GAIA: A New Formula for Reference Evapotranspiration

Abstract

Estimating evapotranspiration is crucial for irrigation and agricultural applications. Although the FAO-56 Penman–Monteith method is highly accurate under conditions of abundant data, its extensive requirements limit its practical application. In the Mediterranean region, most empirical formulas used to estimate evapotranspiration are temperature-based and require calibration to be effective. The current study aims to introduce a novel formula to determine reference evapotranspiration using temperature, relative humidity, and extraterrestrial radiation daily in the Mediterranean region and evaluate its performance. Multi-linear regression was applied to agrometeorological data from the California Irrigation Management Information System (CIMIS) to develop the ETo formula. The formula was then validated using data from 252 stations in four countries (Greece, Spain, Portugal, and Cyprus) over the growing period of six years (2018–2023). The GAIA formula consistently outperformed formulas with the same or fewer variables, including Berti and Ahooghalaandari, across all metrics. The largest differences were observed in RMSE and the index of agreement. There is a strong correlation between GAIA and the FAO-56 Penman–Monteith formula (coefficient of determination = 0.88). While the GAIA formula shows a high coefficient of determination, its performance is somewhat lower than that of Copais and Valiatzas, particularly in terms of Pearson correlation and the coefficient of determination. A key difference is that Copais and Valiatzas rely on incoming solar radiation, whereas GAIA uses extraterrestrial radiation. Relative humidity was found to be the most influential variable, accounting for over 71% of the variance in ETo. Effective evapotranspiration (ETo) calculation methodologies, especially in areas with limited agrometeorological data, can significantly enhance irrigation system efficiency and promote sustainable water management. The GAIA formula offers a cost-effective method for estimating reference evapotranspiration (ETo) during the growing season with enhanced accuracy, eliminating the need for expensive equipment. However, its primary limitation is that it is validated in the Mediterranean region and within a specific geographical latitude range.

Keywords

• evapotranspiration
• agrometeorology
• agricultural water management
• statistical analysis