Wetland hydrology as a function of hydrogeomorphic (HGM) subclass

Abstract

Characterizing wetland hydrology is key to assessing relative function over a range of wetland types. However, hydrologic data are often lacking. 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 floodplain (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 wetlands the greatest (−70 cm). Comparisons of the percent occurrence of a piezometric head (from comparisons between paired piezometer and slotted wells) indicated that riparian depressions and slopes had significant ground-water inputs (47 and 48%, respectively), whereas the mainstem floodplain (31%) and headwater floodplain wetlands (23%) were more surface-water-driven systems. Water occurred within the root zone (30 cm) most often for riparian depressions (80% of observations), intermediate for slopes (48%), and least for mainstem floodplains (17%) and headwater floodplains (6%). Headwater floodplain wetlands were never inundated by overbank flow during this study but instead received water from snowmelt and overland flow after rain events. Mainstem floodplain wetlands were inundated by floods during major storm events. The upper 30 cm of soil (i.e., the root zone of plants) was almost continually saturated in riparian depressions, but rarely for both floodplain systems. Slope wetlands were intermediate between riparian depressions and floodplain systems in the amount of time water was present within 30 cm of the ground surface. Riparian depressions and slopes had lower pH than floodplain systems, and pH did not vary significantly by month for any HGM subclass. Floodplain systems (both headwater and mainstem) had greater values of specific conductance than either riparian depressions or slopes; riparian depressions were the only HGM subclass to show seasonality in specific conductance. Factors other than HGM subclass that may have influenced the hydrologic pattern and water quality parameters included bedrock geology, disturbance levels, and watershed attributes.