Interannual to Multidecadal Climate Variability Effects on Sustainable Groundwater for Agriculture
Geosciences Department, San Francisco State University
Climate variability and change have important implications for recharge, discharge, contaminant transport, and the sustainability of groundwater resources for agriculture. Reliable predictions of groundwater sustainability due to climate change will require improved understanding of the climate forcings on interannual to multidecadal timescales. Climate variability on these timescales has been shown to partially control precipitation, air temperature, drought, evapotranspiration, streamflow, recharge, and mobilization of subsurface-chemical reservoirs. Thus, climate variability can augment or diminish human stresses on groundwater and the responses in storage can be dramatic when different climate cycles lie coincident in a positive or negative phase of variability. Understanding climate variability has particular relevance for management decisions during drought and for groundwater resources close to the limits of sustainability. Results will be presented from recent studies on two important agricultural regions of the United States, including the High Plains and Mississippi Embayment regional aquifers to quantify recharge and contaminant transport responses to climate variability on interannual to multidecadal timescales. Using singular spectrum analysis, the signal of groundwater pumping was removed and natural variations were identified in groundwater levels as partially coincident with the El Nino/Southern Oscillation (ENSO) (2- to 6-yr cycle), the Pacific Decadal Oscillation (PDO) (10- to 25-yr cycle), and the Atlantic Multidecadal Oscillation (AMO) (50- to 80-yr cycle). Recharge in both aquifers was most significantly correlated to the PDO. In the High Plains (450,000 km2), climate varying recharge rates (196 to 476 mm/yr) were found to be substantially larger than previous estimates of diffuse recharge (0.2 to 110 mm/yr), indicating the importance of preferential flow and downward displacement of chloride reservoirs during recharge to the High Plains aquifer. In the Mississippi Embayment aquifer (181,000 km2), future PDO and AMO shifts and continued groundwater-pumping trends are predicted to result in 25 to 50-meter declines in water levels. These studies support the conclusion that understanding natural climate variability is necessary toward predicting groundwater response due to climate change.