Revisiting the parameterization of potential evaporation as a driver of long-term water balance trends

We examine the effects of two different parameterizations of potential evaporation on long-term trends in soil moisture, evaporative flux and runoff simulated by the water balance model underlying the Palmer Drought Severity Index. The first, traditional parameterization is based on air temperature alone. The second parameterization is derived from observations of evaporation from class-A pans. Trends in potential evaporation from the two parameterizations are opposite in sign (+/-) at almost half the stations tested over Australia and New Zealand. The sign of trends in the modelled soil moisture, evaporative flux and runoff depends on the parameterization used and on the prevailing climatic regime: trends in water-limited regions are driven by precipitation trends, but the choice of parameterization for potential evaporation is shown to be critical in energy-limited regions.