| In the 21 st century,the problem of water scarcity is becoming increasingly prominent issue in many countries and regions,especially in the arid and semi-arid areas with more fragile ecological environment.Evapotranspiration(ET),as the main way of water consumption,is not only an important part of material energy exchange process in the earth atmosphere system,but also an important part of water cycle and energy balance.Therefore,it is very important for the management of regional water resources to accurately estimate the evapotranspiration in arid and semi-arid areas and to master the spatial distribution of evapotranspiration.In this paper the Weiganriver and Kuqa river oasis in Xinjiang Uyghur autonomous region was selected as the study area.And remote sensing images(1997,2006 and 2016,from March to November)are combined with measured data from surface meteorological stations,for estimating the daily ET changes in the study area based on the SEBAL model.Finally effects of LUCC on special distribution of daily ET,and correlation between daily ET and land surface parameters were analyzed.The main conclusions of this paper are as follows:(1)the change of LUCC types in Weigan-Kuqa Oasis are more obvious in the period of 1997-2016 years,showing a trend of "one increase four decrease".Among the LUCC types the cultivated land persisted in the increasing tendency,forest and grassland and Gobi showed higher devotion to the expansion of cultivated land area,the results caused extension of oasis to some degree.It shows that a large number of natural land use types have been transformed into artificial land use types.(2)the spatial distribution characteristics of daily ET values of 1997,2006 and 2016 of Weigan-Kuqa Oasis are similar,overall indicated high value in the west and north,low value in the east and south,the high value appears in the water and the vegetation covered oasis inner region,this creates the along the river distribution characteristics.And the low value is dispersed in the low vegetation coverage area in the eastern and southeastern part of the study area.With the increase of cultivated land area,from 2006 to 2016,the area with high ET value extended outwards from the oasis.From March to April,The spatial distribution of daily ET showed periodical changes.From March to July,the average daily ET value increased with the time growth,however,7-11 months gradually decreased with time goes.According to the spatial distribution characteristics,the daily ET of the Tarim River in the southern part of the study area has highest value,while the daily ET in the saline alkali land and Gobi area in the oasis and desert ecotone and the oasis area has low value.(3)From1997 to 2016(3 ~11 months),daily ET showed a significant positive spatial autocorrelation,it means that the points with the similar ET values has higher spatial aggregation degree.1997-2016,the regions with the high ET values were mainly concentrated in the areas with high soil moisture and vegetation coverage around the Weigan and Kuqa rivers and the Tarim River,while the daily ET low concentration areas were mainly concentrated in the East,Southeast and western regions of the research areas with low vegetation coverage.The daily ET high value accumulation area in the northern part of the study area extends outward along the river in the northern part of the study area,and the area in the South gradually decreases.The low value aggregation area shows the trend of decreasing gradually in the eastern and western areas of the study area.In the oasis,the ET high concentration area in 3 ~11 months showed the trend of the river as the center in the oasis,and the southern part decreased gradually,while the ET high concentration area decreased inner part of the Oasis and increased in the south.(4)There are obvious differences in the daily ET of different LUCC types in the Weigan-Kuqa Oasis,and their values are ranked as water body > cultivated land > forest grassland > saline alkali soil > Gobi.The ET value of the study area increased with the increase of water content and vegetation coverage.in 3 ~11 months,the daily ET average of each LUCC type showed an increasing trend with the increase of temperature and sunshine hours and the difference between daily ET of the LUCC types gradually increased,but with the decrease of temperature and sunshine hours,the daily ET of the LUCC type reduced,and the difference between daily ET of each LUCC type gradually decreased.(5)1997~2016,the land surface parameters of Weigan-Kuqa Oasis varied correspondingly with time.In this time period,for the vegetation index(NDVI,SAVI,LAI),the order of the size of the mean value LAI>SAVI>NDVI,and the average value of each vegetation index showed an increasing trend.For water index(MNDWI,NDWI,TVI,VSWI,TVDI),the average values of MNDWI and NDWI are all less than 0,while TVI,VSWI and TVDI are all larger than 0.The order of mean values of each water body index is TVI> TVDI>VSWI>MNDWI>NDWI in sequence.For LST,the mean value increases with time.From March to November,for the vegetation index(NDVI,SAVI,LAI),the vegetation index increased with the increase of temperature,but decreased with the decrease of temperature.The characteristics of the spatial distribution are that the high value of each vegetation index appears in the inner area of the oasis with high vegetation coverage,and the low value is mainly distributed in the saline alkali land and Gobi areas with low vegetation coverage in the eastern and southern part of the study area.For the water index(MNDWI,NDWI,TVI,VSWI,TVDI),MNDWI and NDWI decreased with the increase of temperature and vegetation coverage,but increased with decreasing temperature.On the contrary,VSWI increased with the increase of temperature and vegetation coverage,but decreased with decreasing temperature.TVI and TVDI did not show obvious change rules;for or LST,it increases with the increase of temperature and vegetation coverage,but decreases with decreasing temperature and vegetation coverage.(6)During the whole study period,the surface parameters such as NDVI,SAVI,LAI,MNDWI,NDWI,TVI,VSWI,TVDI,LST and other surface parameters in the Weigan-Kuqa Oasis had higher correlation with the daily ET at the the 0.01 level.From 1997 to 2016,ET was positively correlated with different vegetation indices.There is a positive correlation between MNDWI,NDWI,TVI,TVDI and ET among the water indices,while ET is negatively correlated with VSWI.On 3-11 months,the correlation coefficient of NDVI,SAVI and LAI with daily ET increased and then decreased with time.Instead,the correlation coefficient of MNDWI,NDWI,TVI and daily ET showed a tendency to decrease and then increase with time.The results showed that with the increase of temperature and vegetation coverage,the evaporation effect of soil gradually weakened,and the plant transpiration was gradually enhanced,and the plant transpiration was dominant,but with the decrease of temperature and vegetation coverage,the transpiration of plants gradually weakened,and the soil evapotranspiration increased gradually.For surface temperature,LST has a negative correlation with daily ET,and the absolute value of its correlation coefficient is more than 0.58,indicating that the correlation between LST and ET remains high. |
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