Studies on Evapotranspiration from Crops

Stem heat balance method and Bowen ratio heat balance method were, respectively, used to determine transpiration (T) and evapotranspiration (ET) from a water-shortage free soybean field. Evaporation from the soil surface beneath the soybean canopy was indirectly estimated by subtracting T from ET. The diurnal change in the T/ET ratio was characterized by a parabolic curve with the minimum at noon, and higher values in the early morning and late afternoon, depending on the diurnal march of direct solar radiation intercepted by the canopy. Results showed that the evaporation from the soybean field at the early growing stage with sparse canopy was nearly equal to the evapotranspiration from the field at the later growing stage with dense canopy under a condition of enough soil moisture. The daily evaporation beneath the canopy decreased curvilinearly with the leaf area index (LAI). Combining the Makkink solar radiation model for estimating evapotranspiration and the transmission function of the canopy to global solar radiation yielded a simple model for estimating daily transpiration from the canopy. The transpiration estimated by this model increased curvilinearly with increment in the LAI. This relation was well approximated by a negative exponential function of leaf area index.