Estimation Of Land Surface Evapotranspiration In Yinchuan Plain And Its Application In Eco-Hydrogeology

Yinchuan Plain is in the north of Ningxia Hui Autonomous Region. As a rare oasis of artificial ecosystem, its stability depends on the quantity of regional water resource. In recent years, for the reason of the Yellow River zero flow, the volume of the Yellow River water available for Ningxia has decreased year by year. Irrational utilization of water resource aggravate the deterioration of ecological environment in the Yinchuan Plain, resulting in that major environmental factors lack of coordination, the function of natural ecosystem is weakened, the capacity of the environment is small, and the ecological balance is fragile. As a major component of regional water and energy balances, land surface evapotranspiration is extremely important not only in the processes of water and energy cycle, but also it is an important link between ecological and hydrological processes. Through the research of the temporal and spatial evolution of the land surface evapotranspiration and their impact factors in the Yinchuan Plain, we could understand the process of water cycle in the oasis thoroughly, and the results will provide decision support for rational development and utilization of water resource, allocation of land and water resources as well as protection of ecological environment.We estimated the land surface evapotranspiration of the Yinchuan Plain with MODIS remotely sensed data in the clear sky days and quantitative inversion model– SEBS (Surface Energy Balance System), calculated the evapotranspiration in the date of no available satellite remote sensing images, and the error was evaluated quantitatively, the results show that: (1) The land surface evapotranspiration of the Yinchuan Plain was about 38.16×108 m3 in 2004. (2) The evapotranspiration of the Yinchuan Plain measure up to maximum from early July to mid-August with about 3 mm/d. From mid-November to December and January, the evapotranspiration measure up to minimum, less than 0.5 mm/d.By means of spatial overlay and GIS multi-factor analysis of land surface evapotranspiration, land cover types, groundwater depth and NDVI, we arrive at a reasonable characteristic of the spatial distribution of evapotranspiration in the Yinchuan plain and its relationship with groundwater depth and vegetation cover, as well as the limit depth of phreatic water evaporation on different vegetation cover conditions, the results are as follows: (1) The land surface evapotranspiration has a significant difference between different land cover types in the Yinchuan Plain, and its descending order are: water, croplands, closed shrublands, grasslands, urban and built-up, open shrublands, barren or sparsely vegetated. (2) In the dry season, the spatial distribution of evapotranspiration in the Yinchuan Plain is mainly affected by groundwater depth. In areas of groundwater depth being less than 1.6 m, the evapotranspiration is very small as a result of the soil salinization. The evapotranspiration measures up to maximum when groundwater depth is 1.6 - 2.2 m, and then decreases with the increase of groundwater depth until being 4 m, so the limit depth of phreatic water evaporation of bare soil is about 4 m. (3) In the wet season, the spatial distribution of evapotranspiration in the Yinchuan Plain is mainly affected by vegetation cover and groundwater depth. The evapotranspiration decreases with the increase of groundwater depth until being 6 m, so the limit depth of phreatic water evaporation in the Yinchuan Plain covered by vegetation is about 6 m. (4) The land surface vegetation coverage decreases with the increase of groundwater depth, and the amount of vegetation transpiration is gradually reduced. But the evaporation of bare soil increases as a result of the coverage of bare soil increases with groundwater depth gradually. (5) In the contour map of evapotranspiration changes along with groundwater depth and NDVI, the contour of evapotranspiration decline to the deeper groundwater depth with the increase of NDVI. For the bare soil with NDVI less than 0.2, the limit depth of phreatic water evaporation is about 3 m. The limit depth of phreatic water evaporation increases with the increase of NDVI, and the largest of about 6 m can be achieved.We explored the application of the land surface evapotranspiration estimated by SEBS in eco-hydrogeology: (1) The ecological water requirement is equivalent to land surface evapotranspiration in the Yinchuan Plain, which is about 38.16×108 m3. (2) We established a quantitative inversion model for groundwater depth and estimated the groundwater depth in the Yinchuan Plain, and the error mainly distribute where the groundwater depth is deeper than the limit depth of phreatic water evaporation, such as Helan Mountain alluvial piedmont plain and the groundwater depression cone in the Yinchuan city. (3) The phreatic water evaporation of the Yinchuan Plain was about 14.2×108 m3 in 2004 which was estimated based on the limit depth of phreatic water evaporation as well as the phreatic water evaporation coefficient.