POTENTIAL, ACTUAL, AND EQUILIBRIUM EVAPOTRANSPIRATION IN A WHEAT FIELD

This paper examines the relationship between both potential (E*) and nonpotential evapotranspiration and equilibrium evapotranspiration (EQ) in an irrigated wheat field in southcentral Alberta, Canada. The "control" exercised by surface wetness and root reservoir moisture content in determining the value of the Priestley-Taylor constant-alpha is explored. Also investigated is the relationship between alpha and the vapor flux fraction ET/(R-G) where ET is the actual evapotranspiration, R the net radiation, and G the soil heat flux. It is shown that evapotranspiration occurred at the potential rate (E*) when the available soil moisture (ASM) within the root zone was greater-than-or-equal-to 3 percent. Alpha varied from 0.84 for a dry soil to 1.49 for a saturated soil. The mean-alpha for E* was 1.24. Surface wetness sustained evapotranspiration at the potential rate when such wetting exceeded 2mm d-1 following a period of prolonged drawdown of soil moisture. Alpha and ET/(R-G) were positively correlated and this correlation strengthened with increasing soil moisture for constant values of the energy partitioning factor s+gamma/s where s is the slope of the saturation humidity-temperature curve and gamma is the psychrometric constant. ET = EQ when ET/(R-G) lay within the range of 0.59 to 0.82 corresponding to Bowen ratio (beta) values of 0.22 and 0.69, respectively.