BIRDSALL-DREISS DISTINGUISHED LECTURE: IMPLICATIONS OF CLIMATE VARIABILITY FOR GROUNDWATER RESOURCES AND WASTE DISPOSAL IN SEMIARID REGIONS – A LOOK AT ECOLOGICAL CONTROLS FROM ANNUAL TO MILLENNIAL TIMESCALES

Understanding impacts of climate variability on groundwater recharge is essential for management of water resources and waste disposal. Water scarcity is a critical issue in semiarid regions and potential contaminant transport by recharge to groundwater is also important because of waste disposal. A key question is how do climate variability and related vegetation dynamics impact groundwater recharge. The role of vegetation dynamics in regulating the impact of climate variability on groundwater recharge was evaluated. Results from a unique field data set from weighing lysimeters (large, soil-filled concrete containers) beneath nonvegetated and vegetated systems in the Mojave Desert, Nevada unequivocally show that vegetation dynamics controls the impact of elevated winter precipitation related to El Nino Southern Oscillation (ENSO) on groundwater recharge. The lysimeter data indicate that rapid increases in vegetation productivity in response to 2.5 times normal winter precipitation reduced soil water storage to half of that in the nonvegetated lysimeter; thereby precluding deep drainage below the root zone that would otherwise result in groundwater recharge. Satellite vegetation data provided regionalization of the “point scale” lysimeter results. Unsaturated zone chloride and pressure data at sites across the southwestern U.S. indicate that similar feedbacks have minimized inter-stream basin-floor recharge since the last glacial period, 10,000–15,000 years ago. Strong correlations between satellite vegetation productivity and interannual precipitation variability related to ENSO in deserts in Australia, South America, and Africa indicate that the processes described in the southwestern U.S. may apply to deserts globally. The two-way coupling between the water cycle and vegetation dynamics is critical in controlling how climate variability influences water resources, with important implications for waste disposal in semi-arid regions.