Spectral analysis of groundwater-surface water connectivity and its controls over timescales of minutes to years

Floodplains are dynamic places where water table fluctuations mobilize organic carbon and drive changes in redox reactions. These hydrodynamic and biogeochemical processes influence both river water and groundwater quality and operate over a wide range of timescales depending on climate, river regulation, and groundwater extraction. Through publically available USGS data, this study aims to better understand the behavior of river-groundwater connectivity and water table fluctuations by analyzing continuous time-series data of water levels from 32 pairs of stream gages and nearby (<100m) groundwater monitoring wells. Sites are dispersed throughout each major hydrologic unit across the contiguous United States and a variety of climatological and hydrological controls as well as varying land use practices. More than 50% of well sites have a water table that lies within 3 m or less of land surface year-round. Cross wavelet transformation revealed a high coherency between the water table and neighboring stream over the daily to monthly periods. Frequency-domain analysis using the Fourier Transform showed change in power spectral density associated with river regulation. Spectral analysis has the potential to identify the primary control on stream/floodplain connectivity and subsequently floodplain saturation and related soil respiration processes.