Spatio-Temporal Variations And Attribution Analysis Of Pan Evaporation In China Mainland Base On The PenPan Model

Pan evaporation is an important reference index of atmospheric evaporation capacity.Study on pan evaporation is beneficial to the exploration of hydrological cycle process and rational allocation of water resources.The pan evaporation in China mainland were generally measured and modeled.However,measured values of pan evaporation usually had disadvantage due to the conversion between 20 cm and E601 pans.The PenPan model has proved efficient in the simulation of Pan evaporation,which can make up for the shortcomings of actual measurement.In this study,the PenPan model was used to estimate the pan evaporation in China mainland.First,the PenPan model was calibrated(based on observations during 1961-1991)and validated(based on observations during 1992-2001)with the observations of 20 cm pan evaporation from 1961 to 2001 in China mainland.The mainland of China was divided into four different regions according to their climate characteristics,e.g.temperate monsoon zone(TMZ),subtropical monsoon zone(SMZ),mountain plateau zone(MPZ),and temperate continental zone(TCZ).Next,the PenPan model was corrected through linear regression at three different time scales(year,season,and month)to gain the optimal correction scheme for each zone so as to lay a foundation for the application of PenPan model in China mainland.The corrected PenPan model was used to calculate the pan evaporation from 1961 to 2017 and to explore the trend and attribution of pan evaporation.The methos of T-test,Mann-Kendall(M-K)trend analysis,and inverse distance weight(ArcGIS spatial interpolation)were used to analyze the change-point,trend,and spatial variations of pan evaporation in China mainland.Due to the different meteorological factors in different climatic regions,the dominant factors affecting pan evaporation were different.Therefore,we used multiple linear regression and experimental de-trending approach to explore the dominant factors for the yearly and seasonal pan evaporations before and after the change-point in different climate zones.Some main findings are summarized as follows.(1)There were spatial variations in the simulation accuracy of PenPan model in China mainland.The simulations were relatively better in the temperate monsoon zone and subtropical monsoon zone than in the temperate continental zone and mountain plateau zone.There were obvious seasonal variations in the correction coefficient of the PenPan model.The correction coefficient of a single station varied remarkably in time and space,especially in temperate monsoon zone,subtropical monsoon zone,and the eastern part of mountain plateau zone,while the correction coefficients were the smallest in the summer for each zone.(2)The change-points of pan evaporation in different climate zones were identified.The change-point in the temperate monsoon zone was the year of 1987,1989 in the subtropical monsoon zone,1980 in the mountain plateau zone,and 1988 in the temperate continental zone,respectively.In summer,the regions with large portion of radiation component were in the temperate continental zone and the boundaries between the temperate continental zone and the mountain plateau zone.In other seasons,the regions with large portion of radiation component were southern parts of the mountain plateau zone and the subtropical monsoon zone.In winter,the regions with large portion of dynamic component were the southern parts of the mountain plateau zone and the subtropical monsoon zone.In other seasons,the regions with large portion of dynamic component were the temperate continental zone.The region with larger pan evaporation in winter was the southern parts of the mountain plateau zone and the subtropical monsoon zone.As the season went on,the regions with relatively large total pan evaporation transferred gradually from north to south.Generally,the region with the largest overall evaporation was the temperate continental zone in China mainland.(3)The decrease of wind speed and rise of saturated vapor pressure deficit were the key factors affecting the trend of evaporation in different climate zones.Because of the different climatic conditions in different climate zones,in the first period the dominant factor was the decrease of solar radiation,which led to the downward trend of pan evaporation in the subtropical monsoon zone.In the other zones,wind speed was dominant factor.However,there were some differences between the two identification methods of multiple linear regression and experimental detrending approach.In the second period,the dominant factor of the four climate zones all was the increase of vapor pressure deficit.At this time,the phenomenon of “evaporation paradox” disappeared.The results showed that the corrected PenPan model was able to satisfactorily correctly calculate the pan evaporation in China mainland.The spatiotemporal variations and dominant factors of pan evaporation in different climate zones were identified,which can provide a decision support for the spatial and temporal allocation of water resources in China mainland.