Long Term Empirical Relations between Storm Characteristics and Episodic Groundwater Recharge across Geographic and Land-Use Gradients

Abstract:

Shallow aquifers are an important water resource and provide baseflow to streams, yet estimating rates of groundwater recharge is difficult. While climate change is predicted to increase the frequency and magnitude of extreme precipitation events, the resulting impact on recharge remains poorly understood. We quantify empirical relations between precipitation characteristics and episodic groundwater recharge for a wide variety of geographic and land-use types across North Carolina. We extract storm duration, magnitude, average rate, and storm intensity from precipitation records over periods of twelve to thirty-five years at ten locations, for a total of 3,544 individual storm events. Using time-series of water-table fluctuations from nearby monitoring wells, we estimate relative recharge to precipitation ratios (RPR) to identify statistical trends. RPR increases with increased storm duration, whereas RPR decreases with increasing magnitude, average rate, and intensity. Agricultural and urban areas exhibit the greatest decrease in RPR due to increasing storm magnitude, average rate, and intensity, while naturally vegetated areas exhibit a larger increase in RPR with increased storm duration. Though RPR is generally higher during the winter than the summer, this seasonal effect is magnified in the Appalachian and Piedmont regions. These statistical trends provide valuable insights into the likely consequences of climate and land-use change for water resources in humid, subtropical climates in the American southeast. If, as predicted, growing seasons lengthen and the intensity of storms increases with a warming climate, decreased recharge in Appalachia, the Piedmont, and rapidly growing urban areas in the region would further limit groundwater availability.