Evaluation And Improvement Of Methods For Estimating Reference Crop Evapotranspiration Based On Air Temperature

An accurate calculation of reference crop evapotranspiration(ET₀)plays a critical role in crop water management and irrigation schedule.However,FAO56 Penman-Monteith equation(PM)requires data on temperature,actual vapor pressure(e_a),shortwave solar radiation and wind speed at 2 m height.Except temperature,the related dataset were often not available or incomplete and have uncertain quality.Thus the ET₀ estimation using temperature data has drawn much attention in recent year.Under the condition of missing data,accurate estimation of actual water vapor pressure(e_a)and solar radiation was conducive to improve the estimation accuracy of Penman Monteith temperature approach(PMT),and provide a basis for the sustainable utilization of regional farmland water resources.The main conclusions are as follows:1)Based on the data of 886 meteorological stations around the world,evaluated the accuracy of the common methods(e_a1 and e_a2 method)for estimated the actual water vapor pressure without relative humidity,and proposed the improved methods(e_a3 and e_a4 method);Compared with e_a1 method,e_a2 method improves the estimation accuracy of daily e_a in hyper arid to Dry sub-humid area,but decreases the estimation accuracy in humid areas.The e_a3 and e_a4 methods could further improve the estimation accuracy.Among e_a3 and e_a4 methods,the e_a3method had higher accuracy than e_a4 method.The results were similar when estimating monthly e_a.Therefore,when the relative humidity data was missing,we suggested to use the e_a3 method proposed in this study to estimate the daily or monthly e_a.2)Based on the data of 96 radiation stations in Chinese mainland,the accuracy of 78temperature solar radiation models was analyzed,and proposed the improved models(T79?T82),the results showed that the T82 model had higher accuracy in the whole region and each region.It is recommended to use T82 model to estimate daily solar radiation.3)Based on the above methods to improve the PMT model(PMT_New model).And compared the PMT model,Hargreaves samani(HS)model,improved HS model and PMT_Newmodel estimation accuracy.Compared with the traditional HS model,the improved HS model had higher estimation accuracy of ET₀ from arid area to humid area;Compared with the improved HS model,the PMT model improves the accuracy of ET₀ estimation in all climate regions except the humid region.Except Arid and semi-arid climate zones,the PMT_New model can further improve the accuracy of PMT model;The PMT_New model also presented a better the global performance index(GPI)value,73%of which rank the first.Therefore,it is strongle recommended to use the PMT_New model to estimate ET₀,when the limited data is available.