Calculating Readily Available Water

Scheduling irrigation requires an understanding of how much water the soil can hold and how much of that water the crop can use. The amount of readily available water (RAW) will vary with soil type, crop, rooting depth and irrigation system.

This article describes a six-stage process to calculate the volume of RAW in cropping soils.

Water in the soil that is easily extracted by the plant is called readily available water. To schedule irrigation with confidence that the crop is being given enough water it is important to understand how much of the water the soil can hold that is available to the crop.

A plant's roots get the water it needs to grow and produce a crop from the surrounding soil. This water is held by the soil with increasing strength as the soil dries out. This makes it harder for the plant to get the water and therefore affects its growth.

The relationship between crop stress and the amount of water held in the soil is show in Figure 1. Figure 1. The relationship between soil water and crop stress. Some key terms relating to 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.

Beyond refill point, as the soil dries out, the plant needs to work harder to extract water, stressing the crop. The area between field capacity and refill point is called readily available water — water in the soil that is easily extracted by the plant.

Unless the aim is to stress a crop, e.g. with deficit-irrigated wine grapes, always try to maintain RAW.

Calculating RAW: six steps

The amount of RAW varies with soil type, crop, rooting depth and irrigation system. Step 1. Dig a hole. Dig a hole in the root zone of the crop. For perennial crops, dig under the canopy in an area watered by the irrigation system. Try to dig to 1 m or at least 30 cm past the main root zone (where the fibrous roots are).

Step 2. Identify the effective root zone. 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.

The effective root zone of a citrus tree can be seen in the top

30 cm of the soil.

Figure 2. Fibrous roots, which comprise the effective root zone, may only extend a third as far as the deepest roots

Some crops, such as irrigated pasture, citrus, bananas, avocados and low-chill stone fruit, develop a mass of shallow roots with only a few roots penetrating deeper into the soil.

Step 3. Identify different soil layers. If there are different soil layers within the effective root zone, measure the depth of each of these in metres.

Step 4. Identify gravel/stone in each layer. Stone 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.

Sieving shows the proportions of stone and gravel in soi

Grab three good handfuls of soil and, using a 2 mm sieve, remove all stone and gravel. Place the pile of stones and gravel next to the pile of soil and visually estimate the proportions of each (for example, 60 per cent stone and 40 per cent soil).

Step 5. Identify soil texture(s). Identify the texture of each soil layer within the effective root zone. The amount of water held by a soil and available to a plant varies with texture (see table). For example, a loamy soil can hold more readily available water than a sand.

Soil texture can be assessed in the field by the feel of a moist soil sample when worked between your thumb and forefinger. Step 6. Calculate RAW. To calculate RAW, do the following: 1. Identify the depth of the effective root zone. different soil layers within the effective root zone. 3. Determine the soil texture and percentage Forming a ribbon with moist soil stone/gravel of each helps identify soil texture layer. 4. Select the crop water tension group from the table and identify the RAW value for each soil texture layer (mm/m). 5. Reduce the RAW figure(s) by the percentage of stone/ gravel in the soil. 6. Multiply the thickness of each soil layer by its adjusted

RAW value. 7. Add up the RAW for each soil layer to obtain the total root zone RAW.

A citrus crop growing in a sandy loam soil containing 20 per cent stone, with an effective root depth of 300 mm and a strategy to irrigate at 40 kPa would be calculated as follows: 2. Identify the depth of

From the table, the RAW for a sandy loam soil at 40kPa = 60 mm/m.

As the soil contains 20 per cent stone, reduce the RAW by 20 per cent. To do this, multiply by 0.8.

Adjusted RAW = 60 mm/m x 0.8 = 48 mm/m.

Hence, for a rooting depth of 300 mm

Total root zone RAW = 48 mm/m x 0.3m = 14.4 mm.

If the irrigation system wets the entire cropped area, use this figure (RAW mm) to schedule irrigations.

If the system is drip or micro-spray, which does not wet the entire cropped area, convert RAW mm to RAW L. See WA Department of Primary Industries and Regional Development website.

This article was originally produced by the Department of Primary Industries and Regional Development, Western Australia. Available at their website.

Sand Loamy sand Sandy loam Loam Sandy clay loam Clay loam Light clay

Water-sensitive crops such as vegetables and some tropical fruits

30 45 45 50 40 30 25

Most fruit crops and table grapes, 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)

35 50 60 70 60 55 45 35 55 65 85 70 65 55

Annual pastures and hardy crops such as cotton, sorghum and winter crops

40 60 70 90 80 80 70