Topic 3, Module 9: HGM and Hydric Soils

The module focuses on the Hydrogeomorphic (HGM) system and hydric soils. The classification of wetlands in the HGM system is based on landscape position, dominant water source, and hydrodynamics – the magnitude and direction of water inflow and outflow.  Information on these parameters is contained in soils information, which is housed in the Web Soil Survey and the soils database.  While HGM interpretations are not provided directly, knowledge of soils attributes can be readily applied to make HGM class and sub-class designations.  These attributes can be used to aggregate soil map units into HGM site concepts.  A site concept is valid when all map units have similar water budgets, are in the same watershed positon, and have the same water movement vectors.  The HGM system also requires the definition of a Reference Domain, within which HGM classifications are valid.  Since soil map unit concepts are generally consistent within a Major Land Resource Area, this boundary is the first selection for the Reference Domain.  Since landscape position is more meaningfully defined as watershed position, the HUC-12 watershed scale is useful for heads-up testing of map unit aggregations. Since map units often need to be either aggregated, or disaggregated into components, the use of Digital Elevation Data is useful for performing these “lumping” or “splitting” operations.  The final result should be a HGM sub-class with associated map units, or components, which can be mapped across the MLRA extent, and which is useful for land managers and conservation planners.