The Difference Of Spatiotemporal Changes Between Reference Crop And Potential Evapotranspiration

Evapotranspiration is an important part of water balance and energy balance,which plays an important role in the process of water movement.Reference crop evapotranspiration?ET_o?and potential evapotranspiration?ET_p?are two commonly used concepts in the research,which are also often mixed.In a large number of studies on evapotranspiration in China,these two different concepts are used directly as tools to study relevant contents,and there is little discussion on the historical development of the two concepts and their similarities and differences or connections.Because the difference between ET_o and ET_p are not clear,there are few researches on the numerical value and temporal and spatial variation of ET_o and ET_p,and the relevant researches only discuss the change characteristics of evapotranspiration in some regions of China,lacking the analysis of the overall change of the whole country.Through consulting a large number of literatures,the differences and connections between ET_o and ET_p in terms of concept,application and formula are summed up.In addition,48 ET_o calculation formulas and 36 ET_p calculation formulas are sorted out,the monthly meteorological data from 1961 to 2018 of 630 meteorological stations are used,including average temperature,maximum temperature,minimum temperature,sunshine hours,wind speed and average relative humidity,and 337 stations with monthly evaporation data from1961 to 2001 are also used,chose FAO56 PM?1998?equation and Penman?1963?equation as the standard calculation formula of ET_o and ET_p respectively,and the applicability of the two kinds of formulas is compared with each other by using the coefficient of spatial variation?C_v?and the coefficient of determination?R²?as the evaluation index.Furthermore,the same meteorological data is used to analyze the relationship or similarities and differences between the temporal and spatial changes of the two standard calculation formulas according to the monthly scale and the annual scale,and their differences and relations in numerical and spatial distribution characteristics are distinguished.The main conclusions are as follows:?1?Both ET_o and ET_p are originated from evaporation.ET_p was put forward much earlier than ET_o,ET_o is a new concept developed on the basis of ET_p.They are related to each other,both of them are used to describe the change of water quantity and energy,ET_p is more generally used to refer to the evaporation of all wet surfaces,such as open water,ground,reservoir and atmosphere,etc.,while ET_o represents a specific crop evapotranspiration value,which is mainly obtained by specific formula and crop coefficient,so the application object of ET_o is more specific than that of ET_p,and ET_o can also be regarded as a special case of ET_p.The calculation equations of ET_o can be divided into temperature based,radiation based,combination based and evaporation pan based,while the calculation equations of ET_p mainly includes mass transfer based,temperature based,radiation based and combination based.In addition,their application fields are also different.ET_p is more used in hydrology,ecology and meteorology,and ET_o is more used in agricultural water conservancy,especially in irrigation system planning and other related fields.?2?The monthly average ET_o values calculated by four types of equations show different spatial distribution characteristics,the temperature based equations'results are gradually increasing from north to south,the maximum difference can reach 113mm/month.The results of radiation based equations show that the ET_o values of stations in the central region are smaller,but in the surrounding region are larger,the maximum difference can reach251mm/month.The results of the combination based equations show a decreasing trend from north to south and from east to west,the maximum difference can reach 287mm/month.However,most results of the evaporation pan equation show the characteristics of decreasing from north to south in spatial distribution,the maximum difference can reach 248mm/month.Compared with the ET_o value calculated by FAO56 PM?1998?,among the 48 formulas,Xu and Singh equation(ET_o,T,6)in the temperature based,Xu and Singh equation(ET_o,R,7)in the radiation based,Doorenbos and Pruitt equation(ET_o,C,1)in the combination based and Abdel-Wahed and Snyder equation(ET_o,pan,7)in the evaporation pan based have the highest similarity with the standard equation,the trend of monthly average value with time is the closest,and the fitting results of monthly and site by site values are the best.?3?The monthly average ET_p values calculated by four different types of ET_p equations also have different distribution characteristics,among the mass transfer equations,the result values of stations in the north and northwest regions are larger than those in the central and eastern regions,the maximum difference can reach 614mm/month.The temperature based equations'results are gradually increasing from north to south,the maximum difference can reach 256mm/month.In the results of radiation equations,some stations with larger values are distributed in the middle,south and east subregions,while some stations are mainly distributed in the west,north and northeast subregions,the maximum difference can reach 139mm/month.The spatial distribution of the combination equations'results also has similar characteristics,that is,the value of stations in the northeast is smaller,and the value of stations in the northwest is larger,the maximum difference can reach 163mm/month.Compared with the standard equation,Penman?1963?,the results show that the Kuzmin(ET_p,M,10),Hargreaves(ET_p,T,9),Makkink(ET_p,R,1),and Wright and Jensen(ET_p,C,5)equations have the highest consistency with the standard calculation method in the mass transfer based,the temperature based,the radiation based,and the combination based equations respectively.?4?On the monthly time scale,the monthly results of ET_o and ET_p have similar fluctuation characteristics but with different values,the maximum values are 136mm/month and166mm/month respectively,and the minimum values are 27mm/month and 39mm/month respectively.The spatial distribution of mean multi-year monthly results show that the distribution of stations with larger value gradually transfer from the south to the north and then return to the south in the order of January to December,the maximum values are in July,which can reach 283mm/month and 287mm/month,and the minimum values are in January,which are as low as 3mm/month and 5mm/month.In each subregion,the mean multi-year monthly results in each month show random fluctuation,the peak value in subregions I,VI and VII appeared in July,and that in subregions II,IV and V appeared in May.The results of MMK test show that the number of stations with increasing trend in the time series of ET_o value in each month is more than that of ET_o value,the biggest difference appeared in November with163 stations;the results of R/S test show that the number of stations with continuous trend change in the average multi-year monthly value is much more than that of anti-continuous stations,the biggest difference appeared in January with 69 stations;?5?The results of wavelet analysis on the period of each month show that the periodicity between each month is similar,with the longest main period of 15 years in May and the shortest period of 3 years in December;and the fitting results of these two equations show that the highest similarity is June,which R2 value is as high as 0.96,the lowest is October,which R2 value is as high as 0.89.In annual time scale,the results,such as comparing the spatial distribution of the annual values of the two equations'results,the change trend of the annual values of each subregion with time,using MMK method,R/S method and wavelet analysis to test the trend,are highly consistent with the results of the some monthly values.The numerical relationship between these two equations is that the ET_p value is higher than the ET_o value.