Drainage Index Grid (conterminous U.S.) GIS Data
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Author: Bradley Miller
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The Drainage Index (DI) maps the natural, inherent wetness of soils across the lower 48 states. Values range from 0 for the driest soils and exposed bedrock to 99 for open water. DI is derived from soil taxonomic information—especially natural drainage classes—linked to digital soil datasets, then rendered as a raster for spatial analysis. It represents long-term potential wetness under natural conditions rather than today’s managed hydrology.
Applied Relevance
Use the DI to compare wetness potential across regions, screen areas for trafficability and waterlogging risk, guide species and site selection in forestry and restoration, and provide a hydrologic covariate for ecology, geography, and environmental modeling. It complements terrain and climate layers by capturing how soil genesis and morphology influence aeration and saturation.
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Data and Method
DI values originate from a Michigan State University–U.S. Forest Service partnership and are assigned from soil taxonomy to an ordinal 0–99 scale. Those values are joined to mapped soil units and gridded to create a continuous surface suitable for statewide and national analyses. Because the index is ordinal, treat differences as relative rather than absolute magnitudes.
Metadata – Sources – Limitations
The DI reflects inherent conditions; it does not show seasonal water tables, recent drainage, irrigation, or tile effects. Local inclusions within map units, taxonomic updates, and land alterations can introduce mismatches. Use the grid for screening and modeling, and verify site-level decisions in the field.
Schaetzl, R.J., Krist, F.J. Jr., Stanley, K.E., and Hupy, C.M. 2009. The Natural Soil Drainage Index: An Ordinal Estimate of Long-Term, Soil Wetness. Physical Geography 30:383–409.