PRECIPITATION

Introduction

Precipitation variability

Rainfall measurements techniques

Design of precipitation gauging network

Filling up of missing record

Consistency of rain record

Estimation of mean areal rainfall

IDF and DAD analysis

Snow measurement and determination of snow melt

1.0 INTRODUCTION

Precipitation denotes all forms of water that reach the earth from the

atmosphere. The usual forms are rainfall, snowfall, hail, frost and dew.

The essential requirements for precipitation to occur are

The atmosphere must have moisture,

Presence of nucleii around which condensation of vapor takes place

Weather conditions must be good for condensation of water vapour to

take place, and

The products of condensation must reach the earth.

Condensation

Condensation is the change of water from its gaseous form (water vapor)

into liquid water. Condensation generally occurs in the atmosphere when

warm air rises, cools and looses its capacity to hold water vapor. As a

result, excess water vapor condenses to form cloud droplets.

Forms of Precipitation

RAIN Drop size - 0.5 mm to 6 mm. Rain is considered as

light rain intensity < 2.5 mm/h

moderate intensity caries from 2.5 to 7.5 mm/h

heavy intensity > 7.5 mm/h

SNOW in the form of ice crystals, hexagonal in shape;

density of snow = 0.1 g/cm3

DRIZZLE droplets of size < 0.5 mm; Intensity < 1 mm/hr

GLAZE it is the drizzle, which freezes immediately in contact with cold objects of

the earth’s surface

SLEET where rain falls through the air of subfreezing temperature, the drops

freezes to form grains of ice, called sleet.

HAIL It is the precipitating rain in the form of any irregular form of ice with size

> 6 mm

DEW During nights the moisture present in atmosphere condenses on the

surface of the objects forming water droplets called dew.

Types of Precipitation

Precipitation classified according

to the factors responsible for lifting

the air mass

1.0 CONVECTION

Convection refers to atmospheric

motions in the vertical direction.

As the earth is heated by the sun,

different surfaces absorb different

amounts of energy and convection

may occur where the surface

heats up very rapidly. As the

surface warms, it heats the

overlying air, which gradually

becomes less dense than the

surrounding air and begins to rise.

2.0 OROGRAPHIC

Air is lifted by the earth itself. When air encounters a mountain range, for

example, air is forced to rise up and over the mountains and if enough lifting

occurs, water vapor condenses to produce orographic clouds.

FRONT - is the interface between two distinct air masses. Under certain

favourable conditions when a warm air mass and cold air mass meet, the warmer

air mass is lifted over the colder one with the formation of front. The ascending

warmer air cools adiabatically with the consequently formation of clouds and

precipitation.

3.0 CYCLONE

It is a large low pressure region with circular wind motion. Two types of

cyclones are r Tropical cyclones and Extratropical cyclones.

Tropical cyclone: A tropical cyclone, also called cyclone is a wind system

with an intensely strong depression with MSL pressures sometimes below

915 mbars. Areal extent of a cyclone is about 100-200 km in diameter.

Winds are antilock wise in the northern hemisphere. The centre of storm

called the eye, extend to about 10-50 km in diameter.

Extratropical Cyclone

These are cyclones formed in locations outside the tropical zone. Associated with

a frontal system, they possess a strong counter-clockwise wind circulation in the

northern hemisphere. The magnitude of precipitation and wind velocities are

relatively lower than those of a tropical cyclone. However, the duration of

precipitation is usually longer and the areal extent also is larger.

Anti Cyclones

These are regions of high pressure, usually of large areal extent. The weather is

usually calm at the centre. Anticyclones cause clockwise wind circulations in the

northern hemisphere. Winds are of moderate speed, and at the outer edges,

cloudy and precipitation conditions exist.

2. PRECIPITATION

VARIABILITY

GANGA BASIN

The annual hydrograph

characterised by low flows during post- and

pre-monsoon seasons and extremely high

flows during the monsoon season

The annual variability in maximum

flood discharge rates and volumes

of the Ganga River for 19 years

3. RAINFALL MEASUREMENT TECHNIQUES

Non-recording Rain Gauge (Symons’ Gauge)

Measurement time : 8:30 AM

Tipping Bucket Type Rain Gauge

Demerits

1. When tipping of buckets take place, rainfall at that instant is not recorded.

2. Very high intensity rainfall gives close signal, which can make it difficult to record

the number of tips and

3. Calibration of tips may change due to rusting and dirt accumulation.

Size of each bucket = 0.25 mm rainfall

Weighing Bucket Type Rain Gauge

When very heavy precipitation occurs, there is good chance that the bucket will overflow

Syphon Type Rain Gauge

Rainfall Mass Curve From a Syphon Gauge

RADAR MEASUREMENT OF RAINFALL

4. DESIGN OF PRECIPITATION GAUGING NETWORK

Type of regions Minimum area for one stationunder ideal condition in sq. km.

Area to be covered underdifficult condition perstation in sq. km.

WMO recommendations (1969)

1. Flat regions of temperateMediterranean and tropical zones

600-900 900-3000

2. Mountainous regions oftemperate Mediterranean andtropical zones

100-250 250-1000

3. Small mountainous regions withirregular precipitation

25

4. Arid and polar zones 1500-10,000

INDIAN STANDARD (IS:4987-1968)

1. Plain area 5202. Regions of average elevation 1000m

260-390

3 Predominantly hilly areas withheavy rainfall

130

The following considerations are important for setting a rain gauge:

1. The site should be on a level ground, i.e., sloping ground, hill tops or

hill slopes are not suitable.

2. The site should be an open space.

3. Horizontal distance between the rain gauge and the nearest objects

should twice the height of the objects.

4. Site should be away from continuous wind forces.

5. The site should be easily accessible.

6. The gauge should be truly vertical

7. Ten percent of total number of rain gauge stations of any basin should

be self –recording.

8. The observer must visit the site regularly to ensure its proper

readiness for measurement.

Adequacy Of Raingauge Stations

2

VCN

Coefficient of variation

PC M

V1100

Optimal number of stations

M

iiP

MP

1

1

1

1

2

1M

PPM

i

M

Distribution of additional rain gauge stations

1. Arithmtatic mean method

Mx PPPPPM

P .................1

4321

M

Mxx

N

P

N

P

N

P

N

P

N

P

M

NP .................

4

4

3

3

2

2

1

1

2. Normal Annual Precipitation method

n

ii

n

iii

x

W

WP

P

1

1

3. Inverse distance method

2/1 ii DW

5. FILLING UP MISSING RECORD

If the conditions relevant to the recording of a rain gauge station have

undergone a significant change during the period of record, inconsistency

would occur in the rainfall data of that station.

Common causes for inconsistency of record are:

1. Shifting of a rain gauge station to a new location

2. The neighborhood of the station undergoing a marked change

3. Change in the ecosystem due to calamities, such as forest fires, land

slides

4. Occurrence of observational error from a certain date.

The checking for inconsistency of a record is done by the double mass

curve technique.

Based on the principal that when each recorded data comes from the same

parent population, they are consistency.

6. CONSISTENCY OF RAIN RECORD

Double Mass Curve

a

cxcx

M

MPP

N

iiP

NP

1

1

7. ESTIMATION OF MEAN AREAL RAINFALL

• Arithmetic mean

• Thiessen polygons method

• The isohyets method

• The square grid method.

A

AP

P

N

iii

1

A

PPA

PPA

PPA

P

NNN

2..............

22

11

322

211

MASS CURVE OF RAINFALL

The mass curve of rainfall is a plot of the accumulated precipitation against time, plotted in chronological order.

HYETOGRAPH

A hyetograph is a plot of the intensity of rainfall against the time interval.

I-D-F curves

The calculation of maximum rainfall is necessary for the designing of

evacuation works of rainwater in cities, or on the premises of storm flow

correction, or constructions and hydrotechnic installations. For this purpose

one can use the intensity-duration-frequency lines (Figure 3.11). The

intensity of calculated rainfall is a function of the standardized frequency

and the duration of the calculated rainfall.

Figure 3.11 - The intensity-duration-frequency curves (Musy, 2001)

The standardized frequency is the annual number of rains of duration t,

whose intensity exceeds the computed intensity. The computed

frequency is calculated as a function of the class importance of the

analysed objective. Thus for populated centres and industrial units we

have the following values of standardized frequencies (table 3.1).

Table 3.2 Standardized frequencies

Class of the

importance of the

objective

Industrial units and

production units of a

different nature

Populated centres

I 1/5 1/2...1/1

II 1/3...1/2 1/1...2/1

III 1/2...1/1 2/1

IV 1/1...2/1 -

V 2/1

In expressing frequency the numerator represents the numbers of rains

and the denominator represents the number of years. The values in the

table represent frequencies, not probabilities.