Basin-scale spatiotemporal analysis of hydrologic floodplain connectivity

Floodplain inundation often provides water quality benefits by trapping sediment and biogeochemically transforming other pollutants. Hydrologic floodplain connectivity is a measure of water exchanges and interactions between the main channel and the floodplain via surface (inundation) and subsurface (groundwater) connections. Using an automated model combining GIS and numerical analysis software, this study examined floodplain inundation patterns and measured floodplain connectivity for the Mahantango Creek watershed (Pennsylvania, USA). Connectivity was quantified by developing a metric that included inundation area and duration. Long-term hydrographs at each reach in the watershed were developed via QPPQ (Flow-Percentile-Percentile-Flow) methodology using regional regression analysis to calculate the ungauged flow duration curves (FDC). Inundation area (normalized to stream length) was found to increase with drainage area, suggesting larger streams have more area available for biogeochemical activity. Annual connectivity increased with drainage area, suggesting larger streams, having higher connectivity, should be the focus of individual reach restoration projects due to higher potential for water quality benefits. Across the watershed as a whole, however, the total annual connectivity across first order streams was greater than higher order streams, suggesting the collection of small streams in a watershed may have a stronger effect on outlet water quality. Connectivity was consistently higher during the non-growing season, which was attributed to higher flows. Despite higher connectivity during the non-growing season, increased floodplain biological activity may be negated by low temperatures, reducing microbial activity. Correlations between land use and connectivity were also found, emphasizing dynamics between flow, channel morphology, and floodplain inundation.