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Using GIS to Evaluate Water-Level Changes in Gillespie, Co. Texas:

Using GIS to Evaluate Water-Level Changes in Gillespie, Co. Texas: A comparison of two interpolation methods. Adrien Lindley GIS in Water Resources Fall, 2004. Outline. Introduction Base map Construction Interpolating Piezometric Surfaces Raster Calculations Results and Conclusions.

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Using GIS to Evaluate Water-Level Changes in Gillespie, Co. Texas:

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  1. Using GIS to Evaluate Water-Level Changes in Gillespie, Co. Texas: A comparison of two interpolation methods Adrien Lindley GIS in Water Resources Fall, 2004

  2. Outline • Introduction • Base map Construction • Interpolating Piezometric Surfaces • Raster Calculations • Results and Conclusions

  3. Introduction • Goals: • Create a base map with water level elevations for the Hensel sands in Gillespie county. • Create grids depicting the water level elevation for 3 consecutive years using Inverse Distance Weighting (IDW) and Spline. • Calculate the change in water table elevations from year to year. • Evaluate the results.

  4. Introduction • Where is Gillespie County?

  5. Gillespie county, a closer view

  6. Data Sources TWDB HUWCD TNRIS Data Type Aquifer Coverages Groundwater Database (dBase format) Well locations Depth to water DEMs and Digital Orthophoto Quarter Quads TxDOT Gillespie county road coverage Base Map Construction

  7. Database Management • Well databases included: well location, depth of water from land surface elevation, land surface elevation (LSE), date of water level measurement, aquifer code and state well number. • Sorted well data by date of water level measure in Excel. • Selected out all the July water level measurements for each year. • Resorted each database table for the Hensel aquifer. Using the aquifer codes. • Created new database tables for for the years 2001, 2002 and 2003.

  8. Initial base map with all wells

  9. Base map with sorted wells

  10. Interpolating Water Level Data • Spatial analyst • Set extent of analysis to be the same as the data extent • Interpolate to raster water level elevations using IDW and Spline for the 3 years of data • Make permanent the resulting grids

  11. Spatial extent for analysis

  12. Water level elevations for July, 2001 using IDW • Elevation in feet

  13. Water level elevations for July, 2002 using IDW • Elevation in feet

  14. Water level elevations for July, 2003 using IDW • Elevation in feet

  15. Water level elevations for July, 2001 using Spline • Elevation in feet

  16. Water level elevations for July, 2002 using Spline • Elevation in feet

  17. Water level elevations for July, 2003 using Spline • Elevation in feet

  18. Using Raster Calculator • Water level elevation grids are subtracted to yield yearly water level change - =

  19. ResultsCalculations for Yearly Difference

  20. Water level change from 2001 to 2002 from IDW grids • Brown indicates an area where the water table has lowered Elevations in feet Mean water level change = - 1.07 feet

  21. Water level change from 2002 to 2003 from IDW grids • Brown indicates an area where the water table has lowered Elevations in feet Mean water level change = + 3.19 feet

  22. Water level change from 2001 to 2003 from IDW grids • Brown indicates an area where the water table has lowered Elevations in feet Mean water level change = + 2.12 feet

  23. Water level change from 2001 to 2002 from Spline grids • Brown indicates an area where the water table has lowered Elevations in feet Mean water level change = - 2.01 feet

  24. Water level change from 2002 to 2003 from Spline grids • Brown indicates an area where the water table has lowered Elevations in feet Mean water level change = - 4.09 feet

  25. Water level change from 2001 to 2003 from Spline grids • Brown indicates an area where the water table has lowered Elevations in feet Mean water level change = - 6.10 feet

  26. Comparison of raster calculation results • Similar values for 2001 to 2002 • Difference in water table elevations of 7.28 feet for 2002 – 2003. • Which method is more accurate?

  27. IDW Chunky display, obviously not representative of natural conditions Spline Smoother display, probably more representative of natural conditions IDW or Spline?

  28. Conclusions • Limited well control across the area for a specific time and aquifer yields questionable results

  29. Acknowledgments • Special thanks to Paul Tybor and Margaret Ratliff of the Hill Country Underground Water Conservation District

  30. Thanks

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