A rate equation for soil-surface moisture fraction is developed which makes use of the more customary bulk soil moisture content, as well as the soil-surface evaporation rate minus precipitation rate. The two empirical constants involved are estimated from soil measurements of Jackson (1973). The evaporation rate relative to potential evaporation is taken as the ratio of soil-surface moisture fraction to its saturated threshold value. It is shown how the parameterization simulates the drying of the soil-surface on a diurnal and longer time scale following irrigation or precipitation, and estimates large errors of up to a factor of 5 which can be incurred by atmospheric prediction models that treat only the bulk soil moisture in the absence of foliage.Published by American Meteorological Society. Abstract reproduced by permission.