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Development and Application of Watershed Models



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Watershed Management And Modeling Development and Application of Watershed Models for Simulation and Management of Nonpoint Source Pollutants Charles W Downer Coastal and Hydraulic Laboratory Billy E Johnson Environmental Laboratory Engineer Research and Development Center Watershed Management And Modeling Non point Source Pollution According to the US EPA non point source pollution NPSP Comes from many diffuse sources Caused by rainfall or snowmelt moving over and through the ground As the runoff moves it picks up and carries away natural and humanmade pollutants finally depositing them into lakes rivers wetlands coastal waters and even our underground sources of drinking water These pollutants include Excess fertilizers herbicides and insecticides from agricultural lands and residential areas Oil grease and toxic chemicals from urban runoff and energy production Sediment from improperly managed construction sites crop and forest lands and eroding streambanks Salt from irrigation practices and acid drainage from abandoned mines Bacteria and nutrients from livestock pet wastes and faulty septic systems Watershed Management and Modeling Extent of Problem In it s 2004 Water Quality Inventory Report to Congress the USEPA reported A large portion of the nations water bodies do not meet water quality standards 44 river miles 64 ponds lakes and reservoirs 30 of estuauries Water quality impairment attributed to NPSP constitutes a large percentage of the impaired water bodies 65 of river miles 30 of ponds lakes and reservoirs 40 of estuaries Watershed Management And Modeling Sources of Non Point Source Pollution Urban development Agriculture Land disturbance Construction Military training Timber harvest Fire Natural causes Watershed Management And Modeling Programs Related to Control of NPSP TMDL Process Total Maximum Daily Loading Intended to allow states to meet water quality standards The total loading that will allow water quality standards to be met for a given body of water is determined The total loading is attributed to various sources and land owners The reduction in total loading needed to meet the water quality standards is also determined This determines the amount of reduced loading from each source and or land owner The EPA state land owners develop an implementation plan that will result in the water quality standards being met Low Impact Development LID Development is conducted in a way to minimize the effects of urbanization on hydrology and water quality Water sediments pollutants are captured near their source with various management practices rain barrels porous pavement detention basins etc Watershed Management And Modeling Considerations in Modeling NPSP Sources are unevenly distributed on land surface Hot spots Flow path is important Loss of water and constituents along flow path Infiltration Decay Transformation Exchange with land surface Increased hydraulic efficiency shortens flow paths and reduces loses Streams Canals Subsurface pipe networks To asses the effects of potential changes to the system such as future land use change or best management practices a physically based modeling approach is preferred Watershed Management And Modeling Hydrologic Models Lumped Models Input Known Precipitation Processing Output Unknown Watershed Response Outflow w Watershed Management And Modeling Hydrologic Models Distributed Models Fine scale physical processes are simulated at element level Plant interception Infiltration Runoff Evapo transpiration Elemental responses are integrated to determine system response Watershed Management And Modeling Physically Based Distributed Parameter Modeling Approach Physical Bases Allows use of the model with minimal calibration data Allows extension of the model beyond the range of calibration Distributed Approach Incorporates spatial heterogeneity of watershed features Provides fine scale information for project analysis Provides Tool for analysis of watersheds at fine time and space scales A compliment to simpler models which may be used to identify areas that need more rigorous study Watershed Management And Modeling Physically based distributed parameter Modeling Approach Spatially varied heterogeneity Explicitly resolve features in the grid Land use Soil type Depressions BMPs Roads Wetlands Track fate of water sediment contaminants along flow path Infiltration along path Settling erosion along path Reactions along path Watershed Management And Modeling What is GSSHA GSSHA is a complete watershed simulation and management model used for hydrologic hydraulic sediment and quality simulation and management GSSHA is a fully distributed physics based model that utilizes a grid to represent the watershed GSSHA is a product of the US Army ERDC Maintained Supported Distributed GSSHA is a direct descendent of the surface water hydrologic model CASC2D developed at Colorado State University The original version of GSSHA is the result of my dissertation work at University of Connecticut Downer C W Identification and Modeling of Important Stream Flow Producing Processes in Watersheds PhD Dissertation University of Connecticut 2002 Watershed Management And Modeling Gridded Surface Subsurface Hydrologic Analysis GSSHA Watershed Management And Modeling How does GSSHA Work GSSHA works on a uniform spatial grid Basic equations of mass energy and momentum conservation are solved with finite volume and finite difference techniques Point processes are solved at the grid level Point responses are integrated to get the system response Cascading planes in two dimensions CASC2D Computational Grid Watershed Management And Modeling GSSHA Includes special features to allow the model to explicitly resolve issues related to NPSP Land use change BMPs Storm and tile drains Wetlands Sediment transport Constituent transport Watershed Management And Modeling Land Use Change Changing land uses lead to changing in the physical properties in the watershed and alter the stream response Converting natural areas to agricultural and urban areas results in more compacted soils and smoother surfaces Urbanization also leads to greater impervious area All these changes result in more runoff Drainage capacity is usually increased exasperating the problems of increase runoff All effects sediment and constituent runoff Watershed Management And Modeling Spatial Hydrology Dealing with Runoff Processes Changes Spatial effects of land use changes Where you put a commercial zone


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