Temporal And Spatial Distribution Of Evapotranspiration In The Three-Rivers Headwaters Region

The Three-Rivers Headwaters Region is located in the hinterland of the Qinghai-Tibet Plateau and in the south of Qinghai Province.It is the source catchment area of the Yangtze River,the Yellow River and the Lancang River.Known as the "Chinese Water Tower",the Three-Rivers Headwaters Region(TRHR)is the highest natural wetland and the largest nature reserve in China.It is also the most concentrated natural reserve in the world's high-altitude areas and has the typical characteristics of the Tibetan Plateau ecosystem and biodiversity.However,the ecological environment of TRHR is fragile,and the assessment of groundwater evapotranspiration(ET)plays an important role in the evaluation of the water cycle and water resources in the region,and also has certain significance for ecological environmental protection.Therefore,in this paper,the series MODIS data and GLDAS data are used to calculate the regional evapotranspiration in TRHR from 2001 to 2017 using the SEBS model.The spatial and temporal distribution characteristics,change trends of evapotranspiration and the influencing factors are analyzed by the Sen + Mann-Kendall method,the result can provide a reference for the reasonable development and utilization of water resources in TRHR.The results showed that:(1)The regional ET increase in TRHR from 2001 to 2017.The ET increase significantly from 2001 to 2009,and the ET was generally stable from 2009 to 2017.The largest ET of 552.55 mm was observed in 2009 and the smallest value of 275.03 mm was occurred in 2002.(2)During the past 17 years,the annual ET trends of the sources of the Yangtze River,Yellow River and Lancang River are basically the same as that of the three river sources,showing an overall increasing trend.(3)The ET of the sources of the Yangtze River,Yellow River,Lancang River and the Three-Rivers Headwaters Region is consistent throughout the year,mainly from May to September,with the highest ET in June and the smallest ET in January and December.(4)The annual average ET values of different areas are,from the largest to the smallest,namely the Lancang River source,the Yellow River source,and the Yangtze River source.(5)The ET of the water body in TRHR is the largest,and the annual ET can reach up more than 1200 mm.On the contrary,the ET in the bare land area is the smallest,and the annual ET is less than 100 mm.(6)The ET in the three river sources,the Yangtze River source,the Yellow River source and the Lancang River source region showed a significant growth trend,and the areas accounted for 62.62%,63.25%,58.73%,and 68.37%,respectively.The areas with slight significant growth reached 28.03%,and the area ratio was 28.03%,28.21%,32.69%,and 23.37%,respectively,and the areas with significant reduction doesn't exist.(7)The climate in the study area is the main factor affecting the change of ET.The variation of ET is positively correlated with precipitation and air temperature and the coefficients of determination are 0.80 and 0.89 respectively.The ET increases with the increase of precipitation and temperature.(8)ET is positively correlated with vegetation index and soil moisture.The determination coefficients are 0.62 and 0.60,respectively,which means the ET increase with the vegetation index and soil moisture increase.