Climatic Assessment of FAO WaPOR Reference Evapotranspiration Product

Articles in Press

Document Type : Research Paper

Authors

Water Management and Engineering Deprtment. Collage of Agriculture. Tarbiat Modares University, Tehran. Iran.

Abstract

This study evaluates the accuracy and performance of the FAO WaPOR version 2 model product for reference evapotranspiration (ET₀ WaPOR) in comparison with corresponding estimates derived from the FAO Penman-Monteith equation using meteorological station data across various climate zones in Iran. A total of 42 stations located in northern and northeastern provinces of the country, with recorded data from 2009 to 2022, were selected for the analysis. Comparisons were conducted at daily, dekadal, and monthly time scales. To assess the role of climate, regions were classified based on the UNEP aridity index. Furthermore, the impact of radiative and convective components on evapotranspiration estimation accuracy in the WaPOR model was investigated. Results showed that the WaPOR model performed well in arid and semi-arid climates, with an average R² of 0.91 and an average nRMSE of approximately 20%. In contrast, in humid and sub-humid climates, the R² and nRMSE were 0.83 and 26%, respectively. The model's bias was negative in arid climates and positive in humid ones, indicating overestimation under humid conditions. Additionally, the analysis of the radiative and convective contributions to evapotranspiration estimation revealed that model accuracy decreased with increasing radiative dominance, particularly in humid regions. Overall, the WaPOR model, based on remote sensing, can be considered a reliable tool for irrigation planning in arid and semi-arid regions, especially during the wheat growing season. However, its performance in humid climates requires further optimization.

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Iranian Journal of Soil and Water Research

Articles in Press, Accepted Manuscript
Available Online from 18 October 2025

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