Calculating Readily Available Water

Page last updated: Thursday, 20 September 2018 - 10:51am

Scheduling irrigation requires an understanding of how much water your soil can hold and how much of that water your crop can use.

The amount of Readily Available Water (RAW) will vary with soil type, crop, rooting depth and irrigation system.

A six-stage process helps calculate the volume of Readily Available Water in cropping soils.

Introduction

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.

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. 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.

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 (RAW) — water in the soil that is easily extracted by the plant.

Unless you are trying to stress your crop (for example, with deficit-irrigated wine grapes), aim to maintain RAW at all times.

The amount of Readily Available Water varies with soil type, crop, rooting depth and irrigation system.

Follow these six steps to work out your crop’s RAW.

Relationship between soil water and crop stress