Distributed Hydrologic Modeling In Plain River Network Region

Plain river network region(PRNR)is one of the most common landscapes with complex hydrographic and hydrologic characteristics in the middle and lower floodplain of the Yangtze River and the Pearl River Delta of China.These regions have been suffering with many water and environmental issues because of the highly developed agriculture and industry,the intensive man-land interactions and the particular hydrologic characteristics.Distributed hydrologic modeling is one of the most effective techniques for the quantitative evaluation of water resource and water pollution.However,the conventional distributed hydrologic modeling in PRNR faces many limitations because of the large flat area with a complex network of rivers,lakes,reservoirs,and polders in this region.Therefore,developing distributed hydrologic modeling techniques that can be applied to PRNR is significant for solving the water and environmental issues.The western Taihu watershed was selected as the typical PRNR and studies were conducted focus on the limitations of distributed hydrologic modeling in PRNRincluding:1)development of a new watershed delineation approach,2)a hydrologic model for modeling the drainage and irrigation processes within polders,and 3)routine techniques for crossed and looped channel networks in PRNR.The main results and conclusions of these studies are described as follows:(1)An innovative approach based on the actual hydrographic features and digitalelevation model(DEM)is proposed to delineate the watershed in PRNR.It extracts the river nodes and determines the flow directions of the river network based on a vector-based hydrographic feature data model.The river network,lakes,reservoirs,and polders are then used to modify the flow directions of grid cells determined by D8 approach.The watershed is eventually delineated into four types of catchments including lakes,reservoirs,polders,and overland catchments based on the flow direction matrix and the location of river nodes.Multiple flow directions of grid cells are represented using a multi-direction encoding method and multiple outflows of catchments are also reflected in the topology of catchments.The proposed approach allows the representation of crossed and looped channel networks,lakes and reservoirs with multiple inlets and outlets,and polders with multiple pumping stations.The catchments generated by the proposed approach are divided into four types:lake,reservoir,polder,and overland catchment.Moreover,the topology of the catchment is also built based on the connections between hydrographic features.The results of the application in the western Taihu watershed show that the proposed approach is able to keep the complete drainage networks and their topologies and generate reasonable catchments because it does not need to extract the river network from DEM.Instead,it uses the actual river network,as well as the lakes,reservoirs,and polders,to build the final drainage networks.In comparison with the conventional watershed delineation approaches,such as the standard D8 approach and'Stream Burning' approach,the proposed approach shows its advantages for solving the problems in the conventional approaches such as pseudo parallel reaches generation and unreasonable catchments determination.Moreover,the using of actual river networks in the proposed approach avoids the uncertainty of determination of river source.(2)A new module for simulating drainage and irrigation operations of polder areas is developed and integrated into the commonly used Soil and Water Assessment Tool(SWAT)model,referred to as SWATpld model.SWATpld can simulate the drainage processes and irrigation intake processes by calculating the excess water storage into inner rivers and irrigation scheduling of paddy rice in the polder,respectively.The results of the application in Liyang watershed demonstrate that SWATpld model represents an advancement over the original SWAT model in several aspects:1)it is capable to model the isolated effect of the enclosed polder dikes that prevent the natural water interactions between inside and outside the polder;2)it provides an automatic method to predict the drainage operations of the polder which are manually controlled by the polder managers in the real world;3)it replaces existing irrigation application calculation by simulating the water intake from the outer river of the polder to meet the water layer depth requirement in paddies.Thus,the development of SWATpld model significantly improves the applicability of distributed hydrologic modeling in PRNR.(3)A coupled simulation experiment by using SWATpld model and HEC-RAS model is conducted to address the hydrologic processes of sub-watersheds and polders and hydrodynamic processes of crossed and looped channels in Liyang watershed.This coupled simulation technique,which is based on the input/output files linking,calculates the outflows of tributaries and polders.Then these outflows are used as the boundary condition of the HEC-RAS modeling for main channels with crossed and looped structures.The results of the application in Liyang watershed indicate that the coupled technique enhances the accuracy of the simulation for stream flows which is significantly impact by the crossed and looped structures.The coupled simulation predicts the peak flows and captures both the magnitude and timing of stream flows quite well.Moreover,the simulated water distribution among channels is more reasonable after coupled the HEC-RAS model with SWATpld model.This application,which uses both the aforementioned proposed watershed delineation approach and polder module,addresses the distributed hydrologic modeling problems in PRNR and gives an approach to determinate the boundary conditions for river hydrodynamic modeling in PRNR.