U.S.V.I. Relative Erosion Potential - 2000

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Watersheds are an essential unit for analysis, since they link land areas with their point of discharge to the sea. We have implemented a watershed-based analysis of sediment and pollution threat to coral reefs. This analysis incorporates land cover type, slope, soil erodibility factor (k-factor), and precipitation for all land areas, using a simplified version of the Revised Universal Soil Loss Equation (RUSLE) (USDA, 1989) in order to estimate relative erosion rates for each 30m resolution grid cell. These relative erosion estimates are summarized by watershed and by basin. Since not all erosion makes its way to the river mouth, sediment delivery ratios (based on watershed size) were applied in order to estimate relative sediment delivery at the river mouth. It should be noted that relative erosion rates and sediment delivery are being used as a proxy for both sediment and pollution delivery. This information can be used to estimate sediment plumes and impacted reefs (ideally including circulation patterns for the area.) Model results need to be calibrated using available data on river discharge, sediment delivery, and observed impacts on coral reefs. We are working with NOAA's Summit to Sea project to use satellite imagery to maps areas of observed plume and identify habitat change, as well as to help calibrate model results. Model Implementation Step 1) The first step of the analysis involves estimating likely relative erosion rates for each 30 m resolution grid cell using a modified, simplified form of the Revised Universal Soil Loss Equation (RUSLE) (USDA, 1989). Information on slope, land cover type, precipitation, and soil porosity were integrated to develop an indicator of relative erosion potential (REP) for all land areas within the wider Caribbean. Inputs: (REP relies upon four input data sets,4. Slope (percentage) - for each 30m grid cell, derived from the raw Digital Elevation Model. 5. Relative erosion rate by land cover type from NASA Geocover-LC products (2000). Relative erosion rates for each land cover type were calculated using a look-up table (below.) Land cover categories was reclassified to relative erosion rates, ranging from 15 (for forest) to 220 for barren land. These relative erosion rates are based on published work involving conversion factors.Land Cover Type, Grid Code, Relative Erosion RateDeciduous Forest, 1, 15Evergreen Forest, 2, 15Shrub/Scrub, 3, 50Grassland, 4, 125Barren, 5, 220Urban, 6, 210Agriculture, 7, 200Permanent Wetland, 9, 80Mangrove, 10, 80Water, 11, 5Cloud/No Data, 13, 06. Precipitation during the peak rainfall month (in millimeters) - Long-term average monthly precipitation values for the peak rainfall month of the year is an interpolated grid based on data for climate stations from NOAA's National Climate Data Center (NCDC). This variable was chosen because it is more indicative of the rainy season and more extreme events during the year.7. Soil erodibility factor (K-ffactor) - the K-ffactor was obtained from the SSURGO database of the USDA.Relative Erosion Potential (REP) = (%) Slope x maximum monthly precipitation x soil kffactor x relative erosion rate for land cover type / 1000

(NASA Geocover - LC with 1.4 ha minimum mapping unit "http://www.MDafederal.com/geocover/geocoverlc/"; USGS National Elevation Dataset "http://ned.usgs.gov/"; NRCS Soils Database "http://www.nrcs.usda.gov/wps/portal/nrcs/site/national/home/"; NOAA Monthly Surface Data "http://www.ncdc.noaa.gov/oa/climate/climatedata.html" )