Subsurface water processes

Soil moisture

Soil moisture (or soil water) refers to the water that occupies the spaces between soil particles. It is at its maximum when the soil is saturated, that is when all the air between the particles is replaced by water but, if the soil can drain, the spaces will normally also contain air, the water then forming a thin film on and between the soil particles, held by capillary attraction. As the soil dries out this film becomes thinner and progressively less easy for plant roots to extract. The water is free to move through the soil, up or down, by gravity and by capillary attraction;it is taken up by plant roots, evaporates at the surface or recharges the groundwater.

However, there is also water present in soil which is not free to move or to be taken up by plants but which may nevertheless be detected during measurement – but not differentiated from the free water, depending on the measurement technique used. This is the water of crystallisation or water of hydration – water that is chemically bound to minerals within the soil such as gypsum (CaSO4 2H2O); water may also be bonded to organic material to varying degrees of strength. In making measurements of soil water content, soils of great variety are encountered and any instrument or measurement method must be able to handle them all, from pure peat or sand to silt and clay or a mix of them, all varying in pore size or having a variety of pore sizes combined, and all varying in the extent of chemically bonded water.