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. Base Map Construction Data Sources Data Type TWDB Aquifer Coverages
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

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

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