Interpretation of ``On Average'' in Wetland Hydrologic Analyses

The Corps of Engineers wetlands delineation manual states that a site must meet the wetland hydrologic criterion for water table depth and duration under average conditions, defined by the Corps to mean that those conditions occur in at least one of two years over a long term record. Because long-term water table records rarely exist for sites in question, water table measurements of a few months are often made to determine whether wetland hydrology exists. The decision as to whether those measurements reflect "average" conditions is usually based on whether rainfall during the measurement period is "average". This paper examines whether an assessment of hydrologic status based on satisfying a water table criterion in one of two years, on average, is the same as an assessment based on the same criterion, but applied only in periods with average rainfall. The reliability of a field method based on "average" rainfall was examined. The field method excludes from consideration water table records occurring during above average rainfall months when making hydrologic assessments. Results from the examination of the field method showed that by only considering water table records from average rainfall months, many sites which did have wetland hydrology were incorrectly identified as uplands. Sites which meet the hydrologic criterion under actual rainfall often do not meet the criterion when only months of "average" rainfall record are examined. Results indicate that a site that satisfies the wetland hydrologic criterion on a long term average may be much drier than a site meeting the criterion under "average" rainfall conditions.