Evapotranspiration under Depleting Soil Moisture

Data from two hydraulic weighing lysimeters planted to alfalfa were used to calibrate a simplified root extraction sink term in a model of the one-dimensional continuity equation. The model was then utilized to generate the expected response of alfalfa to depleting soil moisture levels for a wide range of possible soil and atmospheric conditions. This information led to the calculation of values for the K_s stress coefficient employed in irrigation scheduling programs. A new relationship describing the reductions in evapotranspiration as the soil moisture reservoir is depleted was formulated. The calibration data were also employed to evaluate the commonly used logarithmic and linear soil moisture stress models. The results indicated that both models reasonably predicted measured data until the available soil moisture in the root zone diminished below approx 50%.