GEOS 2406 GIS Exercise



Using GIS to Construct Water Table Maps and Flow Nets

Geographic Information Systems (GIS) provide useful tools that enable analysis of spatially-distributed data. An easy example is construction of water table maps, which can be important for understanding hydrologic interconnections, climate change, drought, etc.. In this exercise you will use lake elevation data from an area in western Nebraska to construct a flow net. You can use these same techniques to construct flow nets from water level data in wells.

The largest dune field in North America lies in western Nebraska and was formed during the Pleistocene. Today this area, known as the Sand Hills, is a hummocky region that contains numerous small shallow lakes. The lakes occupy the bottom of blowouts and generally have no inlet or outlet stream and are thus classified as seepage lakes. The dune sand in this region is highly permeable allowing water to flow in and out of the lake through the groundwater system. Thus, the lakes are, for the most part, “windows” into the groundwater system and we can use the lake elevations in the same way we use the elevation of groundwater in wells. This is not always the case. Some seepage lakes become isolated from the groundwater system due to the accumulation of less permeable lake sediments and organic-rich peat. For comparison, see also an application of this kind of mapping in evaluating Saudi Arabian water resources in the Empty Quarter (Sultan, et al., 2008)

1 Procedure

1. Start both ArcCatalog and ArcMap.

2. Download the basic data (sandHillsExerciseFiles.zip) from the class homepage. Open the ZIPfile and extract the files to a convenient directory, preferably in your UTD home directory, otherwise in a directory called “Geospatial” on the local computer User drive.

3. Download the local highways shapefile for plotting:

1. Point a browser at .

2. In the “Zoom to an XY Point” dialog enter the coordinates “-102.299312, 42.146984” (longitude, latitude; Figure 1)

3. Use the Zoom tool at the upper left to zoom in until the scale is around 1:200,000

4. If you wish, enable the hydrography by checking box next to “Hydrography/National Atlas and NHD Waterbodies” (under the “Display” tab) [pic]

Figure 1: USGS Seamless GIS data browser, navigating to Sand Hills region

5. Switch to the “Download” Tab, and check the “National Atlas Roads” box

6. Use the “Define Download Area” tool to drag a rectangle similar to the one in Figure 2

7. An “National Map Seamless Server Request Summary Page” window will open. Select “Download” in the box for “National Atlas Roads”.

8. Open the resulting ZIPfile, extract the all the files to your working directory

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Figure 2: Defining download area in Seamless

4. In ArcCatalog navigate to your working directory . Select the DEM file “NED-DEM\ned_20845919”) and drag into a blank project in ArcMap (Figure 3). Drag the LakeElev and Lakes shapefiles into the ArcMap project window (have the DEM on the bottom of the list, LakeElev on top). Points should appear in the center of all the lakes with elevation data (enable labels by right clicking on LakeElev and selecting “Label Features”). Zoom in and examine the map[pic]

Figure 3: Opening DEM data using ArcCatalog and ArcGIS

5. Turn off the viewing of the DEM by deselecting the checkbox next to ned_20845919. Only the points data will now be shown. Examine the elevation data associated with the points by opening the attribute table. To open the attribute table, right click the mouse on LakeElev and select Open Attribute Table from the pull down menu.

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Close the attribute table by clicking in the x box in the upper right corner of the table.

6. Under the Tools pull down menu, select extensions and make sure that the 3d Analyst and Spatial Analyst boxes are checked. Then make sure the toolbars for these two extension are turned on in the Toolbars option under the View pull down menu,

7. Select the Spline command within Interpolate to Raster under the Spatial Analyst pull down menu (Figure 4). This brings up the Spline dialogue box. This operation will convert the points data to raster data with the value of each cell the elevation at that point. The computer will use a spline curve to determine the values of cells between the known point elevations.

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Figure 4: Spatial Analyst/Interpolate to Raster/Spline dialog

In the Spline dialogue make sure the following has been set (Figure 5):

Input points is LakeElev.

Z value field is Elevation.

Spline type is Regularized.

Output cell size is 0.0006.

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Figure 5: Spline settings dialog

Click OK and a raster image of the groundwater head surface will be calculated.

6. Change the display of the spline surface to a continuous color banding. To do this double click on Spline of LakeElev in the map legend to bring up the Layer Properties dialogue (Figure 6).

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Figure 6: Setting the water table display

Select Stretched and change to the red to blue color ramp and click the Invert checkbox. Click OK. This will apply the color ramp making blue correspond to low elevations and red to high elevations.

8. Turn off the viewing of the LakeElev points. Add contours to the map. To add contours use the Contour option within the Spatial Analysis item under the Spatial Analyst pull down menu (Figure 7).

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Figure 7: Generating contours from a raster (surface)

This will bring up the Contour dialogue box (Figure 8).

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Figure 8: Contour dialog

Make sure that Spline of LakeElev is selected as the Input surface and that the contour interval is 2, then click OK. The contour lines will now be drawn on the map and a line file will be added to the map legend.

9. Add flowlines to the contour map. This can easily be done using the steepest path tool in 3D Analyst (Figure 9). Click on the tool then click anywhere on the map and a line will be drawn down the steepest path which by definition means that lines will be drawn perpendicular to the contour lines (equipotential lines in this case). Include arrowheads by selecting “Set Default Symbol Properties” in the Drawing Toolbar menu.

10. Repeat the contouring of LakeElev using IDW (inverse distance weighted method). In the table of contents name the two contour datasets to indicate the method used.

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Figure 9: Steepest path tool

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Figure 10: Adding arrowheads to pathlines

For Grading:

Turn in a printed copy of your map and answer the following questions.

Questions

1. What type of dunes are present, and what is the prevailing wind direction?

2. Flow lines converge at some locations on the map. What does this indicate about the groundwater flow system? What must be happening at these locations?

3. Flow lines diverge (flow in opposite directions) at some locations on the map. What does this indicate about the groundwater flow system? Hints: these lakes form important wetlands for migratory water fowl; at least one lake nearby to the northwest is named “Alkali Lake”

4. Are the water table contours related to, independent of, or weakly related to the topography (DEM)?

5. Compare the contours from the spline and IDW methods of interpolation (e.g. plot contour lines of one method on top of color-filled contours of the other). What are the main differences between the two results, and which appears more realistic to you (and why)?

6. Turn in a copy of your final map showing water table contours (spline), flowlines, lakes and local roads. Also turn in a comparison map of the two interpolation methods (either a separate map or included with first map, if the combined map is readable).

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