Terrestrial Ecosystems - Topographic Moisture Potential of the Conterminous United States
Dates
Publication Date
2009
Time Period
2009
Summary
The U.S. Geological Survey (USGS) has generated topographic moisture potential classes for the contiguous United States. These topographic moisture potential classes were created as part of an effort to map standardized, terrestrial ecosystems for the nation using a classification developed by NatureServe (Comer and others, 2003). Ecosystem distributions were modeled using a biophysical stratification approach developed for South America (Sayre and others, 2008) and now being implemented globally (Sayre and others, 2007). Substrate moisture regimes strongly influence the differentiation and distribution of terrestrial ecosystems, and are one of the key input layers in the ecosystem delineation process. The method used to produce these [...]
Summary
The U.S. Geological Survey (USGS) has generated topographic moisture potential classes for the contiguous United States. These topographic moisture potential classes were created as part of an effort to map standardized, terrestrial ecosystems for the nation using a classification developed by NatureServe (Comer and others, 2003). Ecosystem distributions were modeled using a biophysical stratification approach developed for South America (Sayre and others, 2008) and now being implemented globally (Sayre and others, 2007). Substrate moisture regimes strongly influence the differentiation and distribution of terrestrial ecosystems, and are one of the key input layers in the ecosystem delineation process. The method used to produce these classes is based on the derivation of ground moisture potential using a combination of computed topographic characteristics and mapped wetlands boundaries. This method does not use climate or soil attributes to calculate relative topographic moisture potential since these characteristics are incorporated into the ecosystems though other input layers.The source data for this assessment is a national Compound Topographic Index (CTI) dataset (USGS Earth Resources Observation and Science Center, 2003), which was derived from the USGS 30-meter National Elevation Dataset (NED). The CTI index is a topographically derived measure of slope for a raster cell and the contributing area from "upstream" raster cells, and thus expresses potential for water flow to a point. This potential accumulation at a point was compared to independent estimates of water accumulation by obtaining geospatial data from a number of sample locations representing wetland/upland boundaries from the National Wetland Inventory (NWI) dataset. Where these "shorelines" (the interface between wetlands and adjacent land) occurred, the CTI values were extracted and a histogram of their statistical distributions was calculated. Based on an evaluation of these histograms, CTI thresholds were developed to separate wetlands from uplands. A similar process was used to assess the distributions of CTI values for known locations of mesic and dry uplands. After the range of CTI values for these three different substrate moisture regimes (wetlands, mesic uplands, and dry uplands) was determined, the CTI values were recalculated to topographic moisture potential. The final step in the generation of the national topographic moisture potential data layer was to partition the dry uplands class into two classes, a dry uplands class, and a very dry uplands class. Very dry uplands were defined as dry uplands with relatively steep, south-facing slopes, and identification of this class was based on the slope and aspect datasets derived from the USGS NED. The resulting Topographic Moisture Potential dataset for the contiguous United States contains four classes: wetlands, mesic uplands, dry uplands, and very dry uplands.
