Climate change, whether caused by natural variability or human activity, has important implications for recharge and the sustainability of ground-water resources. Reliable predictions of ground-water sustainability under future climate change will require a better understanding of the role that natural variability caused by climate cycles on interannual to multidecadal timescales has in controlling spatiotemporal changes in recharge. Climate cycles on these timescales has been shown to partially control patterns in precipitation and air temperature, as well as streamflow and drought, which in turn can affect evapotranspiration and ultimately recharge rates and mechanisms. Because these climate-varying conditions can augment or diminish human stresses on ground water, the responses in water levels and ground-water storage can be dramatic when different climate cycles lie coincident in a positive (wet/cool) or negative (dry/warm) phase of variability. Thus, understanding climate cycles on these timescales has particular relevance for management decisions during drought and for ground-water resources close to the limits of sustainability. The objective of this study was to quantify how recharge to the High Plains aquifer (USA) responds to natural climate cycles on interannual to multidecadal timescales. Using singular spectrum analysis of long-term hydrologic time series, the signal of ground-water pumping was removed and natural variations were identified in all tree ring, precipitation, and ground water time series as partially coincident with known climate cycles. These cycles included the El Nino/Southern Oscillation (2 to 6 years), the Pacific Decadal Oscillation (10 to 25 years), and the Atlantic Multidecadal Oscillation (50 to 80 years). Climate-varying recharge and water-level fluctuations were most significantly correlated to the Pacific Decadal Oscillation. Using a novel recharge estimation method, climate varying recharge rates (196 to 476 mm yr^-1) were found to be substantially larger than previous estimates of diffuse recharge (0.2 to 110 mm yr^-1), indicating the importance of preferential flow during recharge to the High Plains aquifer. The results indicate the importance of interdecadal-climate cycles as controls on rates and mechanisms of climate-varying recharge and support the conclusion that understanding natural climate variability is a necessary step toward predicting ground-water response under climate change. Such understanding may help managers to better plan for the long-term sustainability of ground-water resources.

¹ U.S. Geological Survey, Denver, Colorado
² U.S. Geological Survey, San Diego, California