Soil Moisture Deficit Calculator

Soil moisture deficit is the depth of water needed to return the root zone to field capacity. It is a bookkeeping quantity in a water balance: rain and irrigation reduce it, evapotranspiration increases it, and drainage occurs only at zero. A depth in millimetres is convenient because one millimetre over one square metre is exactly one litre, so the deficit reads directly as an irrigation dose. Field capacity is the water held against gravity a day or two after wetting, near a matric potential of -33 kPa, while the permanent wilting point is taken at -1500 kPa, the suction below which most crops cannot extract water. The difference is the total available water, roughly 60 to 100 mm per metre of depth in sand, 140 to 190 in loam and 160 to 220 in clay. Readily available water is the fraction removable before stomatal closure limits transpiration, commonly half the total, though FAO guidance sets the depletion fraction p between 0.2 for shallow-rooted vegetables and 0.7 for tolerant cereals. Multiplying by bulk density converts gravimetric water content to a volumetric basis, so if moisture is already volumetric, from a capacitance or neutron probe, the factor must be omitted or the deficit is overstated by about 40 percent at a density of 1.4 g per cubic centimetre. The model treats the root zone as one uniform store, whereas real profiles are layered and roots draw most water from the upper portion. Root depth also changes through the season, and stones, compacted layers and capillary rise shift the effective reservoir.

The two limits that bracket plant-available water were defined by agricultural scientists in the early twentieth century. Lyman Briggs and Homer Shantz of the United States Department of Agriculture introduced the wilting coefficient in 1912 from experiments in which sunflowers and other indicator plants were grown until permanent wilting. The upper limit came from Frank Veihmeyer and Arthur Hendrickson at the University of California, who developed the field capacity concept in orchard irrigation studies from the late 1920s and set out the available water range in a series of papers, most influentially their 1949 review. Lorenzo Richards contributed the pressure plate apparatus in the 1940s, which allowed field capacity and wilting point to be fixed at defined suctions rather than by growing plants. Turning these limits into a running deficit required a way to estimate atmospheric demand. Charles Warren Thornthwaite proposed a monthly water balance bookkeeping procedure in 1948, and in the same year Howard Penman of Rothamsted Experimental Station published his combination equation for evaporation from open water, bare soil and grass, introducing the root constant used in early British soil moisture deficit accounting. John Monteith extended Penman's equation with canopy resistance in 1965, and the resulting Penman-Monteith formulation was adopted as the international reference by Richard Allen, Luis Pereira, Dirk Raes and Martin Smith in FAO Irrigation and Drainage Paper 56 in 1998. Sensor-based scheduling has since supplemented but not replaced the balance approach.