urban areas agricultural forested water

These are land use maps for the White Clay Creek watershed in southeastern Pennsylvania and northeastern Delaware. The left image was derived from a 1996 Landsat TM satellite image, while the right image is the SLEUTH predicted land use for the year 2025. These results are not intended as a verified study of the area. White Clay Creek was chosen merely to serve as an example of implementation. It has some interesting features, however. The eastern region of the basin displayed rampant suburbanization during the 1990s and the presence of a natural preserve area can be seen by the forested areas in the west. More information about the basin can be found in the US Department of the Interior's White Clay Creek Watershed Management Plan (White Clay Creek Wild and Scenic River Study Task Force, 1998).

On the left is the 1996 direct runoff image for this same area. On the right is the image for the predicted runoff field in 2025. Note that the output from the SCS Direct Runoff module is at the same resolution as the original land use maps above (25 meter). To generate this output, the SCS Direct Runoff module was run with separate input layers for the percentage of each hydrologic soils group within each pixel. The module's default curve number table was used with a storm depth of 3 inches. These images have been scaled such that, in both, white would represent the maximum possible runoff value. A general correspondence between land use and direct runoff can be seen, with the darker forested areas generated the least runoff. The water channel itself, as it intercepts the rainfall directly, reflects the highest runoff values. Note the blocky transitions in the southeast, where certain regions of a given land type are seen to produce more runoff than similar adjoining areas. This fact reflects the influence of the soil data layers and would not be seen if only one soil type was chosen to represent the entire basin. These images should be used in conjunction with the ascii files that contain the raw data. To generate the gif images, the runoff depths were divided by the depth of the design storm (scaled to the maximum possible value) and then multiplied by 100.

Some suggestions for application of the SCS Direct Runoff module to the SLEUTH model output:

• The influence of the curve number could be explored by entering user-defined CN values rather than allowing the module's default table to be used. The module's table is based on literature-compiled values generally representative of the mid-Atlantic region.
• Different design storm depths could be used. For example, the differences in runoff between the storm that generates the 5-year flood and the 20-year flood could be explored.
• Direct runoff layers could be generated for each year of the SLEUTH model output and a gif animation created.