Correcting <scp>Hargreaves‐Samani</scp> formula using geographical coordinates and rainfall over different timescales

The United Nations Food and Agricultural Organization (FAO) approved the Hargreaves-Samani formula (HAR-85) as an alternative to standard Penman-Monteith method (FAO-PM) for estimating grass reference evapotranspiration (ETo). With much less data demand, HAR-85 is unequivocally useful where meteorological variables are often scarce, incomplete or unavailable. Herein, we evaluate against FAO-PM across 2.505 million km2, representing Sudan South encompassing wide hydroclimate domains including Nile River. We further propose simple year-round seasonal adjustment models correcting entire study area. express HAR-85's error in multiple linear regressions terms of latitude, longitude, altitude and/or monthly rainfall. Varying periods, odd, even all years, used evaluation development validation processes investigate influence changing period. A suit eight performance indicators shows dependency original bias on geographical location, rainfall amount, season year All amplify southward from hyper-arid region dry sub-humid zone. For example, mean (MBE) ranges −0.51 1.29 mm/day, respectively. Study area-wide, least represents during hottest month transitional (between wet dry-cool seasons) with MBE 0.65 0.70 Conversely, it most wettest month, smallest 0.32 mm/day. Beholding this spatiotemporal trait, final yearly developed herein enormously moderate predominant overestimation HAR-85. former model explains 46.7% variance whereas 36.9% 62.3% variation explainable by latter models. These narrow among stations −0.71-2.17 −0.80-1.20 −0.65-0.99 Without undermining accuracy, can still be feasibly recommended general use