ASSESSMENT AND CLASSIFICATION OF SEASONAL GROUNDWATER LEVEL DYNAMICS IN UNCONFINED AQUIFERS OF THE UNITED STATES

In many aquifer systems worldwide, groundwater levels experience significant seasonal fluctuations in response to a host of environmental, climatic, and anthropogenic forcings and these fluctuations affect both the availability and hydrologic and ecologic influence of groundwater on intra-annual timescales. Here we present the first analysis of seasonal groundwater level dynamics in unconfined aquifers across the United States (US). We use functional data analysis (FDA) to characterize seasonal water level dynamics at >1000 sites and find that they cluster into four distinct hydrologic regimes: (A) Mild peak in winter and mild minimum in spring; (B) Distinct summer peak and winter/spring minimum; (C) High amplitude seasonal signal with early-spring peak and early-fall minimum; (D) Strong spring peak with mildly pronounced winter minimum. We examine the regional distribution of the hydrologic regimes, identify factors that influence regime development, and interpret each regime’s environmental implications. We find that physiographic factors such as relative topographic position (valley vs. ridge) and climatic conditions including potential evapotranspiration (PET) and precipitation seasonal timing and amounts differ systematically between regimes, suggesting these factors exert mechanistic control on regime emergence. Furthermore, we interpret the seasonal dynamics in the context of regional hydroclimate to characterize groundwater recharge processes and seasonal timing. We reach a number of conclusions including (1) summer recharge is dominant within regime C, which has wet summers and dry winters and frequently occurs in Florida and Minnesota; (2) late-winter to early-spring recharge is typical across much of the northeastern US (regimes C and D) where snowmelt and precipitation can effectively recharge prior to the onset of high ET during late spring through summer; (3) summer recharge is common along snowmelt-fed rivers of the western US. Our analysis of seasonal groundwater dynamics provides useful insight into how groundwater systems respond to environmental forcings and how they may respond to future environmental change.