(I) Excel

1-Depletion Fraction & Deficit Fraction

2-Readily Available Water (RAW)

(II) WaterSoftware

1-ETC – Calculator

2- Climwat

For Calculation of Crop Water

Requirements & Irrigation Schedule

Requirements For Calculation of Crop Water Requirements & Irrigation Schedule

حساب األحتياجات المائية للمحاصيل وجدولة مياه الرى: متطلبات

Introduction : Definition

Readily available water (RAW) is the water that a plant

can easily extract from the soil. RAW is the soil moisture

held between field capacity and a nominated refill point

for unrestricted growth. In this range of soil moisture,

plants are neither waterlogged or water-stressed.

❑ Water in the soil that is easily extracted by the plant is called

Readily Available Water (RAW). To schedule irrigation with

confidence that you are providing the crop with enough water

you need to understand how much of the water your soil can

hold that is available to your crop

❑ Readily available water(RAW). RAW is expressed inmillimetres per meter (mm/m) and indicates the depthof water (mm) held in every meter (m) of soil depth that canbe readily removed by the plant. RAW should be calculatedfor the plant's effective root zone.

The relationship between crop stress and the amount of water held in the soil is show in Figure 1. Some key terms relating to Readily Available Water (RAW) are field capacity and refill point:Field capacity is the maximum amount of water a soil can hold after drainage.Refill point is when the plant has used all readily available water.

Total Available Water (TAW)Refill point

The relationship between crop stress and the amount of water held in the soil is show in Figure 1. Some key terms relating to Readily Available Water (RAW) are field capacity and refill point:Field capacity is the maximum amount of water a soil can hold after drainage.Refill point is when the plant has used all readily available water.

Readily Available Water (RAW

Refill point

Readily Available Water (RAW

Readily Available Water (RAW) Vs Total Available Water (TAW)

So, for a sand (S) at a tensiometer reading of –40 kPa,you would need to supply 0.36 mm of water for eachcentimetre depth of soil to bring the soil to fieldcapacity. At –1500 kPa (much drier – beyond thevalues on a tensiometer gauge) you would need tosupply 0.62 mm of water for each centimetre depthof soil to reach field capacity. Soil water content.Depending on the type of crop, RAW for horticulturalcrops is usually the amount of water held betweenfield capacity (–8 to –10 kPa) and –20 to –60 kPa. Themost common refill value for perennial horticulture inthe Mallee is –40 kPa

Steps in identifying Readily

Available Water

Step 1: Dig a hole

Step 2: Identify the effective root zone

Step 3: Identify different soil layers

Step 4: Identify gravel/stone in each layer

Step 5: Identify soil texture(s)

Step 6: Calculate RAW

Steps in identifying Readily Available Water

Step 1: Dig a holeDig a hole within the root zone of your crop. For perennial crops, dig under the canopy in an area watered by the irrigation system. Try to dig to 1 metre or at least 30cm past the main root zone (where the fibrous roots are).

The effective root zone is where the main mass of roots is found This is typically one to two-thirds of the depth of the deepest roots

Step 2: Identify the effective root zone

Steps in identifying Readily Available Water

Fibrous roots which comprise the effective root zone may only extend a third as far as the deepest roots

Step 2: Identify the effective root zone

Steps in identifying Readily Available Water

Step 3: Identify different soil layersIf you have different soil layers within the effective root zone measure the depth of each of these in metres.

Step 4: Identify gravel/stone in each layerStone and gravel reduce the amount of water that can be held by a soil. A very stony soil will hold much less water than the same soil without stones.Grab three good handfuls of soil and using a 2mm sieve remove all stone and gravel (Figure 4). Place the pile of stones and gravel next to the pile of soil and visually estimate the proportions of each (for example, 60% stone and 40% soil).

Sieving shows the proportions of stone and gravel in soil

Steps in identifying Readily Available Water

Step 5: Identify soil texture(s)Identify the texture of each soil layer within the effective root zone

Step 6 : Calculating Readily Available Water

(6-1) Select the crop water tension group from Table 1 and identify the RAW

value for each soil texture layer (mm/m).

(6-2) Select soil texture Table 2

(6-3) Reduce the RAW figure(s) by the % stone/gravel in the soil.

(6-4) Multiply the thickness of each soil layer by its adjusted RAW value.

(6-5) Add up the RAW for each soil layer to obtain the total root zone RAW.

Water tension (0kPa at saturation point)

To –20kPa To –40kPa To –60kPa To –100kPa

Water-sensitive

crops such as

vegetables and

some tropical

fruits

Most fruit crops

and tablegrapes,

most tropical

fruits

Lucerne, perennial

pastures, crops

such as maize and

soybeans, wine

grapes (except

where partial root

zone drying is

being practised on

wine grapes)

Annual pastures

and hardy crops

such as cotton,

sorghum and

winter crops

kPa = kilo PascalWater tension (0kPa at saturation point)

Readily Available Water ( for different crops ) as a function of suction

الدوليالنظاممنمشتقةوحدةهيPa(الرمز(pascal)باسكال

علىالعموديةللقوةقياسوهو،،اإلجهادأوللضغطللوحدات

زالمساحةوحدة

وحدةوهي،(N/m2)1المربعالمترعلىنيوتن1=باسكال1

.للضغطجداصغيرة.باسكال100,000نحوتعادل(بار)جويضغط1

1=باسكال1000

تعريف بسكال

Soil TextureReadily Available Water (mm/m)

To –20kPa To –40kPa To –60kPa To –100kPa

Sand 30 35 35 40

Loamy sand 45 50 55 60

Sandy loam 45 60 65 70

Loam 50 70 85 90

Sandy clay loam 40 60 70 80

Clay loam 30 55 65 80

Light clay 25 45 55 70

Table 2 : Select soil texture

conversion soil water content from m3/m3 to mm

In my guess, 1 m3/m3 can be converted to 1000 mm of water, if 1mm of water content is defined as 1mm of water drawn from 1m deep soil.

Assuming in an unit area (S, m^2) of soil, amount of water in depth (h, m) can be extract from soil of a depth (H, m)volumetric water content (m3/m3) = S*h/S*H = h/Hso: h (m)= volumetric water content * H (m)So if my guess of the definition of mm is right, H = 1m =1000mm,h (mm)= 1000 * volumetric water content.

Volumetric water content is a numerical measure of soil moisture. It is simply theratio of water volume to soil volume

Example calculationA citrus crop growing in a sandy loam soil containing 20% stone, with an effective root depth of 0.3m and a strategy to irrigate at -40kPa would have the following

calculations:1. From Table 1, the RAW for a sandy loam soil at -40kPa =

60mm/m.2. As the soil contains 20% stone, reduce the RAW by 20%. To

do this, multiply by 0.8.3. Adjusted RAW = (60mm/m) x 0.8 = 48mm/m.4. Hence, for a rooting depth of 0.3m:5. Total root zone RAW = ( 48mm/m )x 0.3m = 14.4mm.

https://www.agric.wa.gov.au/citrus/calculating-readily-available-water

Write the example in Excel Sheet

Converting Readily Available Water to litres for drip irrigation

Drip irrigation is common in modern orchards. For this method of irrigation it is easier to use litres rather than the more traditional unit of millimetres when describing readily available water in the plant root zone. Using litres also allows simple calculation of irrigation time.

Where irrigation water and plant roots are evenly distributed over the whole planting area, water storage and plant water use can be measured in millimetres. Drip irrigation distributes water over a small part of the whole block and roots follow this water distribution. In these cases, it is often easier to use litres to describe both water use and storage in the plant root zone.This also allows simple calculation of irrigation time as the discharge from drip systems is commonly reported in litres per hour.

Introduction

https://www.agric.wa.gov.au/spring/converting-readily-available-water-litres-drip-irrigation?page=0%2C1

Rule to remember1mm depth of water = 1L applied to 1 square metre.

The volume of root zone that is wetted by the drip system will depend on the size and shape of the wetting pattern.

Water held in root zone

❑ Overlapping drippers

Volume stored (L) = wetted width (m) x wetted length (m) x root zone RAW (mm)

For example, for a width of 1.5m wetted, 3m

tree spacing and root zone RAW of 14mm, the

volume of Readily Available Water = 1.5 x 3 x 14

= 63 litres of RAW per tree.

https://www.agric.wa.gov.au/spring/converting-readily-available-water-litres-drip-irrigation?page=0%2C1

❑ Non-overlapping drippers

or example, if a root zone with a RAW of 14mm is wet by a dripper with a cylindrical wetting pattern and a radius of 0.2m, the volume of readily available water will be:

πr2 x root zone RAW (mm)(πr2 is the area of a circle where pi (π) equals 3.14.)3.14 x (0.2 x 0.2) x 14 = 1.8L/dripper.

Write the examples in Excel Sheet