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

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.

Introduction Where is Gillespie County?

Gillespie county, a closer view

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

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.

Initial base map with all wells

Base map with sorted wells

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

Spatial extent for analysis

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

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

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

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

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

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

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

Results Calculations for Yearly Difference

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

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

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

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

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

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

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?

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

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

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

Thanks