| The Heihe River is the second largest inland river in the arid region of northwestern China. Water resources are the primary limiting factor for the sustainable development of economy and society in the Basin.With the development of economy and increase of population,the water is needed more and more in this region.As a result,the already-existing conflict between economic use of the water in the middle reaches and ecological demand of the water in the lower reaches has been recently exacerbated.Water resources shortage had caused severe deterioration of the eco-environment in the lower reaches and threatened the ecological security of entire Heihe River Basin,even northwestern and northern China.So the ecological restoration has become urgent affair in the lower reaches.Riparian ecosystem has played an important role in stabilizing regional ecosystem.Though,it has degenerated gradually because groundwater level fell down. To restore the major riparian ecosystem in the region,stream water transport from the middle reaches of the river was implemented.The implementation of ecological water transport is bringing the changes of groundwater level,especially in the groundwater-fluctuating belt,which is the core region of the lower reaches of Heihe River in the course of vegetation rebuild and ecology restore.Groundwater is key factor to affect growth and distribution of vegetation.The relationship between the status of natural vegetation and the groundwater level becomes the attention-getting focus in the lower reaches of Heihe River,especially in the groundwater-fluctuating belt.Therefore the objective of this thesis is to study the relationship. The water requirement of vegetation is the link of the relationship.The focalization in the study is ecological water requirement.The purpose is to provide scientific support for balancing the water requirement from human being and natural ecosystem,and confirming the target,measure and project of ecosystem protected.First,the range of the groundwater-fluctuating belt was confirmed.Then simulation of the surface flux and the evapotranspiration were conducted using the technology of Remote Sensing and GIS,combining the basic data such as meteorological data,Landsat TM images,current land use maps and the data from field investigate.And then the quantity of ecological water requirement was estimated based on the simulated evapotranspiration and the distributional area of vegetation in the study area.This method to estimate ecological water requirement in the study afforded the new idea and method for the study of regional ecological water requirement. Conclusions could be drawn as follows:(1)The groundwater-fluctuating belt of the lower reaches of Heihe River is a special region where the groundwater level varies markedly under the influence of ecological water transport.The natural vegetation is the important underpinning in the riparian ecosystem. As a result,this region is the core region of the lower reaches of Heihe River during the vegetation rebuilt and the ecology restored.The groundwater level influences the type, distribution patterns and abundance of the natural vegetation species.Thus,the change of groundwater level is primary drive factor of environment evolution in this region.By analyzing the data of groundwater level came from the observation in the lower reaches of Heihe River in 1990 and 2006,the range of the groundwater-fluctuating belt was confirmed,which lies about 20km buffer zone of the Heihe River,covering area of 1.15454×10~4km~2.(2)The simulation of the land surface parameters is foundation to estimate the evapotranspiration.The simulated parameters of land surface as follows:The surface albedo is changing from 0.08 to 0.42.Its histogram presents single peak.The mean of NDVI is only -0.0686 which effected by Gobi and desert.By analyzing the correlativity of LAI from measure in field and RVI,DVI,GVI,NDVI,SAVI and MSAVI,the polynomial model of GVI was selected for the precision of simulation is 0.9419.The precision of estimation is 84.39%and 88.88%for the height vegetation and the vegetation fraction,respectively.The mean of land surface emissivity is 0.9744.The land surface temperature is the result from the balance of the quantity of heat in land surface.It is very important for ecological water requirement of vegetation.The land surface temperature is fluctuant from 291.03K to 319.39K and its mean is 311.05K.(3)The evapotranspiration will affect the balance of water and heat in the system of soil, vegetation and atmosphere.The evapotranspiration equaled to the ecological water requirement of vegetation in a way approximatively.The net radiation flux is changing from 254.08 to 860.73W/m~2 with a mean of 553.94W/m~2 in study area.The minimum net radiation flux appeared in the some area of Xi Juyan Lake,the water area has maximum net radiation flux.The soil heat flux was converged from 100 to 170W/m~2 and the mean is 142.55W/m~2.The soil heat flux is smaller in the inner of oasis and the lower reaches of Xihe River.The mean of sensible heat flux is 220W/m~2.The maximum latent heat flux present to the water area and its surrounding,for example the distributional area of riparian vegetation and oasis.The latent heat flux is minus(-95.67～0W/m~2)in Xi Juyan Lake.The mean of evapotranspiration is 1.61mm in Sept.12,2003 in study area.The validations of estimated results about the parameter of evapotranspiration indicate the simulated results are reasonable and enough for the estimation of ecological water requirement.(4)The quantity of ecological water requirement of Populus euphratica,Tamarix ramosissima and grassland is 1.32×10~7m~3,2.13×10~7m~3 and 13.5×10~7m~3 in September, 2003,respectively.The total ecological water requirement of vegetation is 1.695×10~8m~3. But the area is extended from the data of remote sensing in the pixel scale.The problem was resolved by introducing the vegetation fraction in this study.Corresponding,the quantity of water requirement of Populus euphratica,Tamarix ramosissima and grassland become 11.37×10~7m~3,0.45×10~7m~3 and 3.24×10~7m~3 by introducing vegetation fraction,respectively.The total ecological water requirement of vegetation also diminished to 1.506×10~8m~3.(5)This method estimates the quantity of ecological water requirement from the scale of surface directly which had broken the restriction from scale transform in the course of the ecological water requirement study,and afford the new idea and method for the study of regional ecological water requirement.By comparing the results from this study with other previous research about ecological water requirement in the lower reaches of Heihe River,we found there was a little difference.We will face many challenges in the further study.The high resolution remote sensing image will be needed to improve the estimation precision of vegetation area.Many ecogeographical variables will be required to monitor for validating estimation of important parameters, such as surface flux and the evapotranspiration...
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