Analysis of Current and Future Land Uses with the Spreadsheet Version 

Wildcat Creek Watershed Case Study

The spreadsheet version of L-THIA was used to analyze the runoff generated by the present land use (1999) of the case study watershed and the runoff that will be generated if land uses within the watershed become the proposed land uses. The proposed land use information was obtained from the local planning authority in paper format. The data obtained in paper format posed problems in terms of converting the data into digital format and the accuracy of the converted data. To complicate matters the three maps covering the watershed of interest were at three different scales.

Data conversion was performed in three steps. The first step was to transfer the data from the proposed land use maps to a map made up of a mosaic of the 1997 aerial photographs of the watershed. The next step was to calculate the total area covered by all land uses on the two hydrologic soil types found in the watershed. The two hydrologic soil types found in the watershed are B and C. A map, at the same scale as the photo mosaic map, was obtained from the digital database used for the GIS version of L-THIA. The soil map was then overlaid with the proposed land use information map produced in an earlier step. A third composite map was produced from the two maps that showed the area covered by different land uses in the watershed according to the two soil groups.

The third step was to calculate the area covered by different land uses according to the soil types. This was calculated by overlaying the new composite map with a 0.25 km square grid. The number of squares covering each land use were manually counted and multiplied by 0.25 to obtain the area covered by the land use in square km. The data generated by this method is not as accurate as estimates obtained using GIS. The total area of the watershed by this method comes to 19.25 square km, whereas the total area of the watershed according to the digital data is 18.11 square km. Such differences can be expected considering the crudeness of the method used to estimate the area, however this relatively small difference in area has a small overall effect on the estimated long-term changes in runoff generated by L-THIA. The data generated by the manual method was then used to run the web-based spreadsheet version of L-THIA.

To run the spreadsheet version of L-THIA, the information required is:

  • The location of the area of interest (state and county)
  • Land use areas, which were obtained by the procedure described above 
  • Hydrologic soil groups for land use areas, which was obtained from the digital data base available at Purdue, alternatively such data can also be obtained at the local Soil and Water Conservation District office or Natural Resources Conservation Service office.
Figure 34 shows the present and zoned land use distribution in the watershed. Figure 35 shows the estimated average annual runoff for present and proposed land uses. 

The proposed developments in the watershed result in an estimated 100% increase in the average annual runoff generated in the watershed. This could have several long-term impacts on the hydrology of the region. The overall runoff volume of the watershed increases from less than 450,000 cubic meters to almost 900,000 cubic meters. At the present the watershed is predominantly agricultural. The conversion of agricultural land to commercial, industrial, and high-density residential land uses has drastic consequences on the runoff.

The large areas proposed for commercial development in the watershed are the largest contributors to the increase in runoff. An increase of just 0.7 square km of commercial use results in an increase of 400% in the runoff generated by that particular land use. Other high runoff generating land uses such as industrial and high density residential are also introduced into the watershed. They also account for the large increase in runoff.

Secondly land uses such as commercial, industrial, and high density residential also cause changes in the pollutants that runoff with water into the surrounding water bodies. At present the watershed is mainly agricultural; it accounts for 73% of the total area and 63% of the total runoff. In the proposed development scenario, commercial land use covers only about 5% of the total area but contributes almost 45% of the total runoff. The increase of land uses such as forests in the riparian buffer zone actually contributes to reducing the total runoff in the watersheds.

Is the proposed land use plan the best solution for the long-term hydrology of the region? This can be determined by running L-THIA for different land use change scenarios or keeping the same land uses but changing the location of the land use so it is on a different soil type. Since different soil types in combination with different land uses generate different runoffs, an alternative land use change solution that maintains the same area of each land use as in the current proposed plan can be found using L-THIA that results in a minimum increase in runoff in the watershed. The data to run such an analysis with L-THIA is provided in Table 4. The watershed is located in Indiana in Tippecanoe County. Using the information in Table 4, L-THIA can be used to determine the average annual runoff that would result from this zoning plan. Click here to use L-THIA to generate the runoff for this situation.

Table 4: Land use according to soil types (all data in km2)
 
 
Present 
Proposed 
Land Use  Soil C  Soil B  Soil C  Soil B 
Low Density Residential  0.77  0.67  0.88  1.69 
High Density Residential  0.03  0  1.38  1.06 
Commercial  0.26  0.03  0.94  2.31 
Industrial  0  0  0.38  2.75 
Agricultural  4.19  8.3  3.25  2.63 
Forest  0  2.06  1.25  2.19 
Grass  0.49  0.13  0.00  0.00 
Total  5.74  11.19  8.06  12.63 

 

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