Watershed Parameters for Water Rights in the Trinity River Basin
Prepared by Melissa Figurski
GIS in Water Resources - CE 394K
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Table of Contents Background of Water Availability
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Sunset Over Trinity River Wetlands in East Texas |
Background of Water Availability
Water is a limited resource in Texas. During the summer of 2000, Austin residents encountered mandatory water restrictions because the amount of precipitation through the summer months was much less than the water demand of users. The imbalance between output (water demand) and input (precipitation) to the Austin reservoirs caused reservoir levels to drop and initiated concern for water availability. To alleviate the stress on the water supply, the City of Austin asked for voluntary declines in water use by residents and companies and then required water restrictions as the reservoir levels continued to fall. The restrictions were upheld until precipitation increased and the reservoir levels rose. This situation describes why water availability is an important concern in Texas.
Texas water rights are permits granted by the Texas Natural Resources Conservation Commission (TNRCC) for diversion of water from a channel in Texas. In 1997, the Texas state legislature mandated the TNRCC to develop better water availability models than those available at that time. The intent of a new model is to assist the TNRCC in water resources planning and management decisions. A water availability model uses data for existing water rights to project whether sufficient water will be available on a river segment to grant a new permit to divert water. The TNRCC chose the Water Rights Analysis Package (WRAP) model written by Ralph Wurbs at Texas A&M University as the new water availability model. The inputs to this model for each water right are upstream area, average upstream precipitation and average upstream curve number.
A former graduate student of the Center for Research in Water Resources (CRWR) at the University of Texas at Austin, Brad Hudgens, developed a set of scripts to establish watershed parameters for water rights. The scripts are incorporated in an ArcView 3.2 project interface named WRAP1117.apr which can be downloaded at www.ce.utexas.edu/prof/maidment/grad/hudgens/research.html. Six of the twenty-two basins to be modeled in Texas have been successfully completed using WRAP1117.
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Although 22 of the 23 basins in Texas will be modeled with the help of the CRWR, this term project focuses on the Trinity River Basin. The size of the Trinity River Basin presents challenges to the previously developed method for determining watershed parameters and requires development of a new method for determining the parameters. |
Running one of the scripts in WRAP1117 on the Trinity River took eleven days. If the process needed to be run only once, this might not be an unreasonable amount of time. However, the intention of the model is that the TNRCC will use it to consider the impact of new water right permits on water availability. To do this, the TNRCC will need to determine the watershed parameters of the new permit. If this includes the addition of a stream, the basin would need to be reprocessed. It is not sensible for the TNRCC to wait 11 days to assess the impact of a new water right permit.
This term project is a portion of research for my Master's thesis and is a collaboration of three groups; the TNRCC, the Center for Research in Water Resources (CRWR) at the University of Texas at Austin, and an independent contractor chosen by the TNRCC. The TNRCC provides the location of all water rights in Texas, the CRWR determines the watershed parameters for those water rights, and the contractor uses the parameters in the WRAP model to provide water availability data under various scenarios.
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The goal of this term project is to define watershed parameters for water rights in the Trinity River Basin to be used as input to the WRAP water availability model. A new method will be created because the previous method is not efficient for use on large basins. The new method should have the ability to be updated by the TNRCC in a timely manner.
As mentioned above, the Trinity River Basin is too large to use the conventional method of defining watershed parameters for water rights. The digital elevation model covering the Trinity basin in 30-meter by 30-meter cells contains 237 million cells. It is apparent that the size of the grids are responsible for the long processing time. Segmenting the Trinity into smaller pieces will alleviate this problem. Segmenting the basin does not change the total processing time for the entire basin, but in the future, the TNRCC will only need to reprocess the section containing the water rights they are assessing.
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For large areas, there is a need for a systematic method to distribute data. Many agencies disseminate data in 'packages' that align with hydrologic cataloguing unit (HUC) boundaries. Therefore, the most logical choice for partitioning the Trinity basin appears to be by HUC. Once the basin is separated into the twelve HUCs contained in the Trinity basin, the units can be processed individually with WRAP1117. A primary concern with this method is that processing each HUC individually will not account for contributions from upstream HUCs. To account for contributions from upstream HUCs, the network capabilities inherent in ArcInfo 8 will be used . Thus, the attributes from upstream will be cascaded through the downstream HUCs. |
Separating the data into HUCs requires the delineation of a watershed for each HUC in the Trinity Basin. The grids cannot simply be cut to the boundaries drawn in the above figure because the drawn boundaries may not accurately align with the elevations of our digital elevation model (DEM). The command for delineating watersheds in ArcInfo workstation requires a flow direction grid and a grid of the outlet cells. Creation of the flow direction grid requires the burn and fill grids. The outlet cells are created by locating points at the intersection of the HUC boundary and the stream network through that boundary.
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To begin processing the grids, the completed stream network for the Trinity River is burned into the DEM of the entire basin. The burned DEM for each HUC is then clipped from the basin grid. To ensure correct delineation of the watershed for each HUC, the burned DEM is clipped to a buffered HUC boundary as shown to the left. The buffer for this project is 5000 meters from the HUC boundary. To create the fill and flow direction grids for each HUC, the grids are processed in ArcInfo workstation using the fill and flow direction commands. |
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To delineate the watershed for each HUC, the shapefile of outlet cells is converted to a grid. The grid of outlets and the flow direction grid for each HUC is used in ArcInfo workstation with the 'watershed' command to delineate the watersheds. The watersheds in the previous picture are the final product of the watershed delineation. All of the watersheds did not delineate correctly on the first try.
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In the first picture to the right, the watershed between HUC 12020302 and 12030203 did not delineate correctly. The white area was not delineated as belonging to either basin. The second picture includes the flow accumulation grid under the undelineated section. The purple stream darkens as the flow accumulation increases. The flow should be running towards 12030203, but the flow accumulation grid shows it running the opposite direction. The problem is a result of the buffered area. A point within the interior of 12030203 has an elevation lower than the outermost point on the northern edge of the buffer. This caused the flow accumulation to accumulate towards that lower elevation. Using a larger buffer remedies the problem as it ensures that there is an upstream elevation greater than all elevations in the downstream HUC. |
Missing piece of watershed
Flow accumulation |
Once the watersheds are delineated, the HUCs are ready to be processed with the scripts in WRAP1117. The following picture is the WRAP1117 interface in ArcView 3.2. The pull-down menu shows the functions of the scripts embedded in the project.
The processes discussed below do not describe all of the functions in WRAP1117 but represent the major processes in attaining watershed parameters.
The goal of WRAP1117 is to arrange data from which watershed parameters can be read. The parameters, area, precipitation, and curve number, are based on the flow accumulation function. The largest flow accumulations occur in the river channel as they capture everything from upstream , so it is important that the water rights points are located on top of the grid cells representing the river channel. If the points do not align with the channel, the values read for the parameters will not include upstream contributions.
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A flow direction stream network ensures the water rights points are located on the channel grid cells. Derived from the flow direction grid, the flow direction network traces each vector stream segment through the grid cells from its headwater to the outlet. The dark blue stream-line is the vector stream which meanders to downstream. The light blue line represents the flow direction network. It only has jagged turns as it flows from one grid cell to the next. |
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When adding water rights points to a view, they may look to be located directly on top of the stream network. When zoomed in, it is apparent that the points are not truly on the network.
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In the picture to the left, the water rights are not yet situated on the flow direction network. A script in WRAP1117 snaps the points to the network as shown on the right. The purple points are the unsnapped points, and the red points are the new snapped locations. The points are now located on top of the grid cells representing the river channel and the watershed parameters will be reported correctly. |
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The average precipitation and average curve number grids are calculated using flow accumulation as a weight. Remembering that the flow accumulation in the channel includes everything upstream, weighting the grids with flow accumulation will include the influence of the upstream precipitation and curve numbers.
In the precipitation grid, the stream can be seen running through the darker portion of the grid. The stream is lighter than the surrounding area because it includes the effects of the lighter area of precipitation upstream as well as the darker area immediately surrounding it.
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Each water right is located on cells representing the river channel and the average curve number and average precipitation grids are prepared. The data is now set up for evaluation of the water rights parameters. A script in WRAP1117 reads the flow accumulation grid under each water right to obtain the number of cells contributing to the water right. Multiply this number of cells by the are of each cell results in the upstream area for that water right. The script also reads the average curve number and average precipitation grids under each water right for those values. The following table is a representation of the data presented
The last task in WRAP 1117 is to delineate incremental watersheds for each water right. For quality control, any watershed that is less than 10,000 grid cells, or approximately 3.5 square miles, is visually checked to be sure that it is drawn correctly.
Finally, the shapefile of the water rights parameters for each HUC are merged into a single shapefile. The parameters are now ready to be loaded into ArcInfo 8 to cascade data to downstream HUCs.
The only files needed in ArcInfo 8 to complete the project are the water rights parameters, the Trinity stream network, and the outlets to each HUC. These three files are imported into ArcInfo 8 as feature classes and are loaded into a geometric network.
Although the points have already been snapped in ArcView 3.2, it is necessary to snap them again in ArcInfo 8 to ensure that they will be selected when running a network function. Snapping the points requires creating a duplicate of the feature dataset to be snapped and erasing the features in the original dataset. The features are then reloaded into the original dataset from the duplicate with the 'Load Objects' function in ArcInfo 8. Reloading the points into the network cuts the stream network edges at the points where the water rights are located.
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The geometric network can now be processed. A downstream trace is run from each HUC outlet to the outlet of the entire basin. The points selected along this path are those affected by the upstream HUC. In some cases, the points did not snap to the network and had to be manually selected. The selected water rights are edited in the attribute table to include the data from upstream HUCs.
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The table is now ready to be exported into an Excel spreadsheet and sent to the contractor as input to the WRAP model for water availability.
The TNRCC grants water rights permits for diversion of water from channels in Texas. Demands on the water supply created needs for water availability models for use in water resources planning and management. The selected water availability model requires watershed parameters for each water right as input. A method for obtaining these parameters, WRAP1117, was created by Brad Hudgens at the CRWR, but processing the data was quite time intensive for large basins. A new method was needed to expedite the processing of large basins.
The new method required separation of the Trinity network into HUCs. The segmented HUCs were then run through the WRAP1117 process to get local watershed parameters. To include the influence from upstream HUCs, the watershed parameters and stream network were brought into ArcInfo 8. The network capabilities in ArcInfo 8 were used to cascade parameters from upstream HUCs through all HUCs downstream .
Hudgens, Bradley and David R. Maidment. Geospatial Data in Water Availability Modeling. May 1999.
Special thanks to Dr. Francisco Olivera and Dr. David Maidment for their suggestions with this project.
Last update 12/10/00
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