Glossary of Soil Water Terms

Available – Tightly held is soil moisture between the refill point and the permanent wilting point (-50 kPa to -1500 kPa) which is available for plant growth but is tightly held and thus is moisture that keeps the plant alive and perhaps growing slowly.

Drainable porosity (DP) is the amount of water held between field capacity and saturation which is said to be unavailable to plants as it is assumed to have drained from the soil too quickly for plant use. Drainable porosity is also an indicator of macroporosity and the soils ability drain excess water.

Field capacity is said to be the moisture content after a saturated or near saturated soil has been allowed to drain unimpeded for two days. In reality, many soils don’t actually drain to field capacity over 2 days. Consider a texture contrast soils (duplex) in which sand lies above a clay. The surface sand can only drain as fast as the underlying clays allows, which must not only allow drainage of water within the clay, but also from the overlying sand. To get around this problem soil scientist consider field capacity to be the soil moisture when soil is at -10 kPa, -0.1 bar or when -100 cm (-30 kPa or -300 cm in some regions) suction is applied to the soil. 

Permanent wilting point (PWP) is the moisture content at which a sunflower suffers irreversible moisture stress and dies, which is equivalent to the moisture content at -15 bar, or -1500 kPa suction. In reality most crops suffer irreversible water stress before the permanent wilting point at -1500 kPa is reached. 

Plant available water content (PAWC) is the amount of water held between the permanent wilting point (-1500 kPa) and field capacity (-10 kPa), however within this range, water gets harder and harder for plants to extract as the soil dries.

Readily Available Water (RAW) is the easily extractable soil water between field capacity and a lower moisture determined by the point at which crop growth is unacceptably slowed due to moisture stress (nominally at -50 kPa) but is still able to grow. This lower moisture content is often referred to as the refill point.

Saturation is the total or maximum amount of water which can be held in the soil. In reality, true saturation rarely occurs due to air being trapped in small pockets within the soil.