Evaporation from a Themeda Grassland. II. Resistance Model of Plant Evaporation

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Abstract
A model, incorporating resistances to water vapor transport, was developed to account for controls on grassland evaporation in the Southern Tablelands region of New South Wales, Australia. A relationship describing aerodynamic resistance was derived from short-term data on wind profiles. A 2nd relationship, involving surface resistance as a function of solar radiation and soil moisture content, was obtained using short-term measurements from a lysimeter and of associated meteorological variables. Model simulations corresponded well with evaporation from a lysimeter over a 5 yr period. Sensitivity analyses of parameters in the model indicated that aerodynamic resistance was not critical. Seasonal adjustments in the rainfall intercepted by the canopy improved estimates of weekly evaporation. The role of the resistance model in land management is discussed. An example is given of the way in which the model may be used to predict changes in runnoff resulting from the conversion of Themeda [T. australis] grassland to improved pasture.