Sensitivity of Groundwater Components to Irrigation Withdrawals during Droughts on Agricultural-Intensive Karst Aquifer in the Apalachicola-Chattahoochee-Flint River Basin

In southwestern Georgia, increased groundwater withdrawal for irrigation in the Lower Flint River Basin (LFRB) is greatly affecting the supply of freshwater to downstream regions during drought years. This study was undertaken to understand the sensitivity of groundwater components to changes in irrigation pumping scenarios during droughts in the highly connected karst Upper Floridan Aquifer (UFA) and assess the effectiveness of irrigation shutdown on stream-aquifer fluxes. This study was done using the United States Geological Survey (USGS) MODular Finite-Element Groundwater model (MODFE). The model was run for the water year 2011 and changes in simulated groundwater-budget components for monthly irrigation pumping rates of 50%, 75%, 100%, 125%, 150%, and 200% of that indicated by metered irrigation systems were quantified. The effect of irrigation shutdown in the vulnerable Spring Creek subwatershed was also investigated. Results show that increasing irrigation pumping in the study area decreased aquifer storage, induced recharge to the UFA (vertical leakage), and induced recharge from streams into the UFA. During the months of May-September 2011, doubling of irrigation pumping induced nearly double the recharge rate into the UFA from streams and effected a 10% decrease in discharge rate from the UFA to streams. Analysis of irrigation shutdown indicates that eliminating irrigation pumping from the Spring Creek watershed alone facilitated an increase in baseflow of 46x104 m3/day, or about two-thirds the total increase in baseflow that would be realized from eliminating irrigation pumping within the entire LFRB.