Data Used

Land use data for four years were compiled to track changes in land use. Land use change information, along with the hydrologic soil groups (Figure 2) was used to assess the change in runoff volumes and non point source pollutant loadings in the watershed. Four sets of historical, present and proposed land use data were used for the change impact assessment analysis. 

The first set of data is "GAP" data from the US Fish and Wildlife Service. This is the 1990 land use layer used in the analysis (Figure 4). The second set of data comes from USGS digital ortho quadrant aerial photos shot in late 1992 (Figure 5). The third set of data is the most detailed. It was generated by manual photo interpretation of 1997 aerial photos provided by the Tippecanoe County Geographic Information Services Department. These high quality images are the most current aerial photos available and clearly show new residential areas and areas where new subdivisions are being developed. To generate the land use map from the aerial photos, the area for each land use was digitized on screen to produce a GIS file containing the land uses. This was the 1997 land use layer for the watershed (Figure 6). The 1999 or present land use layer (Figure 7) was extrapolated from the 1997 data based on field visits to the study area. Proposed land use changes were based on zoning maps from the local planning authority. This information was obtained in paper format. The procedure used to convert this analog data to digital format to be used in the spreadsheet analysis is described in the following section. 

Higher quality and printable versions of all of the map layouts are available in pdf format. Click on the layout (figure) to download the pdf file. For the four land-use scenarios, all of the layouts for that scenario are contained in one pdf file.
 

Figure 2	Figure 3
Figure 4	Figure 5
Figure 6	Figure 7

There is a clear progression of land use changes in the watershed (Figures 4-7; Table 1). The 1990 dataset is most clearly different from the other land use maps, in part because of the age of the data, but also because of differences in the method of compilation, and characterization of land uses. The 1990 dataset (Figure 4) was collected using satellite imagery, whereas the other data were based on aerial photos. Within the 1990 data some northern parts of the watershed appeared to be forested, but were considered grass, pasture or agricultural from the aerial photos. Other discrepancies included areas in the southern and western regions that were considered pasture according to the GAP data, but agricultural from the aerial photos. Some of the discrepancy is also due to the timing of the photos. Neither the 1992 or 1997 photos were taken during the growing season. This made it difficult to distinguish between a pasture and a field. Also the 1992 land use layer shows a significant commercial area in the southern portion of the watershed. This was a temporary parking lot that was assigned a commercial land use for looking up the curve number (CN). For hydrologic purposes a parking space behaves the same as a commercial space because both of them are large impervious surfaces generating large amounts of runoff. It was changed to agricultural use in the 1997 land use layer.
 

Table 1. Land Use Areas within the Watershed for each Land Use Scenario
Type	GAP Data	1992 Photo	Current
Commerical	0.530	0.297	0.295
Agricultural	15.177	13.334	12.526
High Density Residential	0.062	0.037	0.025
Low Density Residential	0.037	0.928	1.437
Grassland or Pasture	1.375	0.469	0.630
Forest	2.485	3.245	3.290

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