Performance of the FAO-56 Penman-Monteith Method with Limited Meteorological Data in Eritrea: A Case Study of Halhale

The FAO56-Penman-Monteith (FAO56-PM) method is widely used to calculate crop water requirements (Sharma & Changade, 2025).It also serves as a standard benchmark for evaluating alternative ETo methods in data-scarce regions (Ghebretnsae et al., 2025).However, its requirement for comprehensive climate datasets is a significant constraint in countries like Eritrea, where meteorological stations often lack complete records.For such datalimited contexts, researchers are strongly recommended to use the FAO56-PM method with estimated missing variables or the Hargreaves-Samani equation as reliable alternatives (Allen et al., 1998).Research has shown that the relative importance of climatic variables for ETo estimation is region-specific, influenced by the local climate regime, geographic location, season, and other factors.Wind speed was found to have less impact on the accuracy of ET o estimates particularly in humid climates of China (Gong et al., 2006), semiarid climates of Tunisia (Jabloun & Sahli, 2013), and semi-arid part of Manitoba (Ndulue & Ranjan, 2021).Contrarily, wind speed was a major source of error in less humid climate and windy areas of Cte d'Ivoire (Koudahe et al., 2018).Solar radiation and relative humidity were more important than wind speed for an accurate PM-ET o calculation (Gong et al., 2006).Ndulue and Ranjan (2021) works showed that the effect of solar radiation on ETo estimates is higher than that of wind speed and relative humidity in tropical sub-humid of Brazil.Solar radiation worked poorly for humid conditions but yielded quite good results for semiarid conditions of Cte d'Ivoire (Koudahe et al., 2018).Gong et al. (2006) concluded that the accuracy rank of these three climate variables in terms of FAO56-PM ETo estimates differ from region to region.Thus, identifying a proper ETo estimation model in climate data-limiting agricultural area is crucial.The southern Central Highlands (CHLs) of Eritrea have a favorable climate and significant agricultural potential.To support the shift from traditional to commercial farming, the region is prioritizing efficient water use.Accurate estimation of reference evapotranspiration (ETo) is crucial for this, but is hindered by scarce climate data.This study addresses this gap by evaluating the performance of the FAO-56 Penman-Monteith and Hargreaves-Samani ETo methods under Eritrea's data-scarce conditions.The FAO56-PM method was evaluated using limited data from the Halhale manual meteorological station in Eritrea.The station (15.060N, 38.50 W, 1917 m AMSL), located in the country's southern CHLs, complies with FAO agrometeorological recommendations.Over 90% of the area's rainfall occurs in summer (June to August), coinciding with the northernmost movement of the Intertropical Convergence Zone.Data collection and processing followed Allen et al. (1998).Mean monthly values for maximum and minimum temperature, relative humidity, wind speed at 2m, and sunshine hours were used.After quality control for missing data and outliers, mean monthly reference evapotranspiration (ETo) was determined.According to Allen et al. (1998), ETo calculated from mean monthly data is very similar to the average of daily ETo values.