Assessment of 40 Empirical Models for Estimating Reference Evapotranspiration under the Three Major Climate Zones of Iraq

Accurate reference evapotranspiration (ETo) estimation is crucial for water irrigation management and sustainable agriculture planning. The difficulty in obtaining several data requirements for employing the recommended Food and Agriculture Organization Penman-Monteith method (FAO-PM) for reliable estimation of ETo has led to the development of many empirical models. This is particularly crucial for Iraq, located in West Asia (29°15′00″–38°15′00″ N; 38°45′00″–48°45′00″ E), where meteorological data are often limited or missing. The objectives of the present study were to assess the performance of 40 ETo empirical models (13 radiation-based, 13 mass-transfer-based, and 14 temperature-based) against the FAO-PM model and identify alternative models with the minimal available data in three major climatic zones of Iraq: the Mediterranean climate (MCZ), semiarid (SCZ), and arid desert (ACZ). The recent ERA5 data set was adopted. The results indicate that (1) the Rohwer mass-transfer method is the best for estimating ETo for two-thirds of Iraq with a mean correlation coefficient (R2) of 0.97, mean Kling-Gupta efficiency (KGE) of 0.84, mean percent bias (PBIAS) of −8.92%, mean Nash-Sutcliffe efficiency coefficient (NSE) of 0.92, and root mean square error (RMSE)-observations standard deviation ratio (RSR) of 0.27, followed by the Penman (R2=0.90, KGE=0.75, NSE=0.77, RSR=0.46, and PBIAS=6.36%) and Caprio (R2=0.90, KGE=0.66, NSE=0.54, RSR=0.58, and PBIAS=24.64%) models; (2) Caprio is the best radiation-based model for estimating ETo, mainly in the ACZ, whereas Kharrufa is the best temperature-based model for estimating ETo, primarily in the SCZ and ACZ. Overall, the mass-transfer-based models performed better than other-based models for ETo estimation. The outcomes of this study provide a scientific reference for accurate ETo estimation using empirical models under limited data sets, which is valuable for irrigation management in Iraq.Practical ApplicationsAccurately estimating ETo is vital for effective water irrigation management and sustainable agriculture planning. However, the recommended method for estimating ETo, the FAO-PM method, requires various data inputs that may not always be readily available, especially in regions like Iraq. Therefore, this study assessed the performance of 40 empirical ETo models, categorized into radiation-based, mass-transfer-based, and temperature-based models, against the FAO-PM model in three major climatic zones of Iraq: the Mediterranean climate, the semiarid region, and the arid desert. The study found that the Rohwer mass-transfer method showed the best performance in estimating ETo for two-thirds of Iraq. The Penman and Caprio models also performed well in estimating ETo in specific areas. The study revealed that the choice of the ETo model varied depending on the climatic zone. The Caprio model performed best for radiation-based estimation in the arid desert, whereas the Kharrufa model was most effective for temperature-based estimation in the semiarid region and arid desert. Overall, the mass-transfer-based models outperformed other types of models in ETo estimation.