Hydrologic Assessment of Natural and Mitigation Wetland Sites.

The hydrology at a natural or created wetland site controls many soil properties.  The important aspects of that hydrology – frequency and duration of saturation or inundation – reflect the regional climate, the landscape position, and the permeability structure of the surrounding landscape. Precipitation and evapotranspiration patterns affect the timing of water availability. Within a given climatic zone, landscape position and soil permeability control infiltration and surface water routing and whether groundwater recharges or discharges at a site.  Stratigraphic and structural characteristics of the regolith and shallow bedrock dictate routes of groundwater movement.  The most complete analyses of a wetland’s hydrologic processes must account for these factors.

In humid climate zones, wetlands readily form at groundwater discharge sites along valley bottom margins; mitigation wetlands built at such sites that ignore that groundwater input and rely on surface water and precipitation tend to be too wet.  Wetlands also can form at groundwater recharge sites that receive only precipitation, usually on broad flats but even on sandy islands.  In these cases, though, the sites often have aquitards or other hydrologic barriers that retard seepage downward at relatively shallow depths.  Mitigation wetland designs developed for such sites must give special attention to the permeability structure of the new shallow soil profile to preserve sufficiently wet conditions during the growing season.

Hydrologic tools now exist to reconstruct the range and timing of water levels in potential mitigation wetlands at monthly time scales, based on the weather records that exist for the design site.  The Effective Monthly Recharge (Wem) procedure correlates monthly water level readings over a year or more with values derived from time-weighted averages of recharge (precipitation minus ET).  If the wetland hydrology is driven by only precipitation and groundwater, and if the months used for this calibration experienced a wide range of dryness conditions, the correlation-based regression equation can generate a synthetic hydrograph for any historic period with sufficient weather data, before wells were installed at the site.