Effect of potential evapotranspiration estimates on effective parameters and performance of the MIKE SHE-code applied to a medium-size catchment R.F. Va ´zquez * , J. Feyen 1 Institute for Land and Water Management, K.U. Leuven, Vital Decosterstraat 102, 3000 Leuven, Belgium Received 9 November 2001; revised 30 September 2002; accepted 4 October 2002 Abstract The effect of the method of calculating a time series of crop potential evapotranspiration (ET p ) 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 ET p was estimated as a function of the reference evapotranspiration (ET o ) by means of a crop coefficient approach. Three methods for deriving ET o 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 ET o outputs of the generation methods were compared to local point scale ET o 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 ET o 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 ET o data generated by method A. The research also revealed a significant dependency of some of the effective parameters on the different ET p estimates; especially of those parameters related to the computation of the actual evapotranspiration (ET act ). q 2002 Elsevier Science B.V. All rights reserved. Keywords: Evapotranspiration; Distributed models; Performance; MIKE SHE; Evaluation 1. Introduction Actual evapotranspiration (ET act ) plays an important role in the hydrology of a catchment, together with other processes such as precipitation, runoff and infiltration. ET act is the amount of water lost by evapotranspiration under a given set of circumstances where water supply limits evapo- transpiration to below the potential rate (ET p ). Over an extensive range of spatial scales, evapotranspira- tion (ET) or its components can be measured with reasonable accuracy (Stewart, 1988; Katul and Parlange, 1992; Wanchang et al., 1999; Wilson et al., 2001) by indirect methods such as water 0022-1694/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S0022-1694(02)00308-6 Journal of Hydrology 270 (2003) 309–327 www.elsevier.com/locate/jhydrol 1 Fax: þ32-16-329760. * Corresponding author. Tel.: þ32-16-329723; fax: þ 32-16- 329760. E-mail addresses: raul.vazquez@agr.kuleuven.ac.be (R.F. Va ´zquez), raulito_vz@yahoo.com (R.F. Va ´zquez), jan.feyen@agr. kuleuven.ac.be (J. Feyen).