A National Soil Drainage Index Map as a Tool for Forest Insect and Disease Risk Assessments

Frank J. Krist Jr. and Randall J. Schaetzl

USDA Forest Service Forest Health Technology Enterprise Team (1)
Michigan State University (2)

With the widespread availability of published Natural Resources Conservation Service (NRCS) maps in digital format, resource managers can now begin to incorporate soilscape analysis into risk assessment exercises. Recent work, including the compilation of the National Insect and Disease Risk map by the USDA Forest Service, State and Private Forestry Area, Forest Health Protection Unit, has demonstrated the significance of soils and soil patterns in risk analysis. In particular, patterns related to soil moisture content are often a primary factor related to tree stress, and thus to insects and diseases. Current measures available from the NRCS soils data, such as available water holding capacity of the various soil series, do not adequately describe natural soil wetness and are difficult to interpret. In order to address this data gap, a soil drainage index (DI), initially developed by Schaetzl (1986) and elaborated upon in this work, was used in the construction of the National Insect and Disease Risk map. The DI ranges from 0 (for bare rock) to 99 (for open water) and is based on soil and topographic characteristics, including drainage class, slope, texture, and profile thickness, all of which can be easily gleaned from the taxonomic family of each soil series, as indicated on county soil surveys. The DI has the advantage of being based on natural soil wetness or dryness, which is unaffected by climate variation or irrigation. As a result, soils with high water holding potentials can be identified, even in desert regions, and evaluated in an unbiased manner against soils in wetter locations. The USDA Forest Service, in cooperation with Michigan State University, is currently developing an algorithm that can be used to calculate the DI value of any soil series in the USA, and in so doing is developing a national map of DI values. This poster briefly outlines the process used to develop this layer and how this soil DI layer can play a key role in dynamic risk assessments. The poster will also compare the resultant DI map to a national map of available soil water holding capacity.

corresponding author:

Frank Krist
Forest Health Technology Enterprise Team
2150 Centre Ave., Bldg. A., Suite 331
Fort Collins, CO, USA 80526-1891
970-295-5845
fkrist@fs.fed.us