Evaluation of evapotranspiration models for estimating daily reference evapotranspiration in arid and semiarid environments

Abstract

145The United Nations of Food and Agricultural Organization (FAO) proposed a methodology for computing crop evapotranspiration (ET o ) and crop coefficient (K c ) (Doorenbos and Pruitt 1977). These coefficients depend on several factors including crop type, stage of crop growth, canopy height and density (Allen et al. 1998). To schedule irrigation properly, an accurate and standard method to estimate ET o to predict crop water requirements, was stated by several authors (Chiew et al. 1995;Allen 1996). A great number of models was developed to estimate ET o for use in environments that lack direct ET o measurements Pruitt 2004, Gavilán et al. 2006). A major complication in ET o estimation using these models is the requirement for meteorological data that may not be easily available. This restriction at times prohibits use of more accurate models, and necessitates the use of models that have less demanding data requirements.An international scientific community has accepted the FAO56 Penman-Monteith (FAO56PM) model as the most precise one for its good results when compared with other models in various regions of the entire world (Chiew et al. 1995, Garcia et al. 2004, Gavilán et al. 2006. Estimation of reference ET o by globally accepted FAO56PM (Allen et al. 1998) requires the weather parameters like maximum and minimum temperature, solar radiation, sunshine hours, wind speed, relative humidity. However, for many locations, as is the case for Jordan, such meteorological variables are often incomplete and/ or not available. Furthermore, no published studies have examined the applicability of ET o models across Jordan, a country with a great gradient in temperature and precipitation. Moreover, the local calibration and validation of other models is more important in semiarid and arid regions than the temperate climate because most of these models were calibrated and validated in temperate environment (Dehghani Sanji et al. 2003). The objectives of this study were to (1) assess the performance of simpler models that require less readily available data against FAO56PM, and (2) to determine models performance across spaces in Jordan, focusing on arid versus semiarid environment. Evaluation of evapotranspiration models for estimating daily reference evapotranspiration in arid and semiarid environments O.E. MohaweshDepartment of Plant Production, Faculty of Agriculture, Mu'tah University, Al-Karak, Jordan ABSTRACT Daily outputs from eight evapotranspiration models were tested against reference evapotranspiration (ET o ) data computed by FAO56PM to assess the accuracy of each model in estimating ET o . Models were compared at eight stations across Jordan. Results show that Hargreaves modified models were the best in light of mean biased error (MBE), root mean square error (RMSE) and mean absolute error (MAE). The MBE, RMSE, and MAE values ranged from -1.47 to 0.81, 3.87 to 1.14 and 0.87 to 3.15 mm/day for HarM1, and from -1.45 to 0.89, 1.08 to 3.91, and 0.85 to 3.16 mm/day for HarM2, respectively, which would make it th...

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