Different Methods To Estimate The Effect Of Reference Crop Evapotranspiration On Drought Index SPEI

Reference crop evapotranspiration?ET_o?is an important parameter for climate,hydrology and agricultural management.It is affected by regional climatic conditions and is an important component of regional energy balance and water balance.It is of great significance in agricultural water resources utilization,ecosystem process modeling and evaluation of regional wet and dry conditions.In general,the FAO56 Penman-Monteith method(ET_o,s)is used as the standard method for estimating ET_o.However,the method requires more input of meteorological elements,higher data requirements and limited application in meteorological data.Based on the analysis of the spatial distribution characteristics of the meteorological elements,the FAO56 Penman-Monteith method was used to study the effects of the evapotranspiration of the reference crop in China and each sub-region.Spatiotemporal distribution and trend distribution characteristics;to assess the other 10 methods?ET_o,i,i=1,2,…,10??i.e.,Irmark et al.,Makkink,Priestley-Taylor,Hargreaves-Samani,Droogers-Allen,Berti et al.,Doorenbos-Pruitt,Wright and Valiantzas,respectively?applicability of the method in China and different sub-regions,select the entire China and regional applicability are good,and the calculation process is simple,the required input meteorological elements of the algorithm as ET_o optimal alternative method.On this basis,the results of ET_o method with good adaptability in each type of method are selected to calculate the standard precipitation evapotranspiration index?SPEI?,and the SPEI corresponding to each method is compared with the different time scales scale and seasonal scale and the difference between wet and dry.The main results and conclusions are as follows:?1?The annual scale of 339?61%?sites in 552 sites has a downward trend,and 213?39%?sites have an upward trend,indicating that China's annual scale is generally declining.From the spatial distribution of the monthly scale,it is higher in the western region of China?high altitude?and lower in the eastern part of China?low altitude?.?2??Relative error,standard deviation and consistency index?and the scatter plot between ET_o,s and ET_o,i,the results show that: The proposed MHS₂ method(Berti et al.2014,ET_o,6)has high accuracy and simple calculation process,only the temperature?weather data?is used as the input data.In addition,the method has good correlation with the standard method.Therefore,when the meteorological data is missing,the method can be used as the optimal alternative method.?3?The SPEI corresponding to each method has high similarity in the process of time variation,but there are still regional differences and differences between methods.At different time scales,the SPEI corresponding to each method has the smallest difference in V sub-region?the subtropical humid Central and South China?and VII area?the tropical humid South China?,and the difference is most in I sub-region?the temperate and warm-temperate desert of Northwest China?.In the seasons,the size of the SPEI corresponding to each method in different seasons follows: I>II>VI>III>IV>V>VII.Among them,the SPEI corresponding to each method in winter is significant difference in different sub-regions,the overall change is larger than the other three seasons;the summer change is relatively gentle.?4?The spatial distribution of SPEI corresponding to the FAO56-PM method has seasonal differences,and the trend of wet and dry varies with different seasons.The SPEI corresponding to standard method in spring has a significant downward trend in the central region of China?part of the IV and V sub-regions?,showing a significant upward trend in the northeastern region of China,indicating a significant trend of dryness in the central region and significant in the northeastern region Wetting trend.In the summer,the SPEI corresponding to standard method showed a significant upward trend in the V sub-region,indicating a significant wetting trend;the significant descent sites were relatively few and more dispersed.In the autumn,the corresponding changes of SPEI are in contrast to the summer,and the descending stations are mainly concentrated in the V sub-region?central China?,indicating a significant trend of drying in central China;the significant increase in sites is relatively small and scattered.There was a significant upward trend in winter in Northeast China?III sub-region?,central China?V sub-region?and Northwest China?I sub-region?SPEI,indicating a significant wetting trend in these areas in winter.?5?Precipitation is the main factor of the regional wet and dry change.The spatial distribution of the seasonal variation of the SPEI trend in the four seasons is similar to that of the precipitation trend in varying degrees.The precipitation and ET_o are the necessary parameters to calculate the SPEI.The influence of two parameters on SPEI is different under the different sub-regional conditions.