ESTIMATING EVAPORATION FROM BARE OR NEARLY BARE SOIL

A model is presented that uses a daily mean evapotranspiration ET(sub 0) rate to estimate energy-limited (potential or Stage 1) soil evaporation, and it also uses daily mean ET(sub 0) and a soil hydraulic (Beta) factor to estimate soil hydraulic property-limited (Stage 2) evaporation. The model provides good estimates of cumulative soil evaporation on both hourly and daily bases when compared to observed soil evaporation in three field trials. Crop coefficient K(sub c) values from cumulative hourly and cumulative daily soil evaporation estimates and ET(sub 0) data were comparable. Using a soil hydraulic factor ((Beta) = 2.6) in the model gave a fair approximation for the widely used K(sub c) curves for initial growth of crops presented in the United Nations Food and Agricultural Organization's Irrigation and Drainage Paper 24. However, using a site-specific 13 factor should improve soil evaporation and K(sub c) estimates for site-specific applications.