PrePro2004: Comparison with Standard Hydrologic Modeling Procedures Rebecca Riggs April 29, 2005

Hydrologic Modeling  The use of physical or mathematical techniques to simulate the hydrologic cycle and its effects on a watershed  Most commonly used to simulate stormwater runoff in a watershed

HEC-HMS  US Army Corps of Engineers Hydrologic Modeling System  Used to electronically simulate precipitation-runoff processes of dendritic watershed systems  Outputs include peak flows and runoff hydrographs  Outputs used directly or in conjunction with other programs

3 Components of HMS 1.Basin Model Watershed Parameters 2.Meteorologic Model Precipitation and Evapo-transpiration Parameters 3.Control Specifications Start and End Date/Time of Simulation

Basin Model  Watershed parameters: sub- basins, reachs, junctions, sources, sinks, and reservoirs  Commonly data is derived from contour maps (USGS or survey data), land use maps, and soil maps  Tedious and time consuming

Sub-basins

Reach Parameters

PrePro2004  A GIS pre-processor which extracts hydrologic information from spatial data for HMS modeling  Add-in tool in Arc-GIS interface  Much faster than conventional methods for calculating basin model parameters

Study Area  College Station, Texas  Castlegate Subdivision  Spring Creek, before confluence with Lick Creek

Study Area

PrePro2004 Basic Steps 1.Gather data 2.Fill sinks, create FDIR grid, create FACC grid 3.Construct stream network 4.Add inlet, outlet, and/or reservoir 5.Delineate watershed 6.Extract HMS elements 7.Calculate parameters 8.Calculate curve numbers 9.Export data to IDM 10.Export IDM to HMS 11.Setup HMS project

1. Gather Data  DEM, stream vector data, mask grid, soils vector or grid data, land use vector or grid data  DEM source: USGS seamless  Soils and land use (zoning) from COCS  Stream vector data from US EPA Lower Brazos arc data

2. Fill sinks, create FDIR grid, create FACC grid  Fill: Cell elevation raised to lowest surrounding cell elevation  FDIR: Flow direction grid  FACC: Flow accumulation grid

3. Construct Stream Network Chose cell threshold = stream drainage area

4. Add Inlet/Outlet/Reservoir

5. Delineate Watershed

6. Extract HMS Elements  Under Vectorization tab in Watershed Delineation tool  Creates shapefiles to be directly imported to a target geodatabase  Elements include watershed, reach, junction, source, and reservoirs  Following element extraction can merge basins, but this was not selected in this project

7. Calculate Parameters  Generates longest flow path for each sub-basin, HMSCode, and extracts slope and elevation data from DEM  Data stored in ‘watershed’ and ‘reach’ layers

8. Calculate Curve No. Need soils data, land use data, watershed data (previously created), and CN lookup table

Curve No. Continued Vector data was used, but raster data could be used if available Soils data was given by individual HSG, not a % of each Zoning was used for land use, then equated to the NLCD land use codes manually After calculating CN and impervious cover are stored in the ‘watershed’ layer

9. Export Data to IDM Prior to this step gage weights can be calculated, a frequency storm is used in this project so gage weighting was skipped Exports data from previous steps to project and basin geodatabases (preferably empty) Following this step time of concentration for each subbasin is calculated (Calculate Parameters)

10. IDM to HMS Creates input files for HMS from data stored in basin geodatabase –Basin file –Meteorologic file (if gage data is used) Must propogate fields with chosen methods for loss rate, transform, and routing

11. Import to HMS Import basin and meteorologic files into HMS Import background map and grid data

Results

Basin Comparison

Comments The sub-basins would have been more similar had I added more user defined outlets Time savings is huge Good for large scale projects, but difficult to accurately define watersheds with tool on small scale (development scale)

Questions?