MODELING CATCHMENT EVAPORATION - AN OBJECTIVE COMPARISON OF THE PENMAN AND MORTON APPROACHES

Water-balance data from the Shannon catchment are used to calibrate two simple models of catchment-scale evaporation. The first uses the more traditional approach: evaporation is taken as Penman potential evaporation (PE), unless water is not freely available, in which case the reduction of actual evaporation (AE) from PE is calculated using a simple Thornthwaite-style soil-moisture model. The second model makes use of the relatively recent work by Bouchet and Morton which suggests, inter alia, that AE and PE are inversely related in the absence of an abundant supply of moisture. The resulting objective criteria of efficiency for the final models are similar, but other results from the model calibration process indicate some of the relative strengths and weaknesses of the two models. In particular, it is shown that the Bouchet-Morton approach provides a valuable alternative to the empiricism of the Thornthwaite-style reduction of AE from PE, but that this is achieved at a high cost: the introduction of a strong degree of empiricism into the process of advection modelling.