FAO-56 Penman-Monteith Daily ETo 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 (ETU) 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 ofU 2. LHGuwas 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 ETU better than HG over a wide range of u(2) : for Accra, forU 2 range 0.5-6.0 m/s, the modified coefficient of efficiency,u(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 i(2). LHGu is useful for quickly computing practically accurate estimates of FPM ETU for varying daily u(2) where only temperature data are available.