ARTICLE 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.
RAW explained
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.
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.
The effective root zone of a citrus tree can be seen in the top 30 cm of the soil.
Figure 1. The relationship between soil water and crop stress.
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.
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.
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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.
Figure 2. Fibrous roots, which comprise the effective root zone, may only extend a third as far as the deepest roots
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.
