FAO-56 Penman-Monteith Daily E T o from Linear Regression Calibrated Hargreaves Equation with Wind Terms in Tropics with Limited Data

Hargreaves equation (HG), which lacks a wind speed ( u 2 ) term, was modified, through a linear regression calibration method, into LHGu which has u 2 terms. LHGu is effectively a simplified method for approximating FAO-56 Penman-Monteith equation (FPM) daily reference evapotranspiration ( E T o ) in tropics with only temperature data. In LHGu, the “0.0023” constant term in HG was calibrated as a shifted power function of u 2 , and the calibration constant was parametrized as a quadratic function of u 2 . LHGu was developed using simulated constant u 2 data and historical temperature data for four sites in West Africa: Abidjan, Accra, Daloa, and Lome. LHGu matched FPM E T o better than HG over a wide range of u 2 : for Accra, for u 2 range 0.5–6.0 m/s, the modified coefficient of efficiency, E 1 , varied narrowly (0.83–0.98) for LHGu but widely (0.14–0.95) for HG optimized for u 2 = 2.0 m/s; the corresponding MBE ranges were −0.05–0.01 mm/d for LHGu and 0.02–0.63 mm/d for HG which cannot respond to varying daily u 2 . LHGu is useful for quickly computing practically accurate estimates of FPM E T o for varying daily u 2 where only temperature data are available.