National Water-Quality Assessment (NAWQA) Program - High Plains Regional Groundwater (HPGW) Study

CLIMATE VARIABILITY CONTROLS ON UNSATURATED WATER AND CHEMICAL MOVEMENT, HIGH PLAINS AQUIFER, UNITED STATES

By Jason J. Gurdak1, R.T. Hanson2, Peter B. McMahon1, Bretton W. Bruce1, John E. McCray3, Geoffrey D. Thyne3, and R.C. Reedy4

Responses in the vadose zone and ground-water to interannual, interdecadal, and multidecadal climate variability have important implications for ground-water resource sustainability, yet are poorly documented and not well understood in most aquifers of the United States. This investigation systematically examines the role of interannual to multidecadal climate variability on ground-water levels, deep infiltration (3 to 23 m) events, and downward displacement (> 1 m) of chloride and nitrate reservoirs in thick (15 to 50 m) vadose zones across the regionally extensive High Plains aquifer. Such vadose-zone responses are unexpected across much of the aquifer given a priori unsaturated total-potential profiles indicate upward water movement from the water table toward the root zone, mean annual potential evapotranspiration exceeds mean annual precipitation, and millennia-scale evapoconcentration results in substantial vadose-zone chloride and nitrate reservoirs. Using singular spectral analysis (SSA) to reconstruct precipitation and ground-water level time-series components, variability was identified in all time series as partially coincident with known climate cycles, such as the Pacific Decadal Oscillation (PDO) (10-25 years) and the El NiƱo/Southern Oscillation (ENSO) (2-6 years). Using these lag-correlated hydrologic time series, a new method is demonstrated to estimate climate-varying unsaturated water flux. The results suggest the importance of interannual to interdecadal climate variability on water-flux estimation in thick vadose zones and provided better understanding of the climate-induced transients responsible for the observed deep-infiltration and chemical-mobilization events. Based on these results, we discuss implications for climate-related sustainability of the High Plains aquifer.

1 U.S. Geological Survey, Denver, Colorado
2 U.S. Geological Survey, San Diego, California
3 Colorado School of Mines, Golden, Colorado
4 Bureau of Economic Geology, Austin, Texas

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