Spatio-temporal Characteristics Of ET And Its Relationship Between Climate Factors In The Source Region Of The Yangtze And Yellow River In Recent 17 Years

Located at the eastern margin of the Tibetan Plateau,the source region of the Yangtze and Yellow River(SRYYR)is an important ecological security shelter in southwestern China with its unique natural habitats and abundant natural resources.As the pivotal link of water cycle,Evapotranspiration(ET)is one of the key factors of water cycle research and water resource management,and has great significance in adjusting regional climate,protecting and conserving ecosystem water.Based on the meteorological station data,land use type data and DEM data,and using the methods of grid analysis,trend analysis and correlation analysis,MODIS ET is selected as the main data source to research the spatio-temporal characteristics of ET and its variation under different land use types,elevations and slopes as well as its relationship with climatic factors in the SRYYR from 2001 to 2017.The results indicate that:(1)The interannual value of ET is between 352.76~452.41 mm/a,the multi-year mean value of ET is about 419.42 mm,and the anomaly relative variation is obvious.In the past 17 years,the trend of ET increases and fluctuates,and the trend variation rate is 12.50 mm/10 a.In addition,the regional differentiation of multi-year mean ET is obvious,the western ET is significantly weaker than that of the central and eastern region,and the strongest ET is observed in the eastern part of the study area.(2)During the study period,the ET shows a periodic unimodal trend and peaks in July.Moreover,the seasonal differences of ET are apparent,and the value of seasonal ET has a regular pattern,namely: summer>spring>autumn>winter.The trend of spring ET increases significantly with the change rate of 1.00 mm/a,and the mean value of spring ET gradually increases from west to east in spatial distribution.The trend of summer ET increases slightly with the change rate of 0.17 mm/a,and the mean value of summer ET gradually increases from west to east.The trend of autumn ET decreases slightly with the change rate of-0.02 mm/a,and the mean value of autumn ET in every district are close.The trend of winter ET increases slightly with the change rate of 0.09 mm/a,and the mean value of winter ET decreases from west to east and its spatial distribution isn't balance.(3)In recent 17 years,the value of ET in each type of land use decreases firstly and then increases,and it has a basic regular pattern,namely: forest>grassland>sandy bare land>wetland>other type.In particular,the value of grassland ET has a regular pattern,namely: high coverage grassland>medium coverage grassland>low coverage grassland.Furthermore,the value of ET gradually decreases with the increase of elevation in study area.When slope is between 0 and 20 °,the value of ET decreases with the increase of slope;and when slope is greater than 20 °,the value of ET increases with the increase of slope.(4)Interannual ET is positively correlated with air temperature and wind speed in the same period,and negatively correlated with relative humidity,precipitation and sunshine hours.According to the partial/complex correlation analysis results,air temperature and relative humidity have greatest impact on interannual ET.Spring ET is positively correlated with air temperature,wind speed and precipitation in the same period,and negatively correlated with relative humidity and sunshine hours.The partial/complex correlation analysis shows that wind speed and relative humidity have greatest impact on spring ET.Summer ET is positively correlated with precipitation and relative humidity in the same period,and negatively correlated with air temperature,wind speed and sunshine hours.The partial/complex correlation analysis shows that precipitation and sunshine hours have greatest impact on summer ET.Autumn ET and Winter ET are positively correlated with air temperature,wind speed and sunshine hours in the same period,and negatively correlated with precipitation and relative humidity in the same period.According to partial/complex correlation analysis results,air temperature and relative humidity have greatest impact on autumn ET,wind speed and relative humidity have greatest impact on winter ET.(5)The results of ET driven by climate factors demonstrate that air temperature is the predominantly driving power of climate factors for interannual ET change,relative humidity for spring ET change,sunshine hours for summer ET change,air temperature for autumn ET change,and relative humidity for winter ET change.