Evapotranspiration Model for Semiarid Regions

Abstract

The Thornthwaite method for calculating evapotranspiration (ET) as used in the water management model, DRAINMOD, is modified for semi‐arid conditions since for such conditions the existing method had underestimated the average monthly ET rate by about 50%. The following three modifications are proposed: (1) The constant in the Thornthwaite general equation is increased depending upon the min‐max range of the annual mean air temperature wave; (2) the daylight hours wave is shifted by 45 days and brought parallel to the mean air temperature wave. This shift is made in order to account for the heat energy of the atmosphere surrounding the plants; and (3) actual evapotranspiration (AET) is not equated to zero during irrigation. The AET was measured in a drip irrigated banana field with lysimeters. The simulated monthly adjusted values of AET give satisfactory estimates with an average error of within ±10% when compared to that of the lysimeter data.