Groundwater Response to Climate Change and Anthropogenic Forcing: A Case Study on Georgia, U.S.A.

Abstract

This study analyzed the effects of climate variability and human activities (i.e., irrigation) on groundwater levels from 1981-2017 in four distinct geologic provinces of Georgia. Georgia is located in the southeastern United States and is home to around 10 million people and the country’s 9th most populous metropolitan area according to U.S. census data. The southeastern United States has long been thought to be resilient to the types of groundwater issues that are seen in the western United States and around the world. Global climate trends are expected to cause increases in temperature and greater variability in precipitation. While climate change trends are expected to affect groundwater, the exact effects are unknown and are likely to vary significantly based on location and local geology. Declines in streamflow and aquifer storage have already been linked to agricultural irrigation expansion in southwestern Georgia.

Data from a total of 404 USGS groundwater monitoring wells have been collected and analyzed for water table fluctuations over a period of 37 years (1981 to 2017). Gravity Recovery and Climate Experiment (GRACE) data were used to estimate changes in terrestrial water storage in comparison with water level fluctuations. High resolution PRISM (Parameter elevation Regression on Independent Slopes Model) climate data (precipitation and temperature) were also collected for trend analysis at 43 monitoring sites. Long-term and annual groundwater trends are computed using a combination of time series analysis, the Mann—Kendall test, and autocorrelation analysis. Statistical trends in climate and hydrological data are used to explore the relationship between climate variability, streamflow, and groundwater level. Time series analyses show statistically significant declines in groundwater level in Coastal Plain and Floridan aquifer systems across the state. The deep confined Coast Plain aquifer appears to be decoupled from climate influences and show the strongest declines in water level. The shallower and less confined Floridan aquifer system appears to be coupled with climate yet also shows declines in water level. By contrast, the Surficial aquifer system and the Northern aquifer system both show relatively neutral trends over the same period. The water table in these shallower aquifers correspond to changes in precipitation. Streamflow data from 36 gauging stations in Georgia show decreasing minimum streamflow trend while other statistics show no trends. Statewide precipitation was found to show no significant trend and temperature averages were shown to show slight increases in nighttime averages, indicating non-climate factors has likely caused the groundwater declines in deeper, confined aquifers. Autocorrelation analysis indicates that each aquifer system and precipitation tend to have different durations of memory. The precipitation and unconfined aquifers exhibit the shortest duration of memory ranging from 3 to 6 months, while the deeper confined aquifers show substantially longer memory and significant lag times of 12 to 18 months or greater. Statistically significant decreasing trends in water level and long lag times further indicate groundwater in deeper aquifers is decoupled from climate influences.