Measurement-based Evapotranspiration Spatial Variations And Their Driverrs Over Global Terrestrial Ecosystems

Annual evapotranspiration（AET）,the total water vapor loss to the atmosphere during a year,is a vital process of global water cycles and energy balance.Revealing the differences in AET values and spatial variations would benefit for understanding of regional differences and formation mechanism of AET,which also provides a data basis for regional water balance assessment.Based on the long-term networked observation data,we constructed an AET dataset containing 506 sites combined with climate biological data of each site and public data,which we used to explore the effect of ecosystem types on AET spatial variations,to analyze the differences of different climate regions and the effects of climate factors,and clarify the differences of AET spatial variation in different regions and the global pattern.The results showed that:Based on observational data from forest and grassland sites with no water supply in China,we found that the difference between forest and grassland did not cause significant differences in spatial variation and driving factors of AET.Climate factors play a dominate role in spatial variation of AET between forest and grassland.AET of forest and grassland ecosystems showed a significant latitudinal patterns,but affected differently by the factors such as mean annual air temperature（MAT）and mean annual precipitation（MAP）.AET values of forest and grassland ecosystems were generally similar under similar MAP and aridity index（AI）conditions.Based on observational data in China,we found that the difference in ecosystem types did not significantly change the values and AET spatial variation.Climatic and biological were the dominating factors of spatial variations and differences of AET among different ecosystems.There were significant differences in AET among different ecosystem types and vegetation regions.However,the differences of AET in different ecosystems types within the same vegetation region were only reflected in the wetland and grassland ecosystems in subtropical region and temperate desert region with abundant water conditions.Due to the different geographical locations,the dominanting factors of AET spatial variation in different ecosystem types were differ.However,they all reflected the influence of climatic factors and biological factors.Based on observational data in the Northern Hemisphere,we found that the geographical and climatic factors caused the difference of AET values and spatial variation among different climatic regions.There were significant differences in AET among different climate regions,which decreased from the warm climate region to the cold climate region.Geographical factors including latitude（LAT）dominate this spatial variation of AET,followed by MAP.However,the dominant factor of spatial variation of AET was different in different climate regions because of the differences of hydrothermal and vegetation conditions.Biological factors including the leaf area index（LAI）and maximum leaf area index（MLAI）dominate the spatial variation of AET in tropical and arid regions.Geographical factors dominate the spatial variation of AET in warm regions and polar regions.Climate factors including MAT dominate the spatial variation of AET in inland regions.Based on observational data in the global,we found that the the AET values and spatial variations in different regions were also significantly different.However,they all showed a significant decreased latitudinal pattern.Climatic and biological were the dominating factors of AET spatial variation in each region.There were significant differences of AET in different regions,with the highest AET in the Asia-Oceania region and the lowest AET in Europe-Africa region.The influencing factors of AET spatial variation were differ among different regions.Spatial variation of AET was influenced by MAT,MAP and LAI in Europe-Africa region.Spatial variation of AET was influenced by MAT,MAP and MLAI in Asia-Oceania region.Spatial variation of AET was influenced by MAT,MAP and LAI in America region.The combined effects of climatic factors and biological factors made the global AET showing a significant decreased latitudinal pattern.MAP,MAT,LAI and annual total photosynthetic effective radiation（PAR）were the main factors influencing AET,while MAP played a dominating role.The equation containing MAP,MAT,LAI and PAR explained 55%of AET spatial variation in global.Based on the equation,we got a global AET value of 504.13±303.51 kg H₂O m^-2 yr^-1.There were significant differences in AET among the five continents.The AET of Africa(725.09±171.91kg H₂O m^-2 yr^-1)was significantly higher than that of other continents,while the AET of Europe(405.57±112.43 kg H₂O m^-2 yr^-1)was significantly lower than that of other continents.Therefore,the differences of ecosystem types did not significantly change the value and spatial variation of AET.Geographical and climatic factors caused the difference of AET values and spatial variation among different climatic regions.The combined effects of climatic factors and biological factors was the main reason of AET spatial variation in different ecosystem types and regions,the global AET showing a significant decreased latitudinal pattern.Our results provided a data basis and theoretical support for understanding the spatial variation of AET of global terrestrial ecosystems.