The Drainage Index (DI), originally named the "natural soil wetness index" (Hole and Campbell 1986, Schaetzl 1986), is a measure of long-term soil wetness. It is designed to represent, as an ordinal number, the amount of water that a soil contains and makes available to plants under normal climatic conditions. It is not meant to mimic the concept of "plant available water", which is mostly dependent upon soil texture. The DI only loosely/secondarily takes soil texture into consideration. The main factor affecting the DI is the depth to the water table and the soil volume available for plants to root and grow in. The DI concept was first initiated by Hole (1978) and Hole and Campbell (1985), and expanded upon by Schaetzl (1986). The DI ranges from 0 to 99. The higher the DI, the more water the soil can and does, theoretically, supply to plants. Sites with DI values of 99 are essentially open water. A DI of zero indicates impermeable surfaces like bare bedrock or urban areas dominated by pavement and buildings. The DI is derived from the soil's taxonomic subgroup classification in the US system of Soil Taxonomy, and (optionally) its soil map slope class. Because a soil's taxonomic classification is not (initially) affected by such factors as irrigation or artificial drainage, the DI does not change as soils become irrigated or drained (unless the long-term effects of this involve a change in the soil's taxonomic classification). Instead, the DI reflects the soil's NATURAL wetness condition. Each soil SERIES has, in theory, its own unique DI. Some soil series span two or more drainage classes; in this case the DI that is used is the one that would normally be used for a soil with that subgroup classification. Contact: Estella Smith, Geospatial Technology and Applications Center (GTAC), USFS. estella.smith@usda.gov