Simulation of the evaporation of soil water beneath a wheat crop canopy.

摘要

The evaporation of soil water (E_s) is an important component of the water balance in cropping systems. Although E_s is considered to be non-beneficial (not directly related to crop production), it may indirectly influence water availability for transpiration which is directly related to crop growth and thus yield. Due to the difficulty in measuring E_s, empirical models are usually used, especially Ritchie's two stage model. Ritchie's model assumes that stage 1 evaporation is dependent on radiant energy at the soil surface, and that during stage 2, E_s is independent of radiant energy. During stage 2, the model assumes that E_s is only dependent on soil hydraulic properties, and that cumulative stage 2 E_s is proportional to the square root of time. To evaluate the model, we measured E_s under the canopy of wheat grown on a clay loam soil in Punjab, India, using mini-lysimeters. Soil evaporation during three different potential evaporation (E_o) periods (1.5, 3.0, 6.0 mm d-1) was compared during the 2006-2007 and 2007-2008 wheat seasons. The results suggested that stage 2 E_s was affected by E_o, and cumulative E_s was not described well by a square root of time function. An exponential decay function which describes the decrease in E_s/E_o as a function of a soil dependent constant (b), whose value is directly related to E_o, was developed. Using this function, calculated cumulative E_s for two independent drying periods was close to the observed E_s values with coefficients of determination of 0.82 and 0.95.