FAO-56 Penman-Monteith DailyETofrom Linear Regression Calibrated Hargreaves Equation with Wind Terms in Tropics with Limited Data

Hargreaves equation (HG), which lacks a wind speed (u2) term, was modified, through a linear regression calibration method, into LHGu which has u2 terms. LHGu is effectively a simplified method for approximating FAO-56 Penman-Monteith equation (FPM) daily reference evapotranspiration (ETo) in tropics with only temperature data. In LHGu, the “0.0023” constant term in HG was calibrated as a shifted power function of u2, and the calibration constant was parametrized as a quadratic function of u2. LHGu was developed using simulated constant u2 data and historical temperature data for four sites in West Africa: Abidjan, Accra, Daloa, and Lome. LHGu matched FPM ETo better than HG over a wide range of u2: for Accra, for u2 range 0.5–6.0 m/s, the modified coefficient of efficiency, E1, varied narrowly (0.83–0.98) for LHGu but widely (0.14–0.95) for HG optimized for u2=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 u2. LHGu is useful for quickly computing practically accurate estimates of FPM ETo for varying daily u2 where only temperature data are available