The Natural Soil Drainage Index: An Ordinal Estimate of Long-Term Soil Wetness

Abstract

Many important geomorphic and ecological attributes center on soil water content, especially over long timescales. In this paper we present an ordinally based index, intended to generally reflect the amount of water that a soil supplies to plants under natural conditions, over long timescales. The Natural Soil Drainage Index (DI) ranges from 0 for the driest soils (e.g., those shallow to bedrock in a desert) to 99 (open water). The DI is primarily derived from a soil's taxonomic subgroup classification, which is a reflection of its long-term wetness. Because the DI assumes that soils in drier climates and with deeper water tables have less plant-useable water, taxonomic indicators such as soil moisture regime and natural drainage class figure prominently in the "base" DI formulation. Additional factors that can impact soil water content, quality, and/or availability (e.g., texture), when also reflected in taxonomy, are quantified and added to or subtracted from the base DI to arrive at a final DI value. In GIS applications, map unit slope gradient can be added as an additional variable. The index has myriad applications in forestry, ecology, geomorphology, and environmental modeling, especially when examined spatially; we provide some examples in this paper. The DI has great potential for many landscape-scale modeling and GIS applications where soil water content is an important variable. DI values for all soils currently classified by the NRCS can be accessed from pull-down menus on the DI web site: http://www.drainageindex.msu.edu/