Spatio-Temporal Patterns Of Evapotranspiration And Exploration Of Its Influence Factors In Hengduan Mountain Area

Remote sensing technique has made it intuitive to estimate surface evapotranspiration(ET)from various regions,which can break the limitation of traditional "point" observation,and it has become an important method to reveal the response and feedback of ET to global climate change.ET is a crucial parameter in the hydrological and energy transport and plays an important role to determine regional water resource and ecological balance.The response of ET to climate change has become an important part of global change investigation.In this study,the spatiotemporal patterns and trends of ET in the Hengduan Mountains area(HM)from 2000-2014 are demonstrated based on MOD16(Moderate Resolution Imaging Spectroradiometer,MODIS)ET data.The correlation analysis method is used to study the relationship between ET and climate factors within interannual and seasonal period.Combining with land use/cover types data,ET changes over different types of land use/cover change(LUCC)in HM from 2001 to 2014 is analyzed,and the impacts of climate change and land use change on ET is explored.Results show that:(1)Contrary to the trend of air temperature,ET shows an overall trend of declinein,which means that the phenomenon of "evaporation paradox" existed in HM.In the past 15 years,the annual value of ET in the HM fluctuated from 561.34 to 619.60 mm,and the average value of annual ET for the 15 years is approximately 590.04 mm.The dry season ET is fluctuated in 179.84 to 214.37 mm and the wet season ET from 381.5 to 411.32 mm,in which the multiyear average value of seasonal ET is 193.97 mm and 396.07 mm,respectively.ET showed a "unimodal" change within a year,with the peak approach to July,and the trough around December.The statistical results of ET value on different slopes further indicate that the monsoon plays a positive role on ET in HM,which proves the influence of the northern plateau monsoon on dry season ET and the east Asian and south Asian monsoon on wet season ET.With the increasing of elevation,ET presented a characteristic of “increasing-decreasing-increasing”.(2)From 2000 to 2014,ET values in interannual,dry and rainy seasons are positively correlated with cumulative precipitation(PRE)and average relative humidity(RHU),negatively correlated with average temperature(TEM)and cumulative sunshine hours(SSD),and has an indistinctively negative correlation with average wind speed(WIN).The overall correlation between ET and climate factors in the whole year and the dry season is TEM,RHU,PRE,SSD and WIN in the order of the largest to the smallest,wherea it is RHU,PRE,SSD,TEM and WIN in the rainy season.The driving factor analysis within ET and climate factors is studied,and the resullts show that the main climate driving factor of ET in the whole year and dry season is TEM,whereas it is RHU in the wet season.(3)From 2001 to 2014,the multi-year average value of ET in each type of land use is as follows: bare land > forestland > sparse forestland > cultivated land > grassland.The conversion of forestland to sparse forestland and sparse forestland to grassland was extensive and in which the declining of ET is sharp,indicating that the decline trend of ET in HM is mainly affected by vegetation degradation in terms of land use change.The result of comparing between the effects of climate factors and LUCC on ET shows that the impact of the former is greater than that of the latter.