THE WATER BALANCE OF AN AGRICULTURAL CATCHMENT II. CROP EVAPORATION : SEASONAL AND SOIL FACTORS

Rates of evaporation, soil water deficits and extraction depths for spring cereals and grass were estimated from soil water measurements for a four year period. Estimates of evaporation were also compared with evaporation formulae and evaporimeter values. Evaporation and soil water deficit were highly dependent upon weather, particularly rainfall. Evaporation from grass was noticeably restricted whenever deficits exceed 50 millimeters. Cereals extracted water from a slightly greater depth of soil but developed deficits, up to harvest, very similar to those under grass. Deficits developed in sand and clay soils were also very similar, because water held in large dense clay aggregates is not readily accessible to roots. Although soil type had only minor influence on rates of evaporation, deficits and depths of drying, because of movement of water through fissures, slow drainage and delayed swelling effects soil type has a strong influence on stream behaviour.