Effect of potential evapotranspiration estimates on effective parameters and performance of the MIKE SHE-code applied to a medium-size catchment

The effect of the method of calculating a time series of crop potential evapotranspiration (ETP) on both the model performance and the magnitude of the main effective parameters of a distributed hydrological model was assessed on the basis of an independent multi-calibration. The ETP was estimated as a function of the reference evapotranspiration (ETo) by means of a crop coefficient approach. Three methods for deriving ETo estimates were used in modelling a medium size catchment in Belgium: (A) Food and Agriculture Organisation (FAO) FAO-24 standard approach; (B) FAO-24 approach but used with coefficients, for both the wind and the Stefan-Boltzmann equations, differing from the standard formulation; and (C) FAO-56 standard approach. For assessing the performance of the estimation approaches, the point scale ETo outputs of the generation methods were compared to local point scale ETo guidelines derived from previous research. The outputs of generation method B and of the generation method C showed a close agreement with the local point scale ETo guidelines. Their effect (to a catchment scale) on the performance of the hydrological model also seemed to be comparable. The best model performance was obtained by using the higher ETo data generated by method A. The research also revealed a significant dependency of some of the effective parameters on the different ETP estimates; especially of those parameters related to the computation of the actual evapotranspiration (ETact). (C) 2002 Elsevier Science B.V. All rights reserved.