WETLAND HYDROLOGY AS A FUNCTION OF HYDROGEOMORPHIC (HGM) SUBCLASS

Characterizing wetland hydrology is key to assessing relative function over a range of wetland types. However, hydrologic data are often lac king. To address this lack of information, we categorized a set of 24 reference wetlands by hydrogeomorphic (HGM) subclass from 1993 to 1995 , installed monitoring wells and piezometers, and assessed local water -table levels, pH, and specific conductance by month. Four HGM wetland subclasses were common to central Pennsylvania (riparian depression ( n = 8), slope (n = 7), mainstem floodplain (n = 5), and headwater floo dplain (n = 4)) and formed the basis for our analysis. Median depth to water in the wells differed by HGM subclass. Riparian depressions had the shallowest depth to water (-8 cm) and headwater floodplain wetlan ds the greatest (-70 cm). Comparisons of the percent occurrence of a p iezometric head (from comparisons between paired piezometer and slotte d wells) indicated that riparian depressions and slopes had significan t ground-water inputs (47 and 48%, respectively), whereas the mainstem floodplain (31%) and headwater floodplain wetlands (23%) were more su rface-water-driven systems. Water occurred within the root zone (30 cm ) most often for riparian depressions (80% of observations), intermedi ate for slopes (48%), and least for mainstem floodplains (17%) and hea dwater floodplains (6%). Headwater floodplain wetlands were never inun dated by overbank flow during this study but instead received water fr om snowmelt and overland flow after rain events. Mainstem floodplain w etlands were inundated by floods during major storm events. The upper 30 cm of soil (i.e., the root zone of plants) was almost continually s aturated in riparian depressions, but rarely for both floodplain syste ms. Slope wetlands were intermediate between riparian depressions and floodplain systems in the amount of time water was present within 30 c m of the ground surface. Riparian depressions and slopes had lower pH than floodplain systems, and pH did not vary significantly by month fo r any HGM subclass. Floodplain systems (both headwater and mainstem) h ad greater values of specific conductance than either riparian depress ions or slopes; riparian depressions were the only HGM subclass to sho w seasonality in specific conductance. Factors other than HGM subclass that may have influenced the hydrologic pattern and water quality par ameters included bedrock geology, disturbance levels, and watershed at tributes.