THE APPLICABILITY OF MORTON'S AND PENMAN'S EVAPOTRANSPIRATION ESTIMATES IN RAINFALL‐RUNOFF MODELING1

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Abstract
Estimates of the upper constraint on actual evapotranspiration are required as input data in the majority of rainfall‐runoff models. This paper compares and discusses the applicability of Penman's potential evapotranspiration estimates and Morton's wet environment evapotranspiration estimates in rainfall‐runoff modeling applications. Morton's wet environment evapotranspiration depends only on the atmospheric variables and is the estimate of evapotranspiration that would occur when water supply is not limiting. It is a conceptually more correct representation of the upper constraint on actual evapotranspiration compared to Penman's potential evapotranspiration which is dependent on the water supply to the soil‐plant surfaces. Although Penman's potential evapotranspiration and Morton's wet environment evapotranspiration are two different quantities, comparison of the two estimates using data from different climatic regions throughout Australia indicate that they provide similar magnitudes of the upper limit of actual evapotranspiration at moderate climatic conditions when reliable estimates are required in rainfall‐runoff models. The two estimates can therefore be used interchangeably in rainfall‐runoff modeling applications.

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