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

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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
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Outline

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

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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.

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Introduction

• Where is Gillespie County?

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Gillespie county, a closer view
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Base Map Construction

• 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

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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.

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Initial base map with all wells
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Base map with sorted wells
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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

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Spatial extent for analysis
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Water level elevations for July, 2001 using IDW

• Elevation in feet

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Water level elevations for July, 2002 using IDW

• Elevation in feet

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Water level elevations for July, 2003 using IDW

• Elevation in feet

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Water level elevations for July, 2001 using
Spline

• Elevation in feet

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Water level elevations for July, 2002 using
Spline

• Elevation in feet

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Water level elevations for July, 2003 using
Spline

• Elevation in feet

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Using Raster Calculator

• Water level elevation grids are subtracted to
  yield yearly water level change

-

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ResultsCalculations for Yearly Difference
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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
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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
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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
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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
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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
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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
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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?

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IDW or Spline?

• IDW
• Chunky display, obviously not representative of
  natural conditions

• Spline
• Smoother display, probably more representative of
  natural conditions

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Conclusions

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

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Acknowledgments

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

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Thanks