to observe less than five plants.
iv. Replacement of plants
During the growing season of the crops, some of the fixed plants (annual or
perennial) selected for observation might die or be destroyed. In such a case a new
plant (or plants) should immediately be selected for observation. The new plants should
be at the same phase of development as the previous one and should also be at the
same state and height or very similar.
Registering the phenological information
The phenological data should be registered on special forms.
eg.for monthly returns, called monthly phenological reports. Monthly phenological reports for annual crops
All annual crops both row-grown (maize, cotton, etc...) and those with continuous
surface (wheat, teff, etc...) are reported the same way.
eg. country---------- District------- Station--- field No.-- Crop _____________
Variety----- Date of sowing--------year______month _______Name of observation___________
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Date
of
obse
rvati
on
Phenol
ogical
Phase
Number of plants with the
features of the given phase
Number of
plants with
feature of a
given phase
in % from
the 40 plant
Replications
1 2 3 4 Tota
l
2 Nil - - - - - -
4 Nil - - - - - -
6 Nil - - - - - -
9 Earing 0 1 0 1 2 5
11 " 1 2 0 2 5 12
13 " 4 8 5 7 24 60
13 floweri
ng
0 1 0 0 1 2
16 Earing 10 1
0
10 10 40 100
16 floweri
ng
3 6 3 6 18 45
18 " 6 9 5 8 28 70
20 " 9 1
0
8 10 37 93
23 " 10 1
0
10 10 40 100
25 Nil - - - - - -
27 Nil - - - - - -
30 Nil - - - - - -
field work carried out
1. Weeding on 3rd of April 2.Spraying against blight on 26th April.
As can be seen from the example an observation consists in counting the number
of plants which already have the features of a given phase. The counting and
registration must be made separately for each replication. Later on the percentage of
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the plants is calculated. Thus, not only the date of the first appearance of the phases is
known, but also the rate of their passing. In the period between two consecutive phases
of a given crop, the observer should write "nil" until the next phase appears.
The first phase in most of the annual crops is "emergence". No plants are counted
for this phase. The first time the observation notices the newly emerged plants he
should record only "emergence". The counting of the plants starts with the phase that
follows the emergence.
Monthly phenological reports for perennial plants with a seasonal pattern
Perennial plants (mostly trees and bushes) with a seasonal patter are those whose
phenological phases appear simultaneously on all plants, and on all branches of a given
plant as well. This is how perennial grown in temperate and higher-latitude areas of the
world behave. After the dormant period during the winter season, the phenological
phases appear more or less simultaneously. The development of the plants in these
parts of the world is determined by the temperature condition.
Perennial plants with a seasonal pattern are most fruit-trees apple, citruses, plum,
pomegranate, etc... and certain shrubs.
Each of the ten perennial plants selected and fixed for phonological observations
should be observed as a whole. Two degrees of each phase are observed and recorded:
the beginning and the mass occurrence. The beginning of a given phase should be
registered when its features have appeared on less than one quarter of a given plant.
Example: less than one quarter of all buds on a plant are flowering, or less than
one quarter of all fruit on a plants are ripe. Mass occurrence of a phase should be
registered when its features have appeared on more than one quarter of the plant (for
some agrometeorological purposes mass occurrence is considered to be when its
features are visible on more than 50 percent of the plant).
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Monthly phenological report for penennial plants with a seasonal pattern
eg.Country ----- District ---- Station -----Field No. ---- Plant apple ---- Variety ------Year of
planting ------ Number of trees -------(bushes) observed ------ year -----month April Name of
observer -----.
Date of
observation
Phenologic
al phase
Developme
nt the
phase
Number of trees
(bushes) with the
features of the
given phase
1 Nil - -
4 Nil - -
6 Nil - -
8 Swelling of
the buds
Beginning 6
11 " " 2
11 " Mass
occurrence
8
13 " " " 10
15 " - -
18 Opening of
the buds
Beginning 4
18 " Mass
Occurrence
6
20 " " " 10
22 Nil - -
25 Nil - -
27 Flowering Beginning 3
29 " " 1
29 " Mass
occurrence
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Monthly phenological reports for perennial plants without a seasonal pattern.
In the equatorial belt, temperature conditions do not change essentially from
month to month throughout the whole year, they are within the optimal range for the
plants development and therefore the decisive factor for the development of a new
cycle of perennial is the rainfall regime with the beginning of the rainy season, many
perennial begin a new cycle of development.
The effect of the rainfall pattern on the plants is mainly restricted to incitement of
a new cycle of development. Later on, the rate of development is again mostly
influenced by temperature conditions.
In the equatorial belt of the world, too most perennial plants have certain seasonal
patterns, but usually not as obviously as in higher latitude areas of the world within an
orchard planted with a given variety of tree, the phenological phase often appear at
different times on the different plants, or even on the different branch of the same plant.
Thus, for some plants like coffee, mango or citrus trees, it is a common phenomena to
find several phenological phases existing, at a given moment, on one plant. Such plant
development is considered to be without seasonal pattern.
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Example for monthly phenological report for perennial plants without a
seasonal pattern.Country ........District ........Station .........Field No. ........Plant Coffee
Variety .......Year of planting .....Number of trees ....(Bushes) observed.... Year ......Month
April................. Name of observer .......
Plant
No.1
Plant
No. 2
Plant
No. 3
Plant
No. 4
Plant
No. 5
Plant
No. 6
Plant
No. 7
Plant
No. 8
Plant
No. 9
Plant
No. 10
1 Candl
e
stage
Candle
stage
Candl
e
stage
Candl
e
stage
Candl
e
stage
Candl
e
stage
Candl
e
stage
Candl
e
stage
Candl
e
stage
Candle
stage
4 Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil
6 " " " " " " " " " "
8 " " " " " " " " " "
11 " Floweri
ng
Flowe
ring
" Flowe
ring
" " " Flowe
ring
"
13 Flowe
ring
" " Flowe
ring
" Flowe
ring
" Flowe
ring
" Flowerin
g
15 " Nil Nil " Nil " Flowe
ring
" Nil "
18 Nil " " Nil " Nil " Nil " Nil
20 " " " " " " Nil " " "
22 " " " " " " " " " "
20
25 " Pin
head.
Stage
Pin
head
s.
" Pin
head
S.
" " " Pin
head
s.
"
27 Pin
head
Nil Nil Pin
head
S.
Nil Pin
head
S.
" Pin
head
s
Nil Pin head
s
29 Nil " " Nil " Pin
head
s.
Nil " Nil
Field work carried out
III. OBSERVATIONS ON THE STATE OF THE CROPS
Meteorological factors influence not only the development of the crops but their state as well.
Regardless of the speed of development, the plants could be in different stats due to favourable or
unfavourable meteorological conditions.
The observation comprise seven different agrometeorological observations. All of them, in one way
or another, are related to the stat of the crops. The observstions are:- general assessment of the state of the
plants; density of the sowing area; height of the plants; damage from adverse meteorological phenomena;
damage from pests and disease; extent of weeds and crop yield.
A. General assessment of the state of the plant
The is made on all plants in the field, and not only the 40 annual or 10 perennial
plants selected for the phenological observations. The state of the crop is assessed by
comparing it with the state of the same crop during the years with normal
meteorological conditions and a normal level of agricultural technology for growing this
crop.
The assessment of the plants is made by taking into account many factors: the
health of the plants; the uniformity and density of the sowing area; the number of
weeds in the field; damage by adverse meteorological phenomena; pests, etc. During
the period of vegetative growth, the observer should also take into account the height of
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the plants and the number of tillers(for certain crops), while during the reproductive part
of the growing season, the number and state of the reproductive parts of the
plants(flowers, grains, fruit, pods, etc. ) should be taken into account.
There are five degrees (marks) of assessment:
- Excellent stae :- the plants are strong, healthy, well rooted and well-developed.
The density the sowing area is optimalfor the local conditions, and there are no
missing plants. All parts of the plants, and especially the reproductive ones, are in
excellent condition. There are no weeds. Such a state is typical for years with very
good meteorological conditions and a yield much higher than normal years is to
be expected;
- Very good state :- the state could not be assessed as excellent only because of
some minor shortcomings-some plants are not too healthy or strong; there are
missing plants in some parts of the field; there are some weeds; there is slight
damage from adverse meteorological phenomena, pests. Neverthless, the
expected yield from the crops is still above average for the area;
- Normal state :- a normal yield is expected;
- Unsatisfactory state :- a yield below normal is to be expected;
- Bad state :- a very poor yield is to be expected.
B. Density of the sowing area
The density of the sowing area for each annual crop should be determined at least
three times – at the beginning, middle and end of the growing season. The first
determination should be under taken soon after (within a few days) the full emergence
of the plants. With crops that requiere thinning, the first determination of the density
carried out immediately after the final thinning. The second determination should be
performed at flowering time. The last determination is carried out several days before
the ripenness of the crop, or befor harvesting.
- Method of observation:- depending on the way the crops are planted, there are
three main methods of determining the density of the sowing area. Determination
of crop density for :-
- Raw growm crops
- Crops with a continous surface
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- Crops do not qualify for one of the above mentioned
groups
One example from the three methods:- Crops grown in rows: this group includes all
annual crops grown in rows, plus some perennial crops. The observer should select four
places(replications) in the field which should be located a few metres from the
replications where the phenological observations carried out . From each place, the
observer should measure ten linear meters along a rows, and mark them with small
sticks. Then he should count the number of plants grown within these ten meters. The
sum of the plants from the four places should be multiplied by 25000 and later on
divided by the distance between the rows(in centimetres). The result shows the number
of plants in one hectare.
Example: In a field with maize, the number of plants in ten linear metres in
the first place is 28; in the second place, 31; in the third place, 29; and in
the fourth place, 32. The distance between two adjacent rows is 90 cm. The
number of plants in one hectare will be 33333.
Density of crop = (28+31+29+32) 25000
90
= 120*25000
90
= 3000000
90
= 33333
C. Height of the plants
The hight of the plants should be measured every ten days during the whole
growing period. The height is to be measured on 40 plants which are typical for the
sowing area, at four places in the field, close to the replications for phenological
observations. This is measured with a wooden measuring stick held vertically with the
zero value touching the soil surface. The stem or the leaves of the plant should be
carefully straightend up and the height measured.
According to the methods of measuring height, the plants are divided into two
main groups:
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i) Barley, maize, millet, oats, rice, sorghum, sugar-cane, sunflower, and
wheat: From the emergence of these crops, up to the phase of earing (in
some of the crops, “tasseling” or “heading”), the height should be
measured from the soil surface to the top of the straightened plant. The top
of the plant may be either the top of the stem, or the top of one of the
branches, or the top of one of the upper leaves. After the earring (tasseling,
heading), the height should be measured from the soil surface up to the top
of the ear (without the awns) or, accordingly, up to the top of the tassel, or
up to the top of the sunflower’s head;
ii) On the remainder of the plants, during the whole growing period, the height
has to be measured from the soil surface to the top of the straitened plant.
D. Observation of damage to plants by unfavorable weather phenomena
Recording the extent of the damage on crops by unfavorable phenomena of
weather is essential. In recording the extent of the damage it is necessary to indicate
what part of the plant was affected leaves, buds, flowers, ovaries, fruits or the whole
plant.
i. Unfavorable weather phenomena to plants
The following are the main weather factors that adversely affect crops.
a. Abnormal rainfall condition.
Direct damage to fragile plant organs, like flowers; soil erosion; water
logging; drought and floods; land slides; impeded drying of agricultural
product; conditions favourable to crop and livestock pest development;
negative effect on pollination and polinators etc.
b. Abnormal wind condition.
Pyysical damage to plant organs or whole plants(e.g. defolation,
particularly of shrubs and trees); soil erosion; excessive evaporation.
Wind is an aggravating facto in the event of bush or forest fires.
c. Abnormal air moisture.
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High values create conditions favourable to pes development; low values
associated with high evaporation and often one of the most determinant
factors in fire outbreaks.
d. High tempratures
Increased evaporation; induced sterility in certain crops; poor
vernalization; survival of pests during winter. High temperatures at night
are associated with increased respiration loss. “Heat waves”, lengthy
spells of abnormally high tempratures are particularly harmful.
e. Low temperatures
Destruction of cell structure(frost); desication; slow growth, particularly
during cold waves; cold dews.
f. High cloudiness
Increased incidence of diseases; poor growth.
g. Hail
Hail impact is usually rather localized, but the damage to crops
particularly at critical phenological stages may be significant. Even light
hail tends to be followed by pest and disease attacks.
h. Lightning
Lightning causes damage to buildings and the loss of farm animals. It is
also one of the causes of wildfire.
i. Snow
Heavy snowfall damages woody plants. Un-seasonable occurrence
particularly affects reproductive organs of plants.
j. Volcanic eruptions, avalanches and earthquakes
The events listed may disrupt infrastructure and cause the loss of crops
and farmland, sometimes permanently. A recent example of carbon
dioxide and hydrogen sulphide emissions from a volcanic lake in
Cameroon caused significant loss of human life and farm animals.
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k. Air and water pollution
Air pollutants affect life in the immediate surrounding of point sources.
Some pollutants like ozone, are however known to significant effects on
crop yields over wide areas. In combination with fog, some pollutants
have more marked effect on plants and animals. Occurrences of
irrigation water pollution have been reported.
E. Damage from Plant pathogens
i) Plant pathogens
Plant diseases are caused by different pathogens in groups of viruses, fungi,
nematodes, bacteria etc like organisms.
Meteorological conditions influence not only the development of an
epidemic over a single growing season, but also the survival of the pathogen until
the next season.
Most diseases will have a greater potential to become epidemic under
warmer and moisture conditions. Plant pathogens generally tend to require warm
temperatures and prolonged leaf wetness in order to develop. Only a few
pathogens are favored by dry or cooler conditions.
Meteorological conditions influence every sequence of epidemic release,
transport of pathogenic agents, retention, infection and incubation. In temperate
and moist tropical climates temperature is the most important regulating factors
where as rain fall is vitally important in arid climates. Either temperature or
moisture can be decisive in the contamination, development and spread of a pest
or disease. If one is continuously favorable the other becomes limiting. If both
fluctuate, they must be favorable at critical times. If both are continuously
favorable the pest or diseases infestation or infection becomes serious.
Many disease pathogens are transmitted by animal vectors, usually
insects. The seasonal cycles of insects vary which climate and also from
year to year with weather. The effect of climate upon the activity if vectors
can influence the rate of infection and the size of infected area. The factors
involved here are principally wind, precipitation and temperature.
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ii) The influence of climate on pest and disease
Climate determines the distribution of pest like the distribution of vegetation over the area. In other word climate sets the limits of occurrence of pest. The development of pest depends upon three factors that are environment, host and pathogen.
Disease triangle or disease triode
E- EnvironmentH- HostP- Pathogen
Here the environmental condition plays important roll.There is one more factor that is time so the disease triangle change to this
- The HPE factors have to be at or near their optimum levels for a period of time.
H- the sensivity of the host depends up on the stage of growth and duration of the pest.
P- The severity of infection depends upon virulence of the pest(the ability to infect)
E- Arial + soil evironment= bio-climate of the plant such as air temperature, soil moisture, soil temperature, aeration, reaction of pH, fertility.etc
a) HOW DOES THE ENVIRONMENT ACT ON PATHOGEN AND HOST?
Environmental effects on the pathogen comprise:- Survival of inoculum (genetic material or reproductive bodies) from season to season or year to year. - Activation of inoculum as the season begins.- Rate of multiplication of inoculum to amounts that will support epidemic outbreaks,
including day-to-day effects of the amount of sporulation, efficacy of dispersal and host penetration.
The effect of environment on the host chiefly concerns:-Predisposition to disease, including the weakening of host by
Drought or water logging of the soil or conversely the formation of water-soaked plant tissues under highly humid atmospheric conditions.
Generally weather influences the susceptibility of plants and animals to attack by pests and diseases The nature, number and activity of pests and diseases. Has impact upon the effectiveness of control measures. On the amount and toxicity of spray on harvested crops.
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iii) Classification of plant disease: - Based on the severity of the occurrence or intensity of attack the plant disease classified as follows
- Sporadic- Endemic- Epidemic
Sporadic- the occurrence in patches the severity is not much (the occurrence is irregular in space and time and in a relatively few instances). These diseases are not attracting protection measure. The sporadic condition some times very serious to change to epidemic.
Endemic- diseases are confined to a specific geographic area. These diseases are moderate to sever attack. The best method is releasing resistance varieties.
Epidemic disease- weather sensitive those which occur widely causing extensive damage but they occur periodically (80-90% of the plant). The disease may present constant in the locality but it become series when the environmental factors become favorable for the rapid development of the decease.
The agrometeorologist interested to epidemic disease because they are weather sensitive.The agrometeorologist again concerned on air born diseases.
iv) Role of weather in the development of pest
Temperature - influence all metabolic (physiological or biochemical) reactions- Influence incubation period (time interval between infection and first appearance of disease symptom in the host)E.g. Wheat stem rustTemperature Incubation period
24°C 5 days13°C 12 days4.5°C 21 days
E.g. Fusarium Oxysporum wilt of cotton, watermelon, cowpeas.etc.Temperature Incubation Period
16°C 58 days27°C 12 days
Humidity –temperature and humidity together influences the process of infection, incubation period, and sporulation and determines the number of disease cycles in one crop-growing season.
Rainfall - if it is little with sunny intervals it would create favourable condition for decease whereas if it is heavy it is not favourable for disease development, it washed out the spores.
Wind -if it is calm and the rainfall occurs, it would favour the disease but if it is strong it dried the leaf no condition for germination.
Light - intense light condition is not favorable for fungi but deem and moist condition is favourable.
Insects Temperature-cold blooded (poikilothermic) insects are affected by the fluctuation of temperature.
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Humidity- different stages (Egg, Larvae, Pupa and Adult) of the insect needs different amount of humidity.
Wind- they need optimum wind speed. Fore instance, the locusts couldn’t fly if the wind speed exceeds 16 to 20 km/hour.
v) Plant disease and insect pest responses to climate variability
Plant diseases responses to climate variability
VirusesWeather influences the spread of a virus not only through the vectors but also through the host plant. Fast growing plants in a favourable environment are more susceptible to insect colonization or to virus infection than slowly growing plants under limiting environmental conditions (Broadbent, 1967). Temperature can influence either the vector or the pathogen. Moisture has mainly a secondary effect on virus diseases, influencing the plant and the vector activity
rather than the pathogen itself (Fry, 1982) Water stress has a tendency to promote virus diseases on pasture crops (Beresford and Fullerton, 1989).
FungiClimate factors can affect every phase of the fungus life cycle: sporulation, dispersal, retention, germination, infection and survival between growing seasons. Temperature- Fore instance if we see the case of Phytophora infestans (One of the most studied fungus),
which causes potato blight, are formed in a saturated atmosphere (relative humidity more than 91%) in a temperature range of 3-26°C. Free water or dew on the leaves is necessary for the germination of spores. Direct germination can occur over the temperature range of 9-26°C. The infection process requires temperatures from 10-25°C. The incubation phase is also dependent on temperature (Bajic, 1988).
Moisture is the prime environmental factor affecting sporulation. The germination of fungal spores is normally initiated under condition of wet weather. The infection phase also tends to be triggered by periods of wet weather. Temperature, wetness and humidity are the primary factors in the incubation phase. Moderate temperatures, prolonged wetness on the leaves (rainfall or dew) and high relative humidity are ideal for the development of most fungal leaf diseases (Renfro, 1986). In contrast to other groups of fungi, powdery mildews do not depend on the existence of free water on the leaves, either for germination or for infection. Diseases mediated by powdery mildews would not generally be enhanced by a wetter climate (Beresford and Fullerton, 1989). Some fungal pathogens have overwintering structures, which require winter chilling to complete their development, or to break dormancy (Beresford and Fullerton, 1989).
BacteriaIn present climates, plant pathogenic bacteria cause diseases mainly in subtropical areas, because they need high temperatures and moisture, so they are not such important plant pathogens as fungi and nematodes.
Temperature is the main environmental factor governing bacterial life. Growth as well as other biological events such as fruiting, sporulation, spore germination, mobility and survival are tightly related to temperature, or its variability.
Moisture is a controlling factor in the epidomology of diseases induced by bacteria. Most plant pathogen bacteria do not produce spores and are unable to survive periods of low moisture (Fry, 1982). For instance, Cassava Bacterial Blight(CBB) shows its characteristic symptoms and maximum intensity
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during the rainy seasons when conditions are ideal for its rapid development and spread (Theberge, 1986).
Effects of elevated atmospheric CO2 on the growth and community composition of plant pathogenic bacteria may be important, because bacteria can survive from season to season by living in decaying plant tissues. Hence, the return of plant residues to the soil may increase the population of pathogens as well as the number of useful organisms (Lamborg et al., 1983). In the future, possible climate warming may also increase the severity of bacterial diseases.
( Numerous studies indicate that photosynthesic rate, biomass acumulation and grain yield have in many plants been higher due to the direct effect of CO2 concentration increase)
NematodeNematodes near the soil surface are subjected to extremes of temperature and moisture variations, during which they are either inactive or die. Climate factors such as temperature, rain and moisture have major effect on nematode population diversity. Temperature is a key factor in the development, movement and activity of endo and ectoparasitic
species. Low, optimal and high temperatures for nematode in general are considered to be 5-15, 15-30 and 30-40°C respectively (Wallace, 1963). For instance, a soil temperature of 18°C, with an optimum of 26-28°C is highly conducive to the development of the root knot endonematodes (Collingwood et al., 1988).
Nematode populations increase most rapidly at intermediate levels close to field capacity, because their eggs do not hatch in dry soils. Most nematodes are sensitive to anaerobic condition, so that they may not survive such conditions in soils flooded for a long period (Fry, 1982). Even moderate rainfall, evenly distributed in time is more favourable than large amounts of rain that fall infrequently.
Insect pests responses to climate variability
Adaptability to a wide range of environmental conditions, and prodigious power of reproduction are the two outstanding characteristics of this group.
Insects, being poikilothermic animals, are especially strongly influenced by climate and weather. Light, temperature, humidity, wind and atmospheric circulation pattern, in particular, affect the rate of development, survival, reproduction, migration, and adaptation of insect pests. These in turn determine the population density at any location and the distribution in a region.
Examples
African armyworm (spodoptra exemta)
It feeds on wild grasses as well as cereals and pasture, in eastern and southern Africa, and can cause as much harm as the locust. Outbreaks vary in size from year to year. At a given place there are large seasonal changes in the number of caterpillars, and outbreaks have strong tendency to follow one another in place and time. Thus the first outbreaks in east Africa are often in Tanzania at the end of the year, and later outbreaks are further and further north, reaching Ethiopia by the middle of the year (The infestation period at different areas of the country is mid-April-July). Downwind displacement would take moths towards and into the Inter Tropical Convergence Zones(ITCZ), so that most of the insects, whether caterpillars or moths would be near the ITCZ, where rains fall and grasses grow. Moreover, the following generations would move with the ITCZ as shown by catches from a network of moth traps over east Africa.
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LocustsLocusts are various kinds of grasshoppers which are not only caused local damage in their breeding habitats but also migrate long distances under the influence of favourable wind in the lower atmosphere. There were about fifteen species of grasshoppers in the warmer parts of the world. They have adapted to the erratic seasonal rains characteristic of such areas, which produce typical vegetation on which the locust feed and shelter for survival. Their rate of development increases at higher temperatures and swarms in flight are carried downwind in all but light winds.
The profound dependence of locust on the prevailing weather for breeding, development and movement has been well studied for decades and the results have long been applied to forecasting and control of the species in many countries. Desert locust one of the species of locusts has been the subject of the most study in recent years.
Desert locusts reaching an area of moist soil, usually mature sexually is about one week and egg laying soon thereafter. Widespread rain leads to widespread synchronous egg laying. About 25-30 mm of rain or the equivalent in run-off is usually sufficient for full egg development. Heavier falls may be needed if soil is to remain sufficiently moist for egg laying up to several weeks later. However, too much rain can kill locust eggs by exposing them on the soil surface or by washing them out of the ground, or by causing them to rot.
The hoppers which come out of the eggs take four to six weeks to assume wings, but in case the weather is not warm enough, the hopper period may be prolonged by two more weeks.
Since in the desert regions, which are the habitat of the locust, rainfall is usually light and seasonal in nature, for survival, the locusts have to migrate to areas of favourable rainfall. The locust apparently need certain optimum amount of atmospheric humidity for it’s well being and so avoids areas with conditions of drought and high temperature. On the other hand, it appears to be unable to survive conditions of high humidity in areas of high rainfall. When low temperatures prevail, the activities of the locusts are greatly depressed.
Over most parts of the African Savanna, grasshoppers and locusts destroy many farmlands every year. The locusts generally originate in the Sahara desert margins where there is enough moisture for breeding and for vegetative growth to feed the larvae. The locusts fly in swarm’s southwards with the northeasterly winds during the day when temperatures are between 20 and 40 °C. Locusts find it impossible to hold to a course if the wind speed exceeds 16 to 20 Km/hour.
QueleaThe red-billed quelea is mainly distributed in the Ethiopian rift valley where sorghum is grown abundantly. The depredation of sorghum by the red-billed quelea at its milky stage is very high.The seasonal movement patterns of queleas for breeding are influenced by rainfall. Queleas are dependent on ripening grass seeds for breeding and these become available six to eight weeks after the onset of the main rains.
The Red-billed quelea birds (Quelea quelea) were found to nest in different parts of Ethiopian Rift Valley. The areas include; -The southern Rift, comprising the lower Omo valley and associated plains of southwestern Sidamo; The central Rift, comprising lake Zewai, upper and middle Awash Valleys and; The northern Rift, comprising the lower Awash valley and associated plains the Red Sea coast.
Nesting colonies were found in the southern rift along the border with Kenya and Sudan in May and June (Jaeger and Erickson, 1980). The same investigators also observed juvenile queleas (3-4 months old) in northern Ogaden. Other potential quelea breeding areas found in the northwest and the
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western lowlands because of similarity in rainfall patterns and yearly averages of these lowlands to those of the Awash valley. Thus, depending on rainfall condition the Red-billed quelea breeds in different parts of the Ethiopian rift valley and in other lowland regions.
vi) Estimation of Incidence and intensity of attack of major pests on crops
Crops are liable to be attacked by certain insect pests. In order to cultivate crops
successfully and obtain high yields one should be in constant watch of the occurrence
and intensity of pest attacks. Consequently, it is necessary to estimate and record
periodically, during the development phases of crops, the percentage of affected plants
and the intensity of the attack in the fields under observation so as to get an objective
index of the insect attack.
Pest - Based on the habits of the pest as well as on the parts of the plant attached
the pests are grouped as follows:-
a. Borers
b. sucking insect
c. leaf eating insects
d. other insects
The borers usually attack the shoot or the stem of the plant, tunnelling it and
thereby damaging the plants they are sometimes further classified as
a. top borer
b. stem borer
c. root borer
The sucking insect pests
Consists of bugs and mites which remain on the leaves and tender parts of the
plant and suck the sap. They remain either more or less stationary feeding on the
affected parts or move about from leaf to leaf or from plant to plant. The attacked
portions of the leaves show characteristic punctures and discolored patches, while in the
case of ear-heads they dry up and become chaffy.
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The leaf eating insect pests:- cut tender stems and feed on the foliage leaving characteristics perforations and cut edges.
IV. Crop yield observation
One of the most important agrometeorological observations is the crop yield. This is also the observation where some observers sometimes give incomplete or incorrect reports. Below are the guidelines for proper reporting of the yield:
a. Only the yield obtained from the field where the agrometeorological observations have been carried out should be reported. It might happen that, in a given farm, a crop is grown in several fields. In such a case, the yield from the field with the observed crops should be determined and registered separately. The yield should be from all plants in the field, and not only from those selected for phenological observations;
b. The yield data consist of two components: amount (weight) and the originating area. The weight of the yield is expressed in kilograms, same unit as the farmers or authorities managing the farm. The second component, the area, is expressed in hectares, acres, or some other units. Again, the observer should use the same units as the local farmers.
The two units - the one expressing the amount of the yield, and the one expressing the area - should always be reported together (kilograms per acre, or kilograms per hectare, or tons per hectare, etc.). A common oversight is to report only the amount without mentioning the area from which it has been obtained. Also to be avoided is the use of indefinite units like “bags”;
d. Sometimes, agrometeorological observations are carried out on individual fruit trees, which do not grow in an orchard. As it is very difficult to extrapolate the yield for a larger area (acre, hectare), it could be reported per tree. In such cases, not only the weight, but also the number of fruit should be reported;
e. Besides the yield per unit area, the date of harvesting should also be reported. As some crops are harvested more than once, e.g. cotton bolls or tobacco leaves, every harvesting (picking) should be reported separately in the ten-day reports (see example in table 3.1);
f. Another piece of information to be supplied is the moisture content of some agricultural produce such as grains. It is important to know whether the reported yield is from newly harvested grains, or from grains which have been dried for several weeks after harvesting. The agrometeoroloigical observer can determine the moisture content himself, by taking a representative sample of the grains and drying it in the oven up to a constant weight, using the same method as for soil-moisture determination. The moisture content of the agricultural produce should be reported on the reverse side of the ten-day reports;
g. Besides the moisture content of some produce, there are many other details when reporting yield, which make the data more useful (or less useful). Thus, when reporting maize yield, it should be stated whether the yield refers to the grain alone, or the grain with the cobs. When reporting cotton yield, it should be clarified whether it is a question of seed or lint. Many crops whose seeds are in pods or shells, or are covered with husks, can have their yield reported with or without these pods, shells or husks. The observer should do his/her best to report all supporting information that might help the analysis of the data. It should always be kept in mind that some
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of the data will be used twenty or more years later for some research projects and many details that, at present, seem too obvious to be reported, will be forgotten when needed.
V. SOIL-MOISTURE OBSERVATIONS
INTRODUCTION
Soil moisture is a vast subject and is dealt with by specialists from several professions: agronomists, irrigation and civil engineers, hydrologists, geographers and agrometeorologists. Normally, each of these professional groups deals with different parts of the soil-moisture subject, from different standpoints and for different purposes. Quite often, though, their interests and goals overlap.
Agrometeorologists use soil-moisture data for two main purposes: operations and research. The operational activities consist in:
a. Issuing agrometeorological bulletins
The bulletins are issued either monthly or every ten days. Their aim is to inform the agricutlrual authorities of the existing agrometeorological conditions and the state of the crops. The bulletins give data for the existing soil-moisture storages of the various agricultural crops and in the different soil layers;
b. Issuing agrometeorological forecasts
Some of these forecasts are for the expected soil-moisture storages of the various agricultural crops. Soil-moisture data are also included in some other forecasts, such as for crop yield;
c. Answering day-to-day enquiries
Many of these concern soil-moisture data for proper interpretation and assessment of various types of agrometeorological information.
The research purposes are numerous and of different types. Data from direct and detailed soil-moisture observations are used for studying:
a. The regime and balance of soil moisture in the various areas of the country with different climate and with different soil types. These studies deal with the entry (accumulation) of soil moisture, i.e. when it occurs, the amount stored or accumulated, the depth to which the soil is moistened to field capacity. They also deal with the discharge of soil moisture, i.e. its amount and the extent of depletion of soil moisture storage;
b. The regime and balance of soil moisture of different agricultural crops grown in the same area and on the same soil type. These kinds of study show the effect of the specific biological features of each crop on the regime and balance of soil moisture;
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c. The amounts of water lost by evapotranspiration during the whole growing season of the agricultural crop, as well as during the different interphase periods of their development;
d. The amount of soil moisture which the plants use from the different soil layers during the whole growing season and during various interphase periods;
e. The water requirements of the different agricultural crops in the different climatic regions of the country (for the whole growing season and for the interphase periods as well);
f. The extent to which the water requirements of the agricultural crops are satisfied in the different climatic regions of the country with different soil types, for whole growing season and during the different interphase periods;
g. The normal amounts of soil-moisture storages. Like any other meteorological element, the average value for a long period is considered as “normal” and the actual value during a particular year is better assessed when compared with the normal one. The “normals” for soil moisture storages are prepared for each agricultural crop, for any time of the year, for the different areas of the country, and for the different soil types;
h. The relationship between the meteorological factors and the rate of evapotranspiration (or the amount of soil-moisture storage). Such relationships are used for indirect estimation or for forecasting the rate of evapotranspiration or the soil-moisture storage;
i. The relationship between the soil-moisture storage and the state of the plants. Such relationships are used for proper assessment of the state of the agricultural crops;
j. The relationship between the soil-moisture storage and the yield of the agricultural crops, used for forecasting the latter;
k. Utilization of rainfall water - how much of it is transformed into soil moisture. These studies are carried out both for individual cases of rainfall and for all cases of rainfall during the rainy season;
l. The regime and balance of soil moisture in bare agricultural fields. In many parts of the world, a considerable amount of moisture is accumulated in the soil prior to planting time and, therefore, these studies are quite important;
m. The types of soil-moisture regime in the country;n. The amount of soil moisture that penetrates the soil and joins the groundwater.
Four different methods for soil-moisture observations and they are the only methods that are of practical importance. One of them is direct, while three methods are indirect. The direct method is discussed in full detail - not only because it is the most widely used, but also because it is used for calibrating the indirect methods.
Some basic principles concerning, for example, the time and depth of the observations, or the number and arrangement of the replications, are valid for all soil-moisture observations, regardless of the method used. For convenience, however, these principles are discussed with the direct method.
Method for direct soil-moisture determination
The direct method for soil-moisture determination is also called the “gravimetric method”, consisting of taking samples of wet soil, weighing them, removing the water and reweighing them. The difference between the weights of the wet and dry samples is the amount of water, which is expressed as a percentage of the weight of the dry soil.
Instruments used for the observations
Determination of the soil moisture by the gravimetric method requires the following instruments:
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a. Auger for taking soil samples
There are different models of augers. The model described here is considered to be one of the best. It consists of head, central pipe and handle. The head and the handle are screwed on the central pipe, which allows easy exchange of worn parts. The central pipe and the handle are made of water pipes used by plumbers. The head is made of hare, seamless steel and has a diameter of four to five centimeters. Such width is enough for taking adequate amounts of soil samples. Wider heads, used in some countries, cause destruction of the soil and require greater effort in taking the samples.
The head has two sharp teeth. Extremely important for the easiness of taking the soil samples is the angle under which the teeth attack the soil. The angle should be about 35-40o. Smaller angles cause the teeth to slide on the soil rather than to cut it. After a certain period of work (one to two years, depending on the soil type), the teeth become blunt and working with such an auger becomes difficult. The teeth should be sharpened in a workshop or, if this is not possible, a new head should be fitted).
Another very important feature of the head are the slight cuts. These cuts sharply decrease the friction between the head and the walls of the hole in the soil, as well as with the soil that goes into the head.
Starting from the teeth, the head and the central pipe of the auger are marked at 10 cm intervals up to 100 cm. When working with the auger, these marks indicate the depth up to which the auger’s teeth have penetrated into the soil. The central pipe should be 10-15 cm longer than the 100 cm mark. Augers for taking soil sample up to 200-cm depth have a central pipe twice as long; it is marked up to 200 cm, plus 10-15 cm above the last mark.
It should be stressed that the central pipe and the head should lie in a straight line. If the head is slightly bent, taking the soil samples becomes very difficult;
b. Cups for soil samples
The cups (or containers) are made of aluminium or tin alloy. They have tight-fitting lids which prevent evaporation of moisture from the soil sample within. Both cup and lid are numbered with the same number and, thus, lids should not be exchanged. The cups are cylindrical, usually with a diameter of four to five centimetres and are about five centimetres high.
When the soil-moisture observations begin, the cups have to be cleaned and weighed, with an accuracy of ±0.1 g, with their lids. The weight of the empty cups should be noted in a special table. When using them for long periods, the cups sometimes lose some of their weight. Once a year, therefore, all empty cups have to be weighed again, and the correct weights (if changed) should be recorded;
C. Wooden boxes
These boxes are made of plywood and are used for storing and carrying the cups to the observation fields. Each box contains 44 cups (four rows of eleven), which is enough for making
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soil-moisture observations in one field in four replications up to 100-cm depth. There should be special nests in the box for cups. Each cup has its permanent place and a plan should be drawn and stuck on the inside of the box cover.
d. Scales for weighing the cups
Various mechanical or electrical scales can be used. As the accuracy of weighing the cups is ±0.1 g, the scales should have either the same, or better, accuracy (e.g ±0.05 g). A cup full of wet soil rarely exceeds 100 g, but the maximum capacity of the scales should nevertheless be between 200 and 500 g ;
e. Oven for drying the soil samples
Both forced-draught and electrical convection ovens can be used. In stations without electricity, gas ovens can also be used. The oven should be big enough to accommodate all the cupsused in one observation in the station’s programme. The oven should also be able to maintain a temperature of 100-105oC for long periods.
f. Two knives, chisels or similar tools
These are needed for removing the soil samples from the auger and, later on, for cleaning the excess soil from the auger;
g. Large pincers
These are needed for taking the hot cups out of the drying oven.
Depth of the soil samples
Soil moisture observations are carried out at different depths in the soil. Some specific research projects involve observations on the moisture of the soil surface alone. Other research projects require measurements up to 10 m in the soil and subsoil, and even deeper. For irrigation purposes, soil-moisture observations are usually carried out to a depth of 80 cm, despite the fact that the plants consume water from deeper soil layers.
For routine agrometeorological purposes, soil-moisture observations in different parts of the world are performed to a depth varying between one and two meters. The depth depends on the climate, especially the amount of rainfall. In areas with heavier rainfall, the rainwater regularly penetrates deep into the soil, while in more arid areas, the moistening of the soil does not go beyond a depth of one meter. The depth of the soil-moisture observations depends also on the type of crop. Some crops, like beans for example, have shallow roots and normally do not consume water from the deep soil layers. Other crops, like maize, sugar cane, sunflower, beet, etc., develop deeper root systems and regularly consume water from the second-and even the third-metre layer. Some perennial crops like lucerne, vine, etc. are known to consume water from layers as deep as 10m or more.
It should be emphasized, however, that routine agrometeorological observations do not need to cover the whole depth to which plants' roots penetrate and consume water. This is the task of specific research projects. It should be the duty of the local agrometeorologists (Class I) to determine the depth of the routine soil-moisture observations in their countries.
37
Taking the soil samples
When going into the field for soil-moisture observations, the observer should take the following instruments: one or two augers (the first is for samples up to 100 cm depth, while the second, if necessary, is for the second-meter samples); the boxes with the clean cups; and the knives or chisels for taking the samples and for cleaning the excess soil from the auger. The observer should also have a notebook in which the numbers of the cups for each depth of each replication should be written in advance. When in the field, the observer should take care that samples are put in the right cups.
The auger enters the soil by being pressed down and turned around at the same time. The turning is done in a clockwise fashion. A very important point to bear in mind, when taking soil samples, is that the auger should go into the soil vertically, otherwise working could become extremely difficult. Another difficulty in pulling the auger out of the ground occurs when some lumps of soil fall into the hole while the auger is still inside.
After taking one soil sample and before taking the next, the auger should be cleaned - both inside and out. It is preferable to collect the excess soil removed from cleaning in order, later on, after taking samples from all depths, to fill the hole in. the holes should never be left unfilled not only because water will be evaporated from their walls, but also because rainwater might accumulate and influence the water content of the deep soil layers.
Fields for observations
Soil-moisture observations are usually carried out on several crops at each agrometeorological station. The number and kind of crop are determined by the National Meteorological Service but these are always the most important crops for a given area. Due to crop rotation, most annuals are moved every year from one field to another. As a result, the soil-moisture observations also move, following the crops selected. There is no objection to this, as long as the soil in the new field has similar properties. After one or two years, the crops are usually planted back in the original field.
In order to obtain representative and accurate data for soil moisture, the fields for observations should be large enough. The best size is about one hectare, the same as for the phenological observations. As a matter of fact, for a given crop, both observations should be performed in one field. If the agricultural field is very big, an observational field of about one-hectare should be determined within it. If one-hectare fields are not to be found in the area of a given agrometeorological station, the soil-moisture observations could be carried out on smaller fields. It is not advisable; however, to observe soil moisture in fields smaller than 0.4 ha (about one acre).
Soil moisture has not only temporal, but also spatial variation. Samples taken from only one point in a given field cannot represent the soil moisture in the whole field. For each crop, therefore, the soil samples are to be taken in several replications, and the average value from the replications represents the soil moisture in that particular field (Crop). Normally, four replications are enough for obtaining representative data, but for some soils five replications (or more) are required. The number of replications for each agrometeorological station will be determined separately by the Meteorological Service.
The means of taking soil samples in four replications is as follows: the field for observations (0.4 - 1.0 ha) should be divided into four equal plots, which should be numbered permanently I, II, III and IV. One replication with soil samples has to be taken from each plot.
38
The replications are located in the following order: beginning with soil moisture observations, the replications in plots I and II should be located in the upper left-hand corner of the plot, while in plots III and IV, the replications should be located in the lower left-hand corner. In all cases, the replications should be at least three to four meters from the edge of the plots.
During the next soil-moisture observation, the replications in plots I and II should be moved dawn wards, at about 1.0 to 1.5 m from the previous replications; while in plots III and IV, the new replications should be 1.0 to 1.5m upwards from the old ones. The next soil-moisture observations are taken in the same order, till the replications reach the opposite edge of the plot. Then, in all plots, a new line should be started which has to be parallel to the first one, at a distance of about two meters.
The way of taking soil samples in five replications is as follows: the field for observations (0.4 to 1.0 ha) should be divided into five similar plots, which should be numbered permanently I, II, III, IV and. One replication with soil samples has to be taken from each plot. The location of the replications is also shown in the figure. The general regulations for moving the five replications are the same as in a field with four replications.
The replications (holes) in the row-grown crops should be located between the plants in the lines. It is not recommended to bore the holes in the middle or at the bottom of the rows.
For soil-moisture observations on bushes or trees, the holes should be bored approximately midway between the stem and the end of the plant's crown. When this is not possible for plants with low crowns, the holes have to be made as close as possible to the stem.
Sometimes, the auger might not reach the depth required, due to a stone in the soil. When soil-moisture observations are taken to a depth of 200 cm and a stone appears at less than 100 cm, the hole should be avoided, the soil samples already taken should be thrown away, and a new hole started nearby. If a stone appears deeper than 100 cm, the observer should content himself with the soil samples taken to that depth and no new hole need be started. It should be stressed that such a case is allowed only with one of the replications - the rest of the replications should be sampled to the depth required.
When soil - moisture observations are taken only to a depth of 100 cm, one of the replications could be left without taking soil samples from depths of 80, 90 and 100 cm (due to the appearance of a stone). If a stone appears before a depth of 70 cm, a new hole should be bored nearby.
The frequency of the soil-moisture observations depends on their purpose. For some specific research goals, the observations are carried out daily. Other research projects need soil-moisture data only once in a season. For the agrometeorological aims described in section 4.1, the optimal intervals of performing the soil-moisture observations are ten days.
Calculation of the soil moisture
Soil moisture refers to the existing water in the soil, expressed as a percentage of the weight of the absolutely dry soil. The calculation is as follows;
The weight of the cup with the dry soil (after drying) should be subtracted from the weight of the cup with the wet soil (before drying). The difference obtained is the amount of water (in grams) which has been in the soil sample and has evaporated. Later on, the weight of the empty cup should be subtracted from the
39
weight of the cup with the dry soil, in order to obtain the weight of the absolutely dry soil. By dividing the weight of the water evaporated from the soil sample by the weight of the dry soil, and multiplying the result by 100, a percentage is obtained of the soil moisture. An example is:
Weight of the cup with wet soil 128.6gWeight of the cup with dry soil 121.4gWeight of the empty cup 92.5gThe weight of the evaporated water is 7.2g (128.6 - 121.4);The weight of the dry soil is 28.9 g (121.4 - 92.5);The percentage of soil moisture = 7.2 x 100 = 24.9%
28.9
After the soil moisture from all the samples is calculated, the observer should calculate the average soil moisture for each depth. As mentioned above, there are four (or five) replications from each depth, and the observer should calculate the average value from the replications.
Among the various indirect methods for estimation of soil moisture, three are considered to be useful for certain practical purposes. These are the electrical resistance method, the neutron method, and the tensiometers. Each one of these has numerous designs and models. The manufacturers of the instruments always attach detailed instructions for the operation of their model.
Electrical resistance method
Two main devices are used:a. Porous blocks containing electrodes are embedded in the soil. The blocks are rectangular or cylindrical
in shape and are several centimeters in size. They are made of different porous materials centimeters in size. They are made of different porous materials usually gypsum but also of fibreglass, nylon cloth, casting plaster, etc. The electrode system usually consists of two wires, which are fixed one or two centimeters apart in the prous block .
b. Weatstone electrical bridge for measuring electrical resistance ranging from several hundred ohms to several hundred thousand ohms
After being in the soil for some time, the water content of the porous block reaches equilibrium with the water content of the surrounding soil. As the soil dries, the water content of the block also decreases. When the soil moisture increases, so does the water content of the block.
The water content of the block influences the electrical conductivity of the block - the higher the water content, the higher the conductivity (the smaller the electrical resistance) and vice versa. By generating an electrical current and passing it through the electrodes, we can measure the electrical resistance of the block and, thus, estimate the water content of the surrounding soil. In this way, an observation of the soil moisture consists of measuring the electrical resistance of the blocks embedded in the soil.
Prous blocks are embedded at those depths of the soil where soil-moisture data are required. Shows blocks placed at the standard depths of one replication. If, for example, the spatial variability of the soil in a field requires four replications, then four sets of blocks should be placed in the field.
The relationship between electrical resistance and soil moisture should be expressed quantitatively. This is achieved by calibrating the instruments. The calibration provides the estimated values of soil moisture corresponding to the different values of measured electrical resistance. The calibration process consists in
40
making numerous parallel observations by the electrical resistance method and by the gravimetric method, at various soil-moisture levels. The parallel observations can be made either n the field (which is more accurate but takes more time) or in a laboratory to which large soil samples from different depths are brought.
The results of all parallel observations are plotted on a system of coordinates, and later a calibration curve is drawn. Although the curve can often be drawn by visual estimation of its location, the more accurate approach is by the standard methods of regression analysis (the last-squares regression procedures).
Neutron method
Hydrogen atoms slow down fast neutrons and change them into slow neutrons. If a source of fast neutrons is place in the soil, fast neutrons will be emitted into the surrounding soil. Hydrogen is contained in the molecules of the water in the soil and, therefore, the greater the water content of the soil, the more neutrons will be slowed down, and vice versa. The number of slow neutrons is counted and used to estimate the amount of water in the soil.
Tensiometers
A tensiometers consists of a porous cup filled with water embedded in the soil. The porous cup is connected through a tube to a pressure-measuring device above the soil surface (Figure 4.14). After installing the tensiometer in the soil, the water in the porous cup soon comes into equilibrium with the moisture in the surrounding soil. As the soil dries, water begins to flow out of the cup, and the vacuum created in the cup is registered on the pressure-measuring device (the latter could be either a vacuum gauge or a mercury manometer). When the soil becomes wet again, water flows into the porous cup, which decreases the tension. Thus, the changes in the tension reflect the changes in the soil moisture surrounding the cup.
Advantages and disadvantages of the various methods
Historically, the direct method is the oldest one and the indirect methods were developed in order to overcome the drawbacks of the former. This has been achieved to a great extent but, unfortunately, the indirect methods have their own essential disadvantages. It appears that what is an advantage of the direct method is a disadvantage of the methods, and vice versa.
Disadvantages of the direct method
a. Firstly, this method is time-consuming. The time gap between the indirect and the gravimetric methods is wider if a smaller number of observations are performed.
b. The direct method is laborious. Taking soil samples with the auger in sandy or loamy soils is not a problem. In heavy clay soils, however, especially when they are very wet or very dry, taking the samples requires considerable physical effort and two persons are often needed to work with the auger. At the same time, observations by the indirect methods do not require any physical effort.
c. Successive soil-moisture observations in a field are carried out 1.0-1.5 m apart from each other. If the soil is relatively homogeneous small spatial variability), then this is of little or no significance.
41
Nevertheless, there are soils, especially along riverbanks, where sandy and loamy layers of various thicknesses are intermixed. In such cases, soil-moisture observations by the gravimetric method do not give good results. The indirect methods work better in such conditions, especially if they are well calibrated. Calibration, however, is made with the use of the gravimetric method and, therefore, for heterogeneous soils the calibration curves are often not sufficiently reliable;
d. With soils containing many stones or small rocks, the use of the gravimetric method is not recommended. Firstly, the stones will necessitate frequent reboring of holes. Secondly, stones in the soil sample affect the true value of the soil moisture. A stone occupies appreciable weight and volume in the sample, without making a commensurate contribution to the water content of the sample. The indirect methods work better in rocky soils but, as mentioned above, a good calibration curve is difficult to obtain.
Disadvantages of the indirect methods
Some of the disadvantages are common for all three indirect methods discussed in this compendium. Others are typical for one or two of the methods. The common disadvantages will be discussed first.
a. All indirect methods are less accurate that the gravimetric method. There are two main reasons for this. As stated earlier, being indirect, these methods do not determine soil moisture but electrical resistance, number of neutrons and tension. Furthermore, each of the devices for indirect measurements of the soil moisture has what is called an "instrumentation error". The size of these errors depends upon the characteristics of the particular equipment;
b. All indirect methods need calibration. Although some manufacturers of devices for indirect measurement of soil moisture supply them with calibration curves, it is far better to calibrate the instruments in the fields where the observations are to be carried out in the future. A calibration is definitely required when the instruments are to be used in areas where the soil and climate conditions are essentially different from those in the are (or country) of origin of the instrument. Another important reason for having one's own calibration is that the calibration curves supplied by most manufactures are valid mainly for high soil moisture conditions. This is because most of the devices are used in irrigation schemes. For agrometeorological purposes, however, soil moisture is measured within the whole range possible - from almost completely dry to extremely wet.
Calibration in field conditions takes between several months and one year. The gravimetric method does not need calibration - it is used to calibrate the indirect methods;
c. Unlike the instruments used in the gravimetric method, which can last for 20 or 30 years (except the head of the auger, which can easily be replaced), the instruments used in the indirect methods have a much shorter life-span. Porous blocks, especially those used in the electrical resistance method, gradually deteriorate in the soil. Some last only a year. Among the three indirect methods discussed in the compendium, The devices used by the neutron method have the longest life-span, lasting an average of seven to eight years;
42
d. For the large number of observations carried out in an agrometeorological station, the price of the instruments used in the indirect methods is higher than the price of the instruments used in the gravimetric method. The actual prices vary from country to country from model to model, and from year to year. Among the indirect methods, the most expensive are those of the neutron method (about $3,000 to $4,000 in 1980) followed by the tensiometers.
e. All indirect methods have instruments installed permanently in the fields. These are the wires of the electrical resistance method, the tubes of the neutron method, and the whole tensiometers. This hinders the work of the agricultural machines; fieldwork has to be done by hand, which is especially inconvenient when the fields do not belong to the Meteorological Service but to another agricultural authority. The instruments installed in the fields (in four or five replications) are also subject to theft or damage by children, vandals or animals. As seen, the gravimetric method does not need instruments placed permanently in the fields;
f. Unlike the gravimetric method which is very simple and whose instruments require little professional care, the indirect methods are more complex for operation and require more sophisticated maintenance. This is not a disadvantage when the indirect method is used at an agricultural research station, where all kinds of professional staff live and work in the same area with the agrometeoroloigical observers. For a network of agrometeorological stations, however, manned by one or two observers, and located sometimes hundreds of kilometers from their headquarters, this fact could be an essential drawback. In addition, the instruments of the indirect methods are fragile. They often require spare parts, which are unavailable in most developing countries and there are many cases when soil moisture observations are interrupted for more than one year until the necessary spare part is imported;
g. When the permanent parts of the instruments are installed in soils that shrink when they dry up, small gaps are formed between the tubes (or wires) and the soil. When rain occurs, water often penetrates deep along the tubes or wires, and affects the reading of the instruments;
h. In addition to the disadvantages common to all indirect methods, each one of them has some important specific drawbacks:
- The neutron method does not work well for soil layers less than 20-25 cm. Measurements in the to 25-30 cm of the soil are unreliable because some neutrons escape into the air. It cannot accurately detect sharp changes in the moisture content in the soil profile. It requires special protection against radiation and radioactive leakage. It requires transportation of the devices by car to more distant fields, etc;
- The tensiometers' biggest disadvantage is that they work only at high soil-moisture content around or slightly below field capacity. When the soil moisture drops lower the tension increases and air enters the system through the pores of the cup. Tensiometers, therefore, are used mainly for irrigation control. In areas with winter temperatures below freezing point, tensiometers have to be removed from the field and reinstalled the following spring. Essential changes in the soil temperature affect the readings of the tensiometers. Due to the so-called "hysteresis" effect, the calibration curve of soil when it becomes watter. There is a time-lag in response to soil-moisture changes: sometimes eight to ten hours elapse before the device responds to change in the soil moisture;
43
- The readings of the electrical resistance method are influenced by salinity content of the soil- salt increases electrical conductivity and decreases resistance. The soil temperature always influences the readings - if warm, a porous block has higher conductivity and smaller electrical resistance without increase in the moisture content. The method is subject to the hysteresis effect. When used on soils that shrink when drying up, the soil moves slightly away from the porous blocks and, as a result, the latter do not reflect satisfactorily the moisture changes in the soil surrounding them. When the soil moisture is around field capacity, the device does not reflect well the changes in the soil moisture - it works better in drier soils. When the soil moisture content changes, there is a time lag of about one-day before the porous block comes into hydraulic equilibrium with the surrounding soil.
Conclusions
Those who need to measure soil moisture often ask which is the best method. There is no answer to such a general question. Each of the methods for determination or estimation of the soil moisture has its advantages and disadvantages. For some purposes and under certain conditions, a given method might appear the most suitable, while under other conditions and circumstances; the same method might be completely inappropriate. The suitability of a method depends on a variety of natural, scientific and administrative factors, the most important of which are:
- The climate of the area;- The type of the soil;- The number of depths in the soil in which the soil moisture should be determined;- The accuracy of the data required;- Whether the observations are permanent or temporary;- Whether the observations are to be performed throughout the whole year, or only during the
growing season of a crop;- The speed at which the results are required;- The number of crops and replications for soil-moisture observations;- The size of the fields for the soil-moisture observations;- Whether the fields belong to the authority that will carry out the observations, or whether they
belong to another authority;- Whether funds are available for purchase of equipment;- The availability of staff to perform the observations;- The availability of staff to supervise the observations and maintain the instruments;- The number of stations in the country, their accessibility and proximity to headquarters.
The review of the advantages and disadvantages of the different methods made here could help in choosing the proper method. One alternative not yet mentioned is to start with the gravimetric method (it cannot be avoided because of calibration) and later on, if so desired, to test the applicability and suitability of one of the indirect methods.
Water Balance and Soil-Moisture Instruments
44
Standard instruments used for measuring the different elements of the Water balance for climatological and hydrological purposes are also employed in agricultural meteorology. The WMO Guide to Meteorological Instrument and Observing Practices (WMO No. 8), the Guide to Hydrological Practices (WMO No. 168), and WMO Technical Notes Nos. 21, 83 and 97 provide information and guidance concerning instruments such as raingauges, rain recorders, snowgauges, and screened and open pan evaporimeters. However, a few remarks are appropriate here on a number of instrument used for specific work and on their operation.
Inexpensive raingauges and small-size totalizer raingauges are used for studying the small-scale distribution of precipitation, as in limited mesoclimates., forest or crop interception, shelterbelt effects, etc. However, it is advisable to check these raingauges often against a standard gauge because of the possibility of the plastic gauges becaming distorted by strong sunlight, freezing temperatures or wind.
Although it is possible to estimate actual or potential evapotranspiration from observed values of screen or open pan evaporimeters or from other integrated sets of meteorological observations, more accurate, direct observations are often preferred. Actual evporanspiration is measured by using soil evaporimeters or lysimeters, which are field tanks of varying types and dimensions, containing natural soil and a vegetation cover (grass, crops or small shrubs) (see WMO Technical Note No. 83).
When adequate instrumentation facilities and personnel are available it is possible to compute actual evapotranspiration using energy-balance or mass-transfer methods. Composite devices for simultaneous and spatial measurements or net radiation, soil heat continuous measurement of wind, temperature and water-vapour profiles for the aerodynamic method.
Potential evapotranspiration can be measured with evapotranspirometers or lysimeters., containing soil at field capacity and a growing plant cover. A surface at almost permanent field capacity is obtained by regular irrigation or by maintaining a stable water table on the soil surface. A strict control must be kept of infiltration from rainfall or excess water supply. The reliability of observations by both lysimeters and evapotrnspirometers depends upon the conditions at the instrumental surface being identical to the conditions of the surrounding soil.
Dew and fog deposition can be important supplies of water in some limited parts of the world, where the frequency of such phenomena is high and where other water resources are scarce. Many countries have developed instruments to measure these elements, but they are not yet standardized (WMO Technical Note No. 55). Observations of dew and leaf wetness duration may be important in some regions in connexion with plant and animal diseases.
Time and space variation of soil-moisture storage is the most important element of water balance for agrometeorology. Gravimetric observations of soil water content have been in use for agrometeorology. Gravimetric observations of soil water content have been in use for a long time in many countries. The instruments used for the purpose have three basic components in common, i.e an auger to obtain a soil sample, scale for weighing it, and an oven for drying it at 100-105°C. Comparison of weights before and after drying permits evaluation of moisture content which is expressed as a percentage of dry soil or, where possible, by volume (in mm) per meter depth of soil sample. Because of large sampling errors and high soil variability, the use of three or more replicates for each observational depth is recommended (see also WMO Technical Note No. 21).
Several instruments have been constructed to measure soil-moisture variations at a single point, in order to avoid the variability of soils in space and depth. Among those in wider use are tensiometers, porous blocks (electrical resistances) and radioactive probes (see WMO Technical Note No. 97).
45
Tensiometers measure soil-moisture tension, which is a useful agricultural quantity, especially for light, irrigated soils. The instrument consists of a porous cup (usually ceramic or sintered glass) filled with water, buried in the soil and attached to a pressure gauge (e.g. a mercury manometer). The water in the cup is absorbed by the soil through its pores until the pressure deficiency in the instrument is equal to the suction pressure exerted by the surrounding soil.
The electrical resistance method uses blocks of porous materials (e.g. gypsum), the electrical resistance of which changes when moistened, without alteration of the chemical composition.
Radioactive methods include the use of gamma radiation and neutron-scattering probes. Soil moisture is measured with the gamma radiation probe by evaluating differential attenuation of gamma rays as they pass through dry and natural soils. This method generally requires two probes introduced simultaneously into the soil a fixed distance apart, one carrying the gamma source and the other the receiver unit. In the neutron-scattering method, which is more widely used and safer and simpler to operate, the scattering and slowing effects of H+ ions (from water in the soil) on fast neutrons from a source probe are measured. Fast neutrons back-scattered and slowed by collision with H+ ions impact on a built-in receiver in the probe. The impact signal of each neutron is amplified and counted by a digital or spring scaler. The total count per unit time is proportional to the moisture content of a sphere of soil, approximately 15 cm in dimater. Neutron sources of americum-beryllium are safer than those of radium-beryllium; transistorized circuits and digital scales are preferred because they have a more accurate response.
Subjective methods of estimating soil moisture have been used with satisfactory results in some regions where regular observations in a dense network are necessary and suitable instruments lacking. Skilled observers, trained to appreciate the plasticity of soil samples with any simple equipment, form the only requirement for this method. Periodic observations and simultaneous determinations of soil texture at different depths, by competent technicians, allow approximate soil-moisture charts to be contructed.
DETERMINATION OF THE AGROMHYDROLOGICAL PROPERTIES OF THE SOIL
Introduction
Each soil has certain physical and water properties which influence the amount of water held in the soil, its mobility therein, and its availability to the plants. The physical properties are texture, bulk density, partial density, porosity, etc. The water properties are full capacity, capillary capacity, field capacity, wilting point, maximum hygroscopicity, etc.
Whenever soil-moisture observations are carried out, some of the abovementioned soil properties have to be determined. Agrometeorologists rarely deal with all of these properties. Three of them, however, are of greater importance: bulk density, field capacity and wilting point. Many agrometeorologists call them “agrohydrological” properties of the soil.
Before describing the methods for their determination, it should be stated soil-moisture observation discussed in the previous chapter. By using one of the four methods (gravimetric, electrical resistance, neutron, tensiometer), soil moisture data are obtained. In order to interpret and properly present these data, the values of the agrohydjrological properties of the soil are needed.
5.2 Bulk density
46
Bulk density is the weight, in grams, of one cubic centimetre (-g cm -3) of undisturbed, absolutely dry soil. “Undisturbed” means that within the volume of one cubic centimetre, both the solid particles and the pore space are included.
When soil moisture is expressed as a percentage of the weight of the dry soil (20 per cent, for example) , the values indicate only the degree of wetness of the soil. They do not show the water content existing in the soil. The latter is expressed in millimetres of water, in the same way as rainfall. The values of the bulk density of the soil are used to transfer the percentages of soil moisture into millimetres of soil water. This is done with the help of the following small formula:
W = a x b x c10
Where W = total storage of soil water in millimetres; a = soil moisture in percentage of the weight of the dry soil;
b = bulk density of the soil in g cm-3; c = thickness (height) of the soil layer in centimetres;
Example: Determine the total amount of soil water (in millimetres) in a 20-cm soil layer, if the bulk density of this layer is 1.4 gm cm-3, and the soil moisture is 22.2 per cent:
W = (11.1) (1.4) (20) = 621.6 = 62 mm 10 10
Bulk density is determined by two main methods. One is called the core method, and consists of driving a specially designed and constructed long cylindrical metal sampler with a cutting edge into the soil. The advantage of this method is that there is no need to dig a pit into the soil. Disadvantages are that samplers for deep soil layers (2m) are not often manufacture, do not work in wet soils, and are more expensive than the second method.
The second method uses instruments that can be easily and cheaply made in each country. Four small steel rings are needed and a device to knock them into the soil. Each ring is filled with a sample of undisturbed soil. By dividing the weight of this sample by the volume of the ring, the bulk density of the soil is obtained.
The rings have a diameter of six to seven centimeters, and a height of about five centimetres. Thus, the volume of the rings might vary from about 140 to 200 cm3. They are made of seamless steel and have a sharp cutting edge on one side (Figure 5.1). the inside diameter of the rings should be slightly larger than the diameter of the cutting edges, which allows the soil block to enter the ring with less friction. The rings are driven into the soil with the help of a metal device and a hammer.
The bulk density of the soil has to be determined for each depth at which soil-moisture observations are carried out -5, 10, 20, 30 cm etc. A pit has to be dug in the soil, and the determination of the bulk density for the 5-cm depth begins by clearing the top two or three centimetres of the soil from the front wall of the pit. The four rings are then carefully knocked into the soil until they penetrate about half a centimetre below the cleared soil surface. With the help of a spade, the rings are dug out and cleaned of the excess soil. The soil, on both sides of the ring, should be carefully cleared with a large kitchen knife, so that the volume of the soil block is the same as that of the ring. The rings are weighed and a small sample of soil is taken from
47
each to determine the soil moisture. Later on, the soil left in the rings is thrown away and the latter are washed, ready for the next soil depth.
Calculation of the bulk density of the soil
BULK DENSITY OF THE SOIL IN STATION: ……….XX……………………DISTRICT: …..XX……………. FIELD: ….XXX…….. DATE: ..XXX………..
Depth (cm)
Ring number
Weight of the ring with the wet soil (g)
Weight of the empty ring (g)
Weight of the wet soil (g)
Soil moisture (%)
Weight of the absolutely dry soil (g)
Volume of the ring (cm-3)
Bulk Density(g cm-3)
Average bulk density (g cm-3)
5
1234
398.0430.2435.0491.1
142.0161.0157.5215.7
256.0269.2277.5275.4
25.024.424.024.5
204.8216.4223.8221.2
177.5183.1186.9181.3
1.151.181.201.22
1.19
10
1234
422.6462.0435.0497.0
142.0161.0157.5215.7
280.6301.0277.5281.3
24.723.924.124.8
225.0242.9223.6225.4
177.5183.1186.9181.3
1.271.331.201.24
1.26
20
1234
435.7448.5449.9505.0
142.0161.0157.5215.7
293.7287.5292.4289.3
24.323.824.023.9
236.3232.2235.8233.5
177.5183.1186.9181.3
1.331.271.261.29
1.29
30
1234
440.0507.0504.5526.5
142.0161.0157.5215.7
298.0346.0347.0310.8
24.023.823.323.2
240.3279.5281.4252.2
177.5183.1186.9181.3
1.351.531.511.39
1.45
40
1234
468.4502.0492.5552.8
142.0161.0157.5215.7
326.4341.0335.0337.1
23.122.823.022.6
265.2277.7272.4275.0
177.5183.1186.9181.3
1.491.521.461.52
1.50
48
The bulk density at 10 cm depth (as well as all other depths) is determined on the same wall of the pit. The soil has to be cleared to a depth of 7-8 cm and the rings knocked in. For determining the bulk density at a depth of 20 cm, the soil has to be cleared to a depth of 17-18 cm , and then the rings knocked in. This way, the bulk density is determined at all other soil depths (Figure 5.2).
As the rings are about 5 cm high, bulk density is determined for soil layers with the same thickness. Thus, the bulk density for a 5 cm depth is actually for a soil layer between 2-3 and 7-8 cm; the bulk density for a 10 cm depth is actually for the soil layer between 7-8 and 12-13 cm; the bulk density for 90 cm depth is actually for the layer between 87-88 and 92-93 cm, etc.
The calculation of the bulk density for the topsoil layers is shown in Table 5.1. Naturally, it has to continue for all depths at which soil-moisture observations are carried out. The moisture content of the soil in the rings is determined by following the procedure described in the previous chapter.
Although bulk density can be determined at any degree of wetness of the soil, it is preferable to measure it when the soil is moistened to field capacity. This allows a good comparison of the bulk density of the soils in the various areas of the country. Normally, bulk density and field capacity are determined at the same time, and on the same site in the field. This is further expained in section 5.3.
5.3 Field capacity
Field capacity is the maximum amount of water which can be held in the soil after all gravitational water has seeped out, evaporation from the soil surface has been prevented, and there is no direct contact between the soil moisture and the ground water table.
When the whole pore space in the soil is filled with water, the soil is moistened to full capacity. In natural conditions, soils are moistened to full capacity for a short time after heavy rains, or when the snow melts in areas with snow cover. Soil cannot be at full capacity for a longer time because of the force of gravity, the water which is in the large-size pores percolates through the deeper soil layers and the subsoil. This portion of the soil water is called gravitational water (Figure 5.3). After the gravitational water moves away, the rest of the soil water is held in the soil by forces stronger than the force of gravity. When a soil, or a separate soil layer, contains the maximum amount of such water, the soil is moistened to field capacity. The water at, and below, the field capacity is less mobile than the gravitational water and can stay in the soil for a long time. It is, therefore, of great agrometeorological importance.
There are several methods of determining or of estimation field capacity. The direct method consists of selecting a small representative site in the field, watering it to full capacity, waiting for the gravitational water to seep down, and then determining the moisture of the soil. The value obtained will be the values of the field capacity for that soil.
The site for determination of the field capacity should be a square with each side about two metes long. In order to prevent the irrigation water from spilling away from the site, a wooden frame (2m x 2m, and about 25 to 30 cm high) could be placed on the ground. If this is not convenient, a simple earth dike on the edges of the site will be sufficient but should be well trodden.
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FIELD CAPACITY GRAVIATIONAL WATER
AVAILABLE WATER
WILTING POINT
WATER NOT AVAILABLE
ABSOLUTELY DRY SOIL
Figure 5.3 Water properties of the soil and categories of soil moisture
Field capacity should be determined for all soil depths at which soil moisture observations are performed. The amount of water necessary to moisten a certain soil layer can be estimated by determining the soil moisture around the site (a few meters around), and then making a rough estimate of the expected values of field capacity. This estimate can be made by referring to the field capacity values of similar soils in the area, or by looking at the actual soil-moisture data for the same field during the last rainy season or, finally, by using information from the agrometeorological or soil literature. It is always preferable to irrigate the site with slightly more water than necessary. In case the amount of water for irrigation cannot be estimated, then the site should be irrigated for two full days, by maintaining a water layer of 5-10 cm from early morning till late evening. For most soils, this will be enough to moisten them to full capacity to a depth of two meters.
After irrigation, the site should be covered with a plastic sheet or a layer of straw or hay to prevent evaporation. Two days should be allowed for the gravitational water to drain, and then the site should be partially uncovered and the soil moisture determined by the gravimetric method in four replications (Figure 5.4). The holes should later be covered with a clump of muddy soil and the site covered again. After two days, a second determination of the field capacity should be performed (again in four replications) about 40 or 50 cm away from the first holes. After finishing the observation, the new holes and the site should be covered again. Two or three days later, a third and last determination of the field capacity should be carried out on the site.
If the bulk density of the soil is going to be determined on the same site, no third field-capacity observation s made. The determination of moisture from the soil in the rings for bulk density will serve as the third determination of the field capacity.
As mentioned in section 5.1, bulk density of the soil should be determined when the soil is at field capacity. For that purpose, use is made of the same site for field-capacity determination. One or two days after the second determination of the field capacity, a deep pit should be dug in the soil. The depth of the pit should be the depth down to which the bulk density is going to be determined (or even slightly deeper). The pit should be about one meter square with steps down one of the sides. The location of the pit on the site for
50
field capacity is shown in Figure 5.4 (see also Figure 5.2). Next day, the bulk density of the soil can be determined.
5.4 Wilting Point
Wilting point is the amount of soil moisture at which permanent wilting of a plant occurs. Vegetation consumes soil moisture, and if it is not replenished by water from precipitation or irrigation, a time will come when the plants will start to wilt, despite the fact that there is still some moisture in the soil. The moment of permanent wilting occurs when the soil water is attracted to the solid soil particles by forces which are greater than the forces by which the plant’s roots can extract it. Permanent wilting should not be confused with temporary wilting which often occurs in the early afternoon hours of hot, dry days. Permanent wilting means that the plants cannot regain their turgidity even if kept in a place with saturated air (about one hundred per cent relative humidity).
Wilting point is very important because it divides the total amount of soil water into two main categories: that available and that not available to the
STARS
PIT IN THE SOIL
BULK DENSITY ISDETERMINED HERE
SECONC DETERMINATIONOF FILED CAPACITY
FIRST DETERMINATION OF FIELD CAPACITY
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Figure 5.4 - Site for determination of field capacity and pit for determination of bulk density
plants (Figure 5.3). The available soil moisture is between the full capacity of the soil and wilting point. As soils are at full capacity for a very short time during the year, the available soil water is considered, for most practical and research purposes, as the soil moisture between field capacity (not full capacity) and wilting point. Any soil moisture below the wilting point is not available to the plants.
As for the other agrohydrological properties of the soil, there are several methods of determining (or estimating) the wilting point. The direct method is carried out in the following steps:
a. Large soil samples of about two or three kilograms are taken in paper or plastic bags from one of the walls of the pit dug for determination of the bulk density. One soil sample should be taken for each depth at which the soil moisture is observed;
b. The bags should be kept open for a couple of days in the laboratory, so as to let the soil dry. Four cups (containers) made of glass or tin should then be filled with soil from each depth. The cups should be three to four centimetres in diameter, and about eight to ten centimeters high (Figure 5.5). At about three centimeters below the rim of the cups, several seeds of a crop are to be planted and then covered with soil close to the rim. Seeds from many crops can be used for this purpose, but oats or barley are preferable;
c. The soil in the cups is moistened to field capacity with the aid of a pipette. Nitrogen and other nutrients are added to the water in order to encourage good plant growth. The cups are watered every day with small amounts of water but water logging should be avoided. When the plants reach seven or eight centimetres in height, the watering stops and the soil surface in the cups is carefully covered with paraffin or coarse sand in order to limit evaporation;
d. Several days later, the plants will consume the available water and the first features of wilting will appear - the leaves will drop (Figure 5.6). In order to ensure that the wilting is permanent, the plants should be put in a chamber with air humidity close to one hundred per cent for about twelve to fourteen hours (usually an overnight period). If they regain their turgidity, the wilting is not permanent and the plants should be brought back to the laboratory. When the wilting reappears, the plants are put in the humid chamber for another overnight stay. This procedure should be repeated until the plants do not regain their turgidity while in the chamber;
e. When the stage of permanent wilting is reached, the plants should be cut off and discarded together with the top two or three centimetres of the soil in the cup (the soil which is above the planted seeds). The moisture content of the rest of the soil in the cups should be determined, following the guidelines for soil-moisture determination given in the previous chapter. The results will be the wilting point of the soil.