Characterizing hydrologic settings and hydrologic regimes of headwater riparian wetlands in the ridge and valley of Pennsylvania.

摘要

Drawing upon the opportunities to inform and be informed by the stream ecology literature by looking at wetlands in a 4-dimensional, dynamic, River Ecosystem Synthesis approach, this study sought to: better understand the hydrologic setting and the seasonal dynamics of the hydrologic regime for headwater wetlands in the Susquehanna River Basin. The objectives were achieved through a synthesis at the basin scale (Chapter 2) and detailed analyses in a single sub-watershed (Chapter 3) and a subset of wetlands with long-term water level records (Chapter 4). In Chapter 2, this work used the framework of the four-dimensional nature of the RES to generate a revised approach to scaling for the study of wetland services in the Susquehanna River Basin. One aspect of the revised scaling hierarchy was the use of a reach-scale. Chapter 3 used the understanding that in river ecosystems wetlands and, in turn high bio-complexity, occurs where there are laterally and longitudinally unconstrained reaches. This research identified what topographic characteristics defined unconstrained reaches in this physiographic setting based on the known occurrence of wetlands in the study area. Chapter 4 explored the vertical and temporal dimensions of wetlands in this physiographic setting by exploring the relationships between water level, wetland type, and seasonal fluctuations across years with a range of drought and deluge conditions using time series analysis.;Chapter 2 is a synthesis of the efforts of the ecological portion of a study of climate change and ecosystem services provided by freshwater wetlands of the Susquehanna River Basin study to characterize and map a hierarchical landscape classification for use in the study. The scaling hierarchy analysis not only identified a gap in spatial scale of data between disciplines, but it identified the reach as a scale to bridge that gap. Building upon several existing classification schemes, a revised hierarchical landscape classification was generated: Basin, Physiographic Province, Sub-watershed, Channel Reach, and Habitat (macro- and micro-). This work not only proposed the use of a reach scale, rare in wetland studies but very common in stream studies, but articulated a process-based macro- and micro-habitat classification.;Chapter 3 improved the spatial prediction of headwater riparian wetlands through identification of reach settings unconstrained latitudinally and longitudinally that allow for this three dimensional exchange of water. Known locations of mapped, non-open water National Wetlands Inventory (NWI) wetlands, field-identified non-NWI wetlands, and non-wetland locations (n=40, 30, and 35, respectively) were used to build a predictive partition tree. Predictive variables were DEM-derived topographic indices for the stream reaches: valley width, mean stream slope, and contributing area. The partition tree resulted in a 5-node tree (overall R2=0.61). These classes ranged from very high likelihood of wetland occurrence to very low likelihood of wetland occurrence or least constrained to most constrained. This classification is a useful approach to characterizing wetland and non-wetland reach settings, especially in screening out the least likely wetland-supporting or most constrained reaches within a watershed.;Chapter 4 used a suite of time series analyses to explore the hydrographs of five headwater wetlands in terms of their dynamics and response to climatic drivers. Cross correlations between daily differences in water levels and precipitation showed significant correlations for most wetlands under dry and wet conditions on the same day time lag. Of the wetlands evaluated, all experienced a summer drawdown in water level except for the wettest sites in the wettest years. Further, the timing of the beginning of summer drawdown varied greatly for the period of record for the three studied wetlands (slope=61 days; headwater floodplain=58 days; and riparian depression=91 days) with the slope wetland drawing down earlier on average than headwater floodplain or slope wetlands (average day of the year 132, 156, 152 respectively). The moving averages of the water levels generally followed the trends of the downstream stream baseflow, except for the wettest site in the wettest year. Though the hydrologic data are only available as a highly discontinuous record over a 10-year period, more continuous records when analyzed as case studies with time series analyses can give insight into the dynamics and responses of hydrologic behavior of headwater wetlands to climatic drivers. (Abstract shortened by UMI.)