A Simple Model of Flow Reversals in Florida's Karst Springs.

North Florida's karst springs are among the largest and most abundant in the world. Despite relatively stable spring discharges, flow reversals can episodically occur in some springs when river waters backflow into the aquifer during flood events. Reversals are normal features of the springs along the Suwanee River, but the changing incidence of these reversals in response to anthropogenic activities or climate change remains unclear and the mechanisms responsible for these reversals remain poorly described. Here we develop a reduced‐complexity hydrogeological model of the Suwannee River catchment to explore conditions needed to induce spring flow reversals. Our model demonstrates that reversals require two conditions: (a) a hydrogeological setting that combines an upstream catchment with rapid hydrological responses to meteorological drivers, which freely drains to a downstream catchment containing the karst aquifer (i.e., the spring‐fed river segment); and (b) meteorological conditions that create sufficient temporal variability in recharge. Given both conditions, recharge events can propagate from the upstream catchment and fill the downstream river segment faster than it can drain, causing river stage to rise above the aquifer head, resulting in temporary spring flow reversal (or bank storage). Our model accurately predicts significant post‐flood increases in spring flow as bank storage recedes, and using measured electrical conductivity at a major river‐adjacent spring we also quantify the enhancement of limestone dissolution (cave enlargement) due to reversal events. A comprehensive assessment of the incidence and duration of reversal events shows a predominant influence of climate and vegetation changes over that of groundwater pumping. Plain Language Summary: North Florida's karst springs are among the largest and most abundant in the world. Spring flow is very stable and usually into the main river, but reversals of flow direction may occur temporarily, such that water from the river enters the subsurface through the springs. This is a natural feature, but its causes and future behavior under climate change and human activities are poorly understood. In this work, we develop a simple model of the Suwannee River catchment that describes spring flow and reversals, showing that reversals require both appropriate geological as well as meteorological conditions. We also show that spring flow reversals contribute significantly to the limestone cave enlargement near the river, and that the future characteristics of reversal events are most dependent on climate and vegetation changes. Key Points: A reduced‐complexity model explains flow reversals in Florida karst springs due to fast discharge response from upstream surficial aquiferThe occurrence of reversals requires a certain hydrogeological setting and sufficient temporal variability in aquifer rechargeThe model also quantifies enhanced limestone dissolution rates (cave enlargement) near the river due to flow reversals [ABSTRACT FROM AUTHOR]