Water Availability Modeling in the State of Texas CE 394 K.2 - Surface Water Hydrology University of Texas at Austin David Mason

Outline Project Objectives Source Data Collection Analysis of Data Presentation of Results Possible Solutions Conclusions Future Work

Project Objectives Use GIS utilities to create a spatial water rights database Compile watershed data for each water right and stream gage location in selected basins –Drainage area, avg. precip, avg. CN, flowlength to outlet Compare GIS results with established USGS values Study the effects of using higher resolution DEMs

Selected Basins Nueces San Antonio Guadalupe

Source Data Collection Digital Elevation Models (USGS) RF3 Coverages (EPA) Water Right and Diversion Locations (TNRCC) Primary Control Points (Relevant Contractor) Precipitation Grid (Oregon State) Curve Number Grid (CRWR) Digital Raster Graphics (TNRIS)

Process the DEMs

Download and Edit RF3

Water Rights and Diversions -The San Antonio Basin had 488 water right locations when it was sent to TNRCC -When I got it back….488 turned into 788!!

Primary Control Points Primary control points are mainly stream gage locations used to distribute naturalized flows throughout the basin Control points and locations were provided by HDR Arc/Info was used to create an Arcview theme of these control points

Precipitation and CN Grid The precipitation grid received from Oregon State was resampled to the same size as the DEM Once resampled, a calculation was preformed in Arc/Info to produce a grid of the average precipitation across the basin The same basic procedure was used to produce the CN grid

Flow Length Grid The flow length grid is calculated using the Hydrologic extension in Arcview The extension calculates the distance to an assigned outlet for each cell in the grid

Calculating the Parameters Once all the grids have been produced, the next step is to place the points on the flow accumulation grid in order to assure the accurate calculation of the parameters After the points are in place, a script is run that selects the value of each grid cell “under” a point and attributes those values in a table

End Result of a Very Long Process The result is a coverage with avg. CN, avg. precip., flowlength and # of cells in the flow accumulation grid The # of cells are then converted to an equivalent drainage area

Results - Nueces Average Error in Area = 1.36%

Results - San Antonio Average Error in Area = 3.44%

Results - Guadalupe Average Error in Area = 3.57%

Possible Problems DEM Resolution –Currently using 90m, just received 30m Degree of Terrain Relief –Guadalupe is somewhat of a coastal basin God Forbid! USGS May Be Wrong!! –Numbers based on old maps –No one’s perfect!

Conclusions GIS can be used effectively to compute watershed parameters for a large number of control points Although some of the results are somewhat different than USGS values, the new 30m data sets should help to reduce this error

Future Work Find a way to use the new 30m DEM data sets in an efficient manner –the current file sizes are far too large Apply some quality control techniques to assure more accurate results –hand-delineate some of the smaller watersheds to make sure the larger ones aren’t masking discrepancies