Research On The Impact Of Vegetation Dynamics On Streamflow And Ecology In The Upper Heihe River Basin

Upstream inland rivers provide most of water resources for midstream and downstream in arid and semiarid regions due to their abundant precipitation and glacial snow melt.The streamflow from the upper mountainous area in the basin is usually used to maintain the economic-ecological balance of the oasis in the mid-and downstream.However,the upstream is not only the"water tower"area of the inland river basin,but also the"ecological barrier".Its ecological security is also of great significance to the ecological stability of itself and the surrounding areas.In recent years,due to the combined effects of climate change and human activities,the ecological and environmental problems in the upstream inland rivers have become very prominent.For this reason,government departments have carried out a series of ecological rehabilitation measures,which have a relatively obvious impact on the surface vegetation conditions,thereby changing the ecological system conditions,affecting the regional hydrological process,and generating a series of water-ecology-economic chain reactions in the entire region.Therefore,it is urgent to clarify the impact of vegetation dynamics on streamflow and ecological balance,and explore ecological rehabilitation programs that can take into account the functions of"water tower"and"ecological barrier"in the upstream inland river.However,vegetation dynamics effects caused by ecological rehabilitation cannot be well evaluated in previous studies due to the sparseness of observations at regional scales.With increased availability of vegetation data,it is now possible to explore the relationship between water yield and ecological balance affected by vegetation dynamics.This study took the Upper Heihe River Basin(UHRB)as the research object,combined remote sensing-based vegetation dynamics data with Eagleson's Ecohydrological Model at regional scale,and designed simulation experiments to explore the impact of vegetation dynamics on streamflow and ecological balance during the growing season from 1992 to 2015.The main results are as follows:1.The impact of vegetation dynamics on streamflow in UHRB From 1992 to 2015,there is limited evidence that ecological rehabilitation could enhance water yield in UHRB.The fluctuation range of the average annual streamflow change caused by vegetation dynamics in UHRB is only-39.43-27.52 mm.The cumulative change in streamflow caused by vegetation dynamics in the 24 years(1992-2015)was only-15.30 mm.2.Eco-hydrological effects of vegetation dynamics in UHRBRegarding the equilibrium vegetation cover(M_eq)as the optimal ecological standard,the areas where M was lower than M_eq in UHRB accounted for about 42.5%of the entire region.These areas are mostly distributed above 3600 m ASL.,and are key areas that can continue to increase vegetation cover and conserve water sources.From 1992 to 2015,the actual vegetation cover(M)change direction deviated from the M_eq area accounted for 51.31%,of which 64%of the area is vegetation dynamics resulting in reduced water yield.3.Research on Optimization of Land Use/Cover Schemes in UHRBFrom 1992 to 2015,grazing in areas where M is greater than M_eq can increase streamflow by approximately 7.3%/year,and planting trees in other areas can increase streamflow by approximately 2.9%/year.Based on the regional climate and soil conditions,rationally plan the land use,on the basis of maintaining ecological stability,cumulative increase of 3.9-9.0%of streamflow in 24 years.This study is based on the Eagleson's Ecohydrological model and ecological optimal theory to quantitatively evaluate the impact of vegetation dynamics on streamflow in UHRB.The effect of ecological rehabilitation on the regional ecological balance was evaluated by using the M_eq as the standard.According to the comparison results of the impact of vegetation dynamics on streamflow and ecology,design scenario simulation experiments to explore and optimize land use plans.Finally,it is concluded that a reasonable ecological rehabilitation policy can maximize the use of water resources on the basis of maintaining the ecological balance of UHRB by optimizing the land use plan.The results of this study can also provide scientific guidance and theoretical support for ecological rehabilitation planning and water resources management in the upstream inland rivers.