Environmental Factors Driving the Seasonal Dynamics of Groundwater Recharge across the Conterminous United States

Groundwater is a critical resource in the United States. It supplies more than 130 million Americans with their daily drinking supply and irrigates nearly 48% of the country's agriculture. Groundwater also plays a pivotal role in maintaining groundwater-dependent ecosystems and sustaining late-season flows to rivers and streams. Unfortunately, long-term declines in groundwater levels due to changes in land use, overdraft, population growth, and shifts in climatic regimes threaten groundwater availability and quality. Therefore, characterizing changes in seasonal groundwater dynamics is important to managing and sustaining groundwater resources. Fluctuations in the seasonal timing and magnitude of groundwater levels reflect changes in groundwater storage. By characterizing spatial trends in seasonal groundwater dynamics, we can link hydrologic signatures to the underlying processes and environmental factors driving observed trends and improve our understanding of what processes and factors affect heterogeneous changes in groundwater storage. Here we compile and analyze the seasonal dynamics of groundwater levels for more than 1,000 sites from unconfined aquifers across the conterminous United States using normalized mean annual groundwater hydrographs. We observe clear spatial patterns with coherent regional minima and maxima seasonal timing of groundwater levels that closely follow climatic and physiographic regimes influenced by latitude, topography, land use, and hydrogeologic units.