Watershed Hydrological Cycle Under A Changing Environment

Under the changing environment,the hydrological cycle system of watersheds is changing rapidly,which has brought severe challenges to the regional water resources security.In the past,the research on the watershed hydrological cycle was usually static and scattered,mainly focusing on the process of the surface-water cycle and a complete research theoretical system was not formed.Therefore,based on constructing the research framework of the watershed hydrological cycle under the changing environment,this paper improves and proposes new technical methods,taking Taizi River Basin and Liu River Basin as examples,comprehensively considering the response laws of surface-water and groundwater to changing environments at different levels and temporal and spatial scales,quantitatively revealing the water cycle evolution mechanism of typical watersheds under different changing environments,and water cycle evolution model in typical watersheds are built.The research has important theoretical and practical significance.and the main results are as follows:(1)A research framework for the evolution of the hydrological cycle on a basinscale under changing environments was constructed.Starting from the basic concepts of changing environment and hydrological cycle,based on expounding the basic characteristics,summarizing the theoretical basis,revealing the influence mechanism,and proposing the basic model of hydrological cycle evolution,from the perspectives of history,present,and future,this paper discusses the key issues,general ideas and main contents of the hydrological cycle in the watershed scale under the changing environment,comprehensively considers the process of the surface water cycle and the groundwater cycle,and systematically builds a theoretical research framework of the evolution of the watershed hydrological cycle under the changing environment.This research framework greatly helps to update and develop relevant research.(2)Relevant research methods of hydrological cycle evolution on a watershed scale under changing environments are expanded.For specific problems,the related technical methods have been further improved:(1)Two improvements are made to Budyko's coupled equation(Fu equation)by using precipitation regulation function and time-varying parameter w,which expands the application range of the equation.(2)A Cumulative Exchange Fluxes Method is proposed to quantitatively reveal the transformation relationship and dynamic change process of exchange capacity between river and groundwater.(3)A comprehensive assessment method of the hydrological regime using flow duration curve,ecological indicators and indicators of hydrologic alteration is put forward.(4)The multi-scenario ensemble streamflow forecast method and method based on the improved Budyko-Fu equation are proposed.The improvement of technical methods improves the accuracy of evaluation results and provides technical support for studying the hydrological cycle on a watershed scale under changing environment.(3)Hydrological cycle evolution in semi-humid Taizi River Basin under a changing environment was studiedUnder natural conditions,the hydrological cycle in the Taizi River Basin is dominated by precipitation-surface/underground runoff-channel,with 34.67% of precipitation converted to runoff and 65.24% dissipated in the form of evapotranspiration.Under the changing environment,the hydrological cycle elements,paths,and structures have undergone significant changes,which are embodied in the following: The annual precipitation shows a trend of insignificant decline,and the runoff generally shows a decreasing trend;the groundwater level generally declines,and the concentrated mining area forms cones of depression.29.98% of the precipitation is converted into the runoff,and the conversion capacity is reduced by13.75% compared to natural conditions.The contribution rate of climate change to the runoff reduction is 40%-60%,and the contribution rate of human activities to the runoff reduction of some tributaries(Xihe,Lanhe and Tanghe)is 71-84%.By adding new social water intake-water-drainage routes,the watershed has formed a farmland and urban water circulation system.Reservoirs play a key role in changes in the hydrological cycle structure: reservoirs increase runoff significantly in May and June,greatly reduce the peak runoff during flood season,and increase the average flow after flood season.Changes in runoff components lead to ecological surpluses/deficits and the annual setting off days increased nearly 6 times.In addition,the staged water release of the reservoir makes surface water-groundwater interaction in the downstream change in stages,showing three basic patterns.The forecast results of future runoff show that from 2021 to 2070,the runoff of the basin will increase by 5.0-39.0%,and the runoff will increase by 2.5-3.5% and 0.5-1% under the land use planning scenario and natural development scenario,respectively.(4)Hydrological cycle evolution in semi-arid Liu River Basin under a changing environment was studiedUnder natural conditions,the hydrological cycle pattern in the Liu River Basin is dominated by precipitation-evapotranspiration-runoff that 87.05% of precipitation is dissipated in the form of evapotranspiration,and only 12.95% is converted into the runoff.Under the changing environment,the dissipative capacity of water resources increased,5.73% of the precipitation was converted into the runoff,and the conversion capacity decreased by 52.05% compared to natural conditions.Since 1970,the runoff of Liu River Basin has decreased by 56.3%,and the contribution rate of human activities is as high as 86.3%.The upstream soil and water conservation measures are the main reasons for the significant reduction in the runoff capacity of the basin.The response of runoff changes to precipitation changes decreased by 57.25%,and a water cycle model under soil and water conservation was formed.The impact of climate change is mainly reflected in the reduction of flood volume caused by the reduction of precipitation during the flood season.The change of the downstream water cycle is centered on the exploitation of groundwater,which makes groundwater level generally drops by 1-2m,and surface water-groundwater interaction changes significantly and regularly,and the river gradually changed from gaining water to losing water.River seepage and reservoir regulation/storage make the downstream flow cut-off more serious and the monthly ecological flow considering the loss of the river channel is determined,which is 21.36% higher than the theoretical value.Using the calibrated SWAT hydrological model to predict the future runoff changes in the Liu River Basin under soil and water conservation measures,the results show that the future runoff will increase by 7.58-18.19%.On the whole,the hydrological circulation system in the Liu River Basin is highly sensitive to changing environments.In general,new hydrological cycle patterns have been formed in the two typical watersheds under the changing environment.For the Taizi River Basin,the impact of changing environment on the hydrological cycle is direct and macroscopic,mainly reflected in the changes in the path and structure.The impact on the Liu River Basin is indirect and permanent,mainly reflected in the change of the runoff properties of the underlying surface.The study of typical basins not only has important practical significance for the planning,security,and management of water resources in the Taizihe Basin and Liuhe Basin,it also provides a basis,reference and ideas for the research on the evolution of the hydrological cycle in other semi-humid or semi-arid watersheds.