Effects of Angstrom coefficients on ET₀ estimation and the applicability of FAO recommended coefficient values in China

As a commonly used method of calculating global solar radiation(R_s),the determination of the Angstromcoefficients(a and b)values play a vital role to the success of estmiating the reference crop evapo transpiration(ET₀).At present, the values of 0.25 for a and 0.5 for b are used in the ungauged regions of China.Such valuesare recomm ended by the Food and Agriculture Organization of the United Nations(FAO).However,a comprehensiveassessment of the applicability of FAO recommended values in China must be done before they can be used with confidence to calculate global solar radiation.In this study,the effect of a and b on ET₀ estmiation is investigated using the surface clmiatologic and solar radiation data measured at 104 meteorological stations distributed in China.Monthly values of a and b are calibrated with the observations,and their spatial and temporal variations are examined.The applicability of FAO recommended values in ET₀ estmiation is tested over 7 geographic regions.A new regional method for determinating the Angstrom coefficient a and b values is proposed for China ungauged regions.The main conclusions of this study are:(1)ET₀ is sensitive to the variability of a and b.The ET₀ estmiate may contain a large error resulting from the use of FAO recommended values in Rs calculation in certa in regions.(2)The calibrated value of a at most stations is significantly smaller than 0.25 that is recommended by FAO,and the value of b is however much greater than 0.50.For example,in Xinjiang autonomous region and South China,the calibrated values of a and b exhibit are latively uniformed distribution,however this is not the case over other regions.(3)The FAO recommended values are useful in ET₀ estimation with an acceptable accuracy in the areas of Xinjiang autonomous region,Northeast and Northwest China.However,the FAO values can result in severe overestmiation of ET₀ in the regions of South and Southwest China.(4)A new regional method for estimating a and b is proposed,which can give a satisfactory result of ET₀ estmiation when comparing to that of using the FAO recommended values.