Model for predicting evaporation from a row crop with incomplete cover

A model is presented for calculating the daily evaporation rate from a crop surface. It applies to a row crop canopy situation in which the soil water supply to the plant roots is not limited and the crop has not come into an advanced stage of maturation or senescence. The crop evaporation rate is calculated by adding the soil surface and plant surface components (each of these requiring daily numbers for the leaf area index), the potential evaporation, the rainfall, and the net radiation above the canopy. The evaporation from the soil surface E s is calculated in two stages: (1) the constant rate stage in which E s is limited only by the supply of energy to the surface and (2) the falling rate stage in which water movement to the evaporating sites near the surface is controlled by the hydraulic properties of the soil. The evaporation from the plant surfaces E p is predicted by using an empirical relation based on local data, which shows how E p is related to E o through the leaf area index. The model was used to obtain the total evaporation rate E = E s + E p of a developing grain sorghum ( Sorghum bicolor L.) canopy in central Texas. The results agreed well with values for E measured directly with a weighing lysimeter.