ENSO Induced Droughts Impacts on Groundwater Levels in the Lower Apalachicola-Chattahoochee-Flint River Basin By Subhasis Mitra, Puneet Srivastava, Lynn Torak and Sarmistha Singh Biosystems Engineering, Auburn University Georgia Water Science Center, USGS

Introduction There is increased pressure on water resources of southeast USA: Due to increased urbanization and population. Severe climate variability caused by ENSO exacerbates the situation. Cycle of above and below average sea-surface temperatures in the equatorial pacific of the coast of Peru with 3-7 year periodicities. Three phases- El Niño (warm phase); La Niña (cold phase) and Neutral phase. El Niño Southern Oscillation (ENSO) El Niño is expressed in decreased temperature and increased winter precipitation in the Southeastern USA. La Niña is expressed in increased temperature and decreased winter precipitation in the Southeastern USA.

Objective: Quantify the effect of ENSO-induced climate variability on groundwater levels under different overburden conditions. This will allow us to develop forecasting tool to better manage GW & surface water resources and meet anthropogenic water demands. Major mode of climate variability in the southeast United States. ENSO have been found to affect the hydrologic cycle. Groundwater is the major source of water resources in the lower Apalachicola- Chattahoochee-Flint (ACF) river basin. Provides an opportunity to study ENSO effects on GW levels and its interaction with anthropogenic factors.

Study Area Fig. 1. Location of the study area, GW observation wells and geohydrologic zones. The ACF River Basin is at the center of tri-state water crisis Three states namely Georgia, Florida and Alabama went to court for the basin’s finite water resources. The climate of the lower ACF River Basin is humid subtropical with long summers and mild winters. About 4632 mi 2 of land area contributes groundwater and surface water to the Upper Floridan Aquifer (UFA). Nearly 500,000 acres irrigated with around 4000 wells in the Upper Floridan Aquifer.

Upper Floridan Aquifer system (UFAS)  Groundwater levels in the UFA respond to seasonal climatic effects such as precipitation, droughts, stream stage and lake level changes. Fluctuations in groundwater levels in the UFA also depend on: Thickness and location specific hydraulic characteristics of the above lying USU, Proximity to surface streams or lake system. Groundwater irrigation withdrawal for agricultural, industrial and municipal purposes. Fig. 2. Conceptual diagram of groundwater and surface water flow in the inter-connected stream aquifer system in the UFAS. (source USGS) Overburden

Data Collection and Processing Niño 3.4 Monthly Sea-Surface temperature (SST) data were collected from Climate Prediction Center-NOAA. Daily Groundwater Level Data were collected from USGS-Georgia Water Science Center. Twenty-one observation wells with 25 to 30 years of data were used. Groundwater level anomalies were calculated and sorted according to recharge (December-April) and non-recharge seasons (May-November). The software package for Wavelet analysis was used from the Matlab code developed by Aslak Grinsted ( coherence).

Methodology Wavelet Analysis Wavelet analysis examines the relationship between two time series to determine the prevailing modes of variability and their variation over the time period. Used to quantify and visualize statistically significant changes in ENSO SST anomalies and GW level variance during the historical time period. Continuous Wavelet Transform The Continuous Wavelet Transform analyses localized recurrent oscillations in time series by transforming it into time and frequency space. Cross Wavelet Transform and Wavelet Coherence Transform Cross Wavelet Transform examines whether two time series in regions of time-frequency space share high common power and consistent phase relationship, which might suggest causalty. Three wells under shallow ( 50 ft) and deep overburden conditions (>100 ft) were used to study the fluctuations of groundwater levels under different overburden conditions.

Groundwater level fluctuation with Climate Variability Analysis. Non-parametric Mann-Whitney tests were used to evaluate the impacts of ENSO phases on the medians of GW level anomalies in the ACF for recharge (December-April) and non-recharge seasons (May-November). Twenty one observation wells used for climate variability analysis. The year was specially analyzed to study the effects of strong La Niña events (prolonged droughts) on GW level anomalies. Recovery Period Recovery period was calculated using 3 month running averages. Defined by the time required for GW level anomalies to remain above ft for atleast 6 consecutive 3 month running averages of, after the end of the La Niña phase. For calculation of the recovery periods two particular La Niña events, year and , representing short and prolonged La Niña (prolonged drought) were selected.

Results Time (year) Period (years) (a) (b) (d) (c) Figure 3: Significant Wavelet Power Spectra shown within the cone-of- influence for (a) monthly NINO 3.4 sea surface temperatures ( o C), (b) Groundwater level anomalies (ft) for shallow overburden, (c) Groundwater level anomalies (ft) for moderately deep overburden, (d) Groundwater level anomalies (ft) for deep overburden. Figures are color-mapped to indicate low powers in blue and white and high wavelet power with reds and oranges. Black outlines indicate areas significant to 95% confidence. Wells under deep overburden, did not exhibit any areas with high power. Continuous Wavelet Spectra Well under shallow and moderately deep overburden conditions showed regions of high power, though not statistically significant, in the 3-7 year ENSO periodicities. SST Shallow Moderately Deep Deep

Figure 4: Cross Wavelet Spectrum between NINO 3.4 sea-surface temperatures and monthly groundwater level anomalies (ft) for overburden conditions: (a) shallow, (b) moderately deep, and (c) deep. Wavelet Coherence Analysis between NINO 3.4 sea-surface temperatures and monthly groundwater level anomalies (ft) for (d) shallow, (e) moderately deep, and (f) deep. Arrows indicate variable’s phase relationship. Black outlines indicate areas significant to 95% confidence. Arrows pointing anti-clockwise represents anti-phase behavior, while clockwise arrows indicates in-phase behavior. Cross-Wavelet Analysis and Wavelet Coherence Transform The Cross-Wavelet Analysis and Wavelet Coherence Transform between SST and GW level anomalies shows high shared power in the areas that were seen to be sharing high power in the single wavelet spectra. (a) (d) (b) (e) (c) (f) Time (year) Period (years) Well under deep overburden did not show any shared high and significant power in any period suggesting that groundwater levels under deep overburden conditions are not affected by ENSO. Wells under shallow and moderately deep shared high and significant power in the 3-7 year periodicities and the significant areas within this periodicities are positively phase locked. These areas of shared power in Cross- Wavelet Analysis and Wavelet Coherence Transform suggests causalty. Shallow Moderately Deep Deep

Well-IDEl Niño (ft)La Niña (ft)Diff (ft)p 06F G G K K L L L M H L K L Median Table 1: Mann Whitney test results between ENSO phases and monthly groundwater level anomalies for the entire period of record. P values are significant at 0.01 High level of significance (p- value<0.01). The median of GW level anomalies during the El Niño and La Niña phases were above average and below average respectively. Well 15L020 did not exhibit significant difference due to high overburden conditions. Significant differences were found in GW level anomalies for all wells, except for well 15L020.

Recharge (December-April) Well-IDEl Niño (ft)La Niña (ft)diff (ft)p-value 06F G G K K L L L M H L K L Median Table 2: Mann Whitney test results between ENSO phases and monthly groundwater level anomalies for recharge and non-recharge seasons. p values are significant at Non – Recharge (May–November) Well-IDEl Niño (ft)La Niña (ft)diff (ft)p-value 06F G G K K L L L M H L K L Median

Well_Id RechargeNon Recharge Minimum La NiñaYear La NiñaYear F G G K K L L L M H L K K Mean Table 3: Comparison of monthly averaged groundwater level anomalies for severe ( ) and average La Niña phase during recharge and non-recharge seasons. Average GW level anomalies were approximately twice lower during than average La Niña phase in both the recharge and the non- recharge seasons. Minimum GW level anomalies for year were almost 3 times lower than average La Niña phase values with GW level anomalies going below 10 ft at 8 well locations and below 5 ft at 20 at well locations. Wells 08G001, 08K001 and 13M006 groundwater levels fell to approximately below 15 ft during , which demonstrates the effect of severe and prolonged La Niña can have on groundwater levels.

Well-IDYear 2001Year F G G K K L L L L M L K K Mean222 Table 4: Comparison of recovery periods (months) for prolonged (2001) and short (1989) La Niña phase. Year representing severe La Niña phase shows significantly higher recovery times than the year The average recovery time for year was 22 months as compared to 2 months for Might be due to increased irrigation.

Conclusion Wavelet Analysis showed that wells under shallow and moderately deep overburden conditions exhibit ENSO signals while wells under deep overburden conditions does not exhibit such a relationship. Mann Whitney test results validates the above relationship. GW level anomalies tended to be above average during El Niño phase and below average during La Niña events. ENSO signals are stronger during recharge season than non-recharge. Severe La Niña events can severely affect groundwater resources and their recovery periods thereby threatening sustainability. Results indicate a potential for possible groundwater level prediction with respect to ENSO phases.

Future Research Quantify how pumping for irrigation exacerbates the effect of La Niña on groundwater levels, and Develop procedure for forecasting groundwater levels using ENSO forecasts.