A modified Hargreaves equation by free search and its application in Harbin

The reference crop evapotranspiration (ET₀) calculated from Hargreaves equation will be negative as the fixed parameters when mean air temperature is lower than -17.8°C. To improve the application of Hargreaves equation, Free Search was employed to solve the parameters of modifying Hargreaves equations based on 48-year daily meteorological data of Harbin, and the evaluation of the modified Hargreaves equations were verified by three time steps of daily, ten-day and monthly. The research results showed that the modified Hargreaves equations could solve the problem on the negative ET₀, and its accuracy is identical with the verified Hargreaves equations recommended by FAO,when ET₀ values calculated from Penman-Monteith equation was taken as standard. And what´s more, a simple method for developing Hargreaves equation was developed then.