Groundwater-stream connectivity from minutes to months across United States basins as revealed by spectral analysis

Stream corridors are dynamic places where streams and aquifers are connected and interact to various degrees, depending on geology, climate, stream morphology, and water use. Water table fluctuations propagate through the unconfined aquifer and are linked with changes in solute export to streams and biogeochemical transformations in floodplain soils. Through publicly available USGS data, this study aims to better understand the behaviour of stream-groundwater connectivity and water table fluctuations by analysing continuous time series of water levels from 17 pairs of stream gauges and nearby (<100 m) groundwater monitoring wells. Sites are located within 8 of 18 major hydrologic units (HUC-2) across the contiguous United States and span a variety of stream sizes, climates, and land use practises. More than 50% of sites have a water table that remains within 3 m of the land surface year-round. Energy spectral densities and cross-wavelet transformations generally reveal strong coherence between the water table and stream stage over daily to monthly periods. The transfer function, which describes relative variations between the water table and stream stage, shows that 10 of 17 sites are more stream-dominated at daily and monthly frequencies, meaning that water level fluctuations are greater in the stream and propagate into the aquifer. Only 1 of 17 sites is more groundwater-dominated at daily and monthly frequencies, meaning that water level fluctuations are greater in the aquifer. This study shows the utility of frequency-domain analysis for revealing specific timescales of stream-aquifer interaction.

Automated summary

This study analyzes continuous time series of water levels to investigate stream-groundwater connectivity and water table fluctuations. The results show that water level fluctuations are mainly influenced by streams and have an impact on the aquifer in most sites.