Estimating evaporation from bare or nearly bare soil

A model is presented that uses a daily mean evapotranspiration ETo rate to estimate energy-limited (potential or Stage 1) soil evaporation, and it als o uses daily mean ETo and a soil hydraulic beta factor to estimate soil hyd raulic property-limited (Stage 2) evaporation. The model provides good esti mates of cumulative soil evaporation on both hourly and daily bases when co mpared to observed soil evaporation in three field trials. Crop coefficient K-c values from cumulative hourly and cumulative daily soil evaporation es timates and ETo data were comparable. Using a soil hydraulic factor (beta = 2.6) in the model gave a fair approximation for the widely used K-c curves for initial growth of crops presented in the United Nations Food and Agric ultural Organization's irrigation and Drainage Paper 24. However, using a s ite-specific beta factor should improve soil evaporation and K-c estimates for site-specific applications.