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adgeo-18-15-2008 - Adv Geosci 18 1523 2008...

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Adv. Geosci., 18, 15–23, 2008 www.adv-geosci.net/18/15/2008/ © Author(s) 2008. This work is distributed under the Creative Commons Attribution 3.0 License. Advances in Geosciences A global comparison of four potential evapotranspiration equations and their relevance to stream flow modelling in semi-arid environments M. Weiß and L. Menzel Center for Environmental Systems Research, University of Kassel, Kurt-Wolters-Str. 3, 34109 Kassel, Germany Received: 17 January 2008 – Revised: 28 April 2008 – Accepted: 28 May 2008 – Published: 20 June 2008 Abstract. This study compares four different potential evap- otranspiration equations according to Priestley Taylor, Kim- berly Penman, Penman Monteith (FAO-56) and Hargreaves on a global basis to demonstrate their difference, and assess their impact on the calculation of stream flows. The various equations of potential evapotranspiration show great differ- ences in magnitude. But due to the limited availability of validation data, it is difficult to assess which method is the physically most reasonable to be applied. According to this study, the radiation-based Priestley Taylor equation proved to be most suitable for a global application. For the calcula- tion of stream flows, however, the processes involved in the derivation of actual evapotranspiration values from potential evapotranspiration values appear more relevant than the ab- solute value of the potential evapotranspiration itself. 1 Introduction Limited availability of measured climate and discharge data sets for semi-arid and arid areas restricts the reliability of global hydrological modelling in these regions. Validation with available runoff data sets shows an overestimation of the discharge in semi-arid to arid regions for various global hydrological models (Fekete et al., 1999; Nijssen et al., 2003 as cited in D¨oll et al., 2003). Due to several reasons (D¨oll et al., 2003), the quantitative simulation of the discharge can lie outside acceptable limits: (1) The quality of the input data sets highly influences the quality of the simulations. Sec- ondly, a grid-based simulation will not concur with point measurements due to spatial and temporal averaging of in- put data and processes. (3) Further uncertainty arises from the fact that the selection of representative processes might Correspondence to: M. Weiß ([email protected]) not be complete and important processes might be ignored. (4) Processes itself might be reproduced with inadequate al- gorithms. In this study, potential evapotranspiration is exam- ined with respect to the latter case, while the generation of river discharge in semi-arid areas is analysed with respect to case (3). Evapotranspiration as a major component of the water bal- ance has been identified as a key factor in hydrological mod- elling and a wealth of methods have been developed for its calculation (see for example Brutsaert, 1982; Jensen et al., 1990). As a first attempt to improve stream flow simulations in semi-arid and arid areas, the global applicability and re- sults of four potential evapotranspiration equations are com-
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