Comparing evapotranspiration estimates from satellites, hydrological models and field data

This paper provides an overview of an experiment in which eight different m ethods of estimating actual evaporation and transpiration based on field da ta, on hydrological models and on satellite data were compared using a comm on database. The objectives were to compare results and to assess the utili ty of each method for various applications. Evaporation and transpiration a re important components of the hydrological cycle, which cannot be directly measured. Traditionally, actual evapotranspiration has been computed as a residual in water balance equations, from estimates of potential evapotrans piration or, indirectly, from field measurements at meteorological stations . Recently, however, researchers have begun using scintillometers, remotely sensed data and hydrological models to estimate areal actual evapotranspir ation. An experiment was carried out in western Turkey over the summer of 1 998 to compare the newly developed methods with more conventional methods. This paper introduces the different techniques, the experimental sites and the dataset. The results of the different methods are reviewed and compared and recommendations are made as to the suitability of the different method s for different circumstances. Particular emphasis is placed on the data re quirements, the ease of use and the constraints of each method. The results show that the satellite methods and FAO-24 methods have the greatest varia bility. The FAO-56, the models and the field methods show more consistency. More details of each method and the particular results and conclusions are given in the other papers in this special issue. (C) 2000 Elsevier Science B.V. All rights reserved.