Moisture is held in the capillary soil pores. Soil moisture controls many important processes in seedling emergence, evapotranspiration, mineralization of the soil organic fraction, surface run-off, leaching, and crop yield; the extent of these processes cannot be quantified without knowing the moisture status of the root zone (Akinrem and McGinn (1996) Can. J. Soil Sci. 76).

The soil moisture budget is the balance of water in the soil, resulting from precipitation and potential evapotranspiration in combination. Mintz and Serafini (1992) Clim. Dynam. 8 give a simple water budget model.