ENSO Impact on Groundwater Levels in the Lower Apalachicola-Chattahoochee-Flint River Basin

Climate variability affects water resources around the world as well as in the southeastern United States. In the Southeast, with a rapidly growing population, pressure on water resources can further get exacerbated by severe seasonal-to-interannual (SI) climate variability experienced in the region. The SI climate variability in the Southeast is attributed mainly to El Niņo Southern Oscillation (ENSO). Understanding and quantifying the impacts of ENSO-induced climate variability on groundwater levels can provide valuable information on the sustainability of groundwater resources. The study was undertaken to study the impact of ENSO on groundwater levels in the lower Apalachicola-Chattahoochee-Flint (ACF) river basin, an area highly dependent on groundwater for agricultural water use. Twenty-one observation wells with approximately 30 years of monthly groundwater level data were used to study the ENSO-groundwater level relationship. Wavelet analysis techniques were used to study the teleconnection, while the Mann-Whitney tests were used to quantify the impact. Impacts of prolonged La Niņa phase on groundwater levels and their corresponding recovery periods were also studied. The results from wavelet analysis showed high power with 3-7 year periodicities in the wavelet spectra of groundwater level anomaly in wells with small overburden conditions while there was lack of such power in wells with deep overburden conditions. Mann-Whitney tests results validated the relationship and indicated a significant relation between groundwater level fluctuations and ENSO. This relationship was found to be significant during the recharge seasons (December-April) and fairly strong during the non-recharge months (May-November). Comparison of severe (2000-01) and average La Niņa phase found that average groundwater levels dropped approximately twice during year 2000-01 compared to average La Niņa phase during both seasons. Unlike an average La Niņa phase, where groundwater level anomaly are higher for the recharge seasons, for year 2000-01, groundwater level anomaly during the recharge and non-recharge seasons were approximately similar. This could be due to increased irrigation needs in the event of prolonged droughts during non-recharge season. Groundwater level anomalies for the year 2000-01 were almost 3 times bigger than the average La Niņa phase values. Recovery times for year 2000-01 were significantly higher than those during 1988-89 (short La Niņa). The average recovery time for year 2001 was 22 months, while for the year 1988-89, recover time was just 2 months. This increase in the recovery periods can be attributed to increased irrigation in the event of a severe drought.

The sensitivity of these wells to ENSO can be used as an important indicator of the condition of groundwater resources in the study area and hence in ultimately developing a procedure for short-term groundwater level forecasting. In the event of a drought, forecasting of groundwater levels can used to issue restrictions on irrigation water withdrawal