Quantification of Landscape Indicators/Aquatic Resource Associations in the Savannah River Basin
The Savannah River Basin is arrowhead-shaped, trending generally northwest to southeast. The basin spans three ecoregions: Blue Ridge, Piedmont and Coastal Plain. Landcover types are derived from MRLC data, nominal base year 1992. Differences among the three ecoregions are evident in the forest landcover (Figure 1). Deciduous and evergreen forests predominant in the Blue Ridge and Piedmont ecoregions; all forest types account for 64.7 to 83.49% of the total land cover. Forest landcover accounts for 37.20 to 53.35% of the landcover in the Coastal Plain, with evergreen forests the predominant forest type. Wetland landcover types are found primarily in the Coastal Plain, accounting for 11.10 to 35.93% of the total landcover, most of it in woody wetlands. Wetlands comprise less than one percent of the landcover outside the Coastal Plain.
Scientists from the U.S. Environmental Protection Agency (EPA), Region 4, Science and Ecosystem Support Division, enlisted the assistance of the landscape ecology group of U.S. EPA, Office of Research and Development (ORD), National Exposure Research Laboratory, Environmental Sciences Division (ESD), in conducting a landscape assessment of the Savannah River Basin as part of their ongoing Regional Environmental Monitoring and Assessment Program (REMAP) demonstration project. In the Scope of Work provided by Region 4, the goal was stated as "provide technical/scientific assistance ... to EPA Region 4 in assessing current wadeable stream conditions in the Savannah River Basin with landscape factors that may be contributing to these conditions or gradients." Three specific objectives were presented in the form of questions. These were:
- Are both the proportions of land uses and the spatial pattern of land
uses important for characterizing and modeling stream condition in watersheds/ecoregions
of different areas?
- Can land uses near the streams better account for the variability
in ecological condition than land use for the entire watershed/ecoregion?
- Does the size of the watershed/ecoregion influence statistical relationships
between landscape characteristics and ecological condition?
In addition, an assessment of landscape change was to be conducted as part of continuing ESD research in application of change detection techniques. The information presented here addresses only the first objective.
The data analysis plan developed to address the objectives given above called for calculation of a specific suite of landscape indicators for all nine United States Geological Survey (USGS) 8-digit hydrological unit codes (HUC; USGS, 1982), a selected subset of the 94 Georgia and South Carolina subbasins, and the riparian corridors in the HUCs and selected subbasins. The subbasins are generally equivalent in area to USGS 11-digit HUCs. The riparian corridor was defined as 100 meters on either side of stream arcs; this size was selected from a review of state laws and literature available on the Internet (e.g., Santa Cruz County, 1998; U.S. EPA, 1998; South Carolina Department of Natural Resources, 1998). The suite of indicators included: landcover types, u-index, agriculture on slopes greater than 3 percent, agriculture on highly erodible soils, agriculture on moderately erodible soils, agriculture on highly erodible soils with slopes greater than 3 percent, number of occurrences of roads crossing streams, and number of impoundments. Landscape indicator statistics were also computed for the drainage areas and associated riparian corridors of a selected set of sites sampled by Region 4 using REMAP protocols. Region 4 provided an ARC/INFO coverage of the sampling locations and Quattro Pro spreadsheets of the water quality and biotic measurements.