EFFECTS OF SOIL WATER MOVEMENT ON ACTUAL EVAPOTRANSPIRATION ESTIMATED FROM THE SOIL MOISTURE BUDGET

Actual evapotranspiration was estimated from the soil moisture budget for a grass-covered sandy loam soil at Simcoe, Ontario. Soil moisture was measured at 25 sites distributed over a 6-meter-square grid. The coefficient of variation for actual evapotranspiration estimated at all sites averaged 13% and rose as high as 19%. Average actual evapotranspiration exceeded both the Penman and Thornthwaite estimates of potential evapotranspiration for three of the six measuring intervals, due to deep seepage losses. The application of corrections for the vertical water movement, determined from experimentally derived matric suction and hydraulic conductivity data, gave a substantial deep seepage loss for some periods and a capillary uptake of soil water for others. Vertical losses and gains created errors of up to + 28 and − 29%, respectively, in the standard estimates of actual evapotranspiration. The large spatial variations in evapotranspiration estimates resulted from variations in volumetric soil moisture between sample points, apparently creating differences in the magnitude and direction of vertical water movement across the terminal depth. The horizontal flux of water between measuring points was relatively unimportant in accounting for the spatial variations.