Soil Water Capacity & Your Soil's Reservoir

It's the amount of water a soil can store in a form plants can use — the available water capacity (AWC). It is the water held between field capacity (after free drainage) and the permanent wilting point (where roots can no longer extract it). Expressed as mm of water per metre of soil, it sets how big your soil's water reservoir is.

AWC in millimetres = the AWC per metre for your soil texture × the rooting depth in metres. Typical values are about 70 mm/m for sand, 150 mm/m for clay, 170 mm/m for loam and around 200 mm/m for silt loam. So a loam with a 0.6 m root zone holds roughly 170 × 0.6 ≈ 102 mm of available water.

RAW is the share of AWC a crop can use before it starts to suffer water stress — usually about half. RAW = AWC × the allowable depletion (MAD), and a common MAD is 50%. Refilling before you reach the bottom of RAW keeps the crop growing without stress and avoids wasting water by over-filling the profile.

Management allowable depletion is the fraction of available water you let the crop draw down before irrigating, typically around 50% for many field crops. Sensitive or shallow-rooted crops use a lower MAD (irrigate sooner); hardy, deep-rooted crops tolerate a higher one. It directly sets RAW and therefore the irrigation interval.

Irrigation interval in days = RAW ÷ the crop's daily water use (ET). If RAW is 50 mm and the crop transpires 5 mm/day, the soil can carry it for about 10 days before it needs refilling. As ET rises in hot, windy weather the interval shortens; in cool weather it lengthens.

One millimetre of water spread over one square metre is exactly one litre, so the AWC in millimetres equals the litres your soil holds per square metre. Over a hectare (10,000 m²), 1 mm is 10,000 litres or 10 m³ — handy for sizing the volume an irrigation needs to deliver.

Texture controls pore size. Sands have large pores that drain fast and hold little plant-available water; clays hold a lot of water but much is gripped too tightly to release. Loams and silt loams strike the best balance of pore sizes, which is why they store the most readily available water for crops.

Yes. A deeper root zone taps a thicker slice of soil, so it stores more total available water and buffers the crop against dry spells. Organic matter improves structure and water retention too. Both increase the reservoir, lengthen the safe interval between irrigations and make the crop more resilient.

Rooting depth depends on the crop and stage — shallow for seedlings and many vegetables, deeper for established cereals, trees and lucerne; dig a small pit to see where roots actually go. Texture comes from a soil test or a simple hand-feel/ribbon test, then pick the matching AWC class in the tool.

They're reliable planning estimates from standard texture values. Real AWC shifts with structure, stones, compaction and organic matter, and ET varies daily. Use the result to size your reservoir and set a starting interval, then fine-tune with a soil-moisture probe or the feel method as the season unfolds.