The use of surface resistance—soil moisture relationships in soil water budget models

An important feature of a soil water budget is the reduction of transpiration from a canopy below the rate of atmospheric demand with increasing soil dryness. Commonly, an empirical relationship between the ratio of actual evaporation (AE) to potential evaporation (PE) and soil water storage is adopted. Alternatively the Penman—Monteith equation can be used with a specified relationship between surface resistance and soil water storage.Using actual evaporation rates determined from instrumented soil water profiles, a relationship between surface resistance and soil water storage can be inferred, and results are presented for different crops and soil-types in the United Kingdom. These results are compared with the surface resistance values implicit in the performance of two layer soil moisture models adopting an empirical AE/PE relationship with soil moisture deficit. The performance of the two approaches with respect to soil moisture estimation is compared.