Modification of Hargreaves Equation Coefficient to Estimate Reference Evapotranspiration in Gangwondo

Reference evapotranspiration (ET0) can be estimated using Hargreaves equation with temperature data alone. The equation has a tendency to overestimate for regions of high relative humidity and to underestimate forregions of wind speed greater than 3 m s-1. The study was conducted to determine Hargreaves equationcoefficient in 11 regions in Gangwondo by comparing ET0 determined by modified Hargreaves equation withET0 by the FAO Penman-Monteith equation. The Penman-Monteith ET0 values were determined using theweather data including solar duration, air temperature, humidity, and wind speed for 11 weather stations inGangwondo from 2007 to 2017. The modified Hargreaves coefficients for inland regions such as Chuncheon,Hongcheon, Cheorwon, Wonju, Inje, Yeongwol, Daegwallyeong, and Taebaek ranged from 0.00181 to0.00199, less than the default value 0.0023. The modified coefficients for coastal regions such as Gangneung,Sokcho, and Donghae ranged from 0.00275 to 0.00302, greater than the default value. Modified HargreavesET0 using modified coefficients reduced root mean square error (RMSE) from 0.84 mm d-1 to 0.69 mm d-1 andincreased Nash-Sutcliffe coefficient (NSC) from 0.69 to 0.78 compared with original Hargreaves ET0. Therefore the modified Hargreaves coefficients obtained from the study can provide more accurate ET0estimates than using original default value by considering regional climate characteristics in Gangwondo. KCI Citation Count: 4