Abstract
The performance of different soil-moisture deficit models was assessed by comparing model predictions with over 3000 neutron-probe observations of soil-moisture content at six grassland experimental sites operated by the Institute of Hydrology. The models were formulated using combinations of different equations for estimating potential evaporation and different regulating functions relating actual to potential evaporation via the moisture status of the soil. The inclusion of sophisticated evaporation equations (Priestley-Taylor, Penman, Thom-Oliver) gave no improvement in SMD prediction over a proposed simple evaporation formula requiring no direct meteorological measurements other than rainfall. The success of this formula demonstrates the conservative nature of annual potential evaporation within the U.K. both spatially and temporally and also suggests a possible natural feedback mechanism between atmospheric demand and grass transpiration.
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