A mechanistic numerical model was designed to compare evaporation of soil moisture under steady versus fluctuating evaporativity. The model, written in IBM System/360 CSMP language, requires data on soil hydraulic characteristics and on potential evaporation. It calculates the rate and cumulative quantity of evaporation, as well as the change in profile water content and distribution, as functions of time. Computations carried out for a 10-day simulation illustrate the use of the model. The results confirm experimental findings that the diurnal cycle of evaporativity causes nighttime resorption of moisture and hence an appreciably higher average wetness in the soil surface zone. The results also predict that under cyclic evaporativity, the total evaporation tends to be lower than under the equivalent steady evaporativity.