Distribution Of Evapotranspiration And Shallow Groundwater In Plain Area Over Qaidam Basin

In an arid inland basin, evapotranspiration(ET) is the major loss related to the water balance and precisely estimation of ET is essential for evaluation of available water resources in these areas. The study of distribution of actual evapotranspiration and the shallow groundwater in a regional scale have great significance on environmental protection and utilization of water resources. Based on SEBS algorithm(Surface Energy Balance System), the ET was estimated using DEM data, MODIS data and GLDAS data over the plain in Qaidam basin from 2001 to 2011 at a spatial resolution of 0.005°(about 500 m). Furthermore, the long-term variation and spatial distribution of ET and its relationship with pan observed evaporation were studied. The main control factors of evapotranspiration in arid area were studied by discussion quantitative relationships between evapotranspiration and meteorological factors, vegetation, land cover types and depth of groundwater. The distribution of shallow groundwater was studied using geographic data, hydrological data, evapotranspiration, MODIS and Landsat TM data. The results indicated that:(1) The maximum of the mean daily ET of plain in Qaidam basin varied between 2.62 mm and 3.20 mm during the year 2001 to 2011.The annual actual ET increased with time and the value varied from 29.23 mm in 2001 to 101.12 mm in 2011, with an average annual growth rate of 13.21%. On the whole, the value of evapotranspiration of eastern area was obviously higher than that of western region in the study area.(2) The evaporation coefficient is 0.11 in the eastern region of study area, while that of the western region is 0.04.(3) Evapotranspiration was positively correlated with air temperature, precipitation and relative humidity, while it has no obvious correlation with wind speed in plain area. It was showed that air temperature and precipitation were the main meteorological control factors of evapotranspiration.(4) The analysis of NDVI and its corresponding ET presented that the NDVI threshold between bare soil and vegetation is 0.055 in the Qaidam basin. The evapotranspiration of study area increased with the increasing vegetation area, improving growth conditions and trends of vegetation. The mean daily ET of different classes of fraction of vegetation cover(FVC) increased with time and the value of mean daily ET was relatively larger in higher FVC area. The vegetation of low and medium FVC area provided great contribution to the regional evapotranspiration in the vegetated area.(5) The water bodies had highest mean daily ET(2.82 mm/d) of the study area from June to September in the year 2011. The mean daily ET of bare soil, grassland, natural vegetation, farmland and forest in the study area were 0.50 mm/d, 0.91 mm/d, 0.57 mm/d and 0.80 mm/d, respectively.(6) The threshold depth of groundwater for phreatic evaporation in Wulan basin was about 4m, while that of Wutumeiren area is approximately 3m. The corresponding mean daily ET was about 1.4 mm and 0.5 mm in June 2011, respectively.(7) The distribution of containing water probability of Wulan basin derived from the containing water probabilistic model of shallow groundwater was consistent with actual situation, indicating that the model could be applied in the study area.(8) The shallow groundwater mainly distributed in the area with higher fraction of vegetation cover and mean daily ET, small slope and relief of topography in the study area.