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5 Findings and Recommendations This chapter summarizes the major findings related to the three primary themes of the work- shop and recommends directions for future research. Finding ~ Our ability to quantify spatial and temporal variability in recharge and discharge is inadequate and must be improved given the importance of groundwater in the hydrologic cycle, the contribution of groundwater to base flow in streams and inflow to lakes, and society's reliance upon groundwater for water supply. Moreover, the spatial distribution of recharge fluxes influences the vulnerability of aqui- fers to contamination and the discharge of groundwater into wetlands influences associated ecological and biogeochemical processes. A key science question is how landscape heterogeneity controls spatial and temporal variability of recharge and discharge. Addressing this question will require consideration of geology, biology and climate including variability in soils, topography and vegetation. There are no uniformly applicable methods for measuring and quantifying recharge/discharge fluxes in space and time, so our understand- ing of distribution and process is limited (chapter 2~. Recommendation 1-1 Experimental benchmark sites should be established with the goal of improving both measure- ment techniques and the understanding of the processes of groundwater recharge and discharge. These sites should include a wide range of geologic, climatic and landscape types and should be integrated with existing NSF, USDA/ARS and similar experimental watersheds. The proposed experimental benchmark sites program should also work cooperatively with field programs connected with large- scale hydrocTimatic studies for example, the WCRP Global Energy and Water Experiment (GEWEX) and with studies conducted under the NSF-supported CUAHSI (Consortium of Universities for the Ad- vancement of Hydrologic Science, Tnc.) initiative. Recommendation 1-2 A study/workshop should be initiated with the goal of developing scientific and implementation plans for such experimental benchmark sites, perhaps as part of CUAHSI. Such an activity would de- terrnine which sites would be most valuable to improving the science of groundwater discharge and research, the relevant science questions specific to particular sites, the range of measurement and mod- 56

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i' Findings and Recommendations 57 cling that would be undertaken and an evaluation of the historical data available for designing experi- ments. Finding 2 The roles of groundwater storage, and recharge and discharge fluxes in the climate system are under-appreciated and poorly understood. Because groundwater is the largest reservoir of fresh water in the hydrologic cycle, characterization of the linkage between groundwater and climate is crucial. Groundwater plays an important role in the carbon cycle and related subsurface biogeochemical proc- esses, and therefore the variability and fluctuation in groundwater levels can influence climate. For example, the net accumulation (or depletion) of peat (and the sequestration or release of its stored car- bon) depends on the depth to the water table, and whether peat is under aerobic or anaerobic conditions. Climate change may cause changes in the temporal and spatial distributions of groundwater recharge and discharge and, therefore, availability of the groundwater resource. Better understanding of the linkage between groundwater resources and paleoclimatic conditions would be helpful in understanding past climate and its variability and would supplement information derived from study of tree rings and ice cores. Recommendation 2-1 Research should address the relationship between Tong-term fluctuations in groundwater levels in aquifers at a regional scale and climatic variability. Such efforts would include the preservation and study of historical data on groundwater levels, and related hydrologic data such as streamflow records and lake levels, in areas unaffected by direct human influence. These efforts should include a broad range of techniques including paTeocTimatic research such as reconstruction of paleolake levels and iso- tope geochemistry of old groundwater to provide insights into climatic variables such as paleo- temperature. Recommendation 2-2 Research is needed to allow for better representation of groundwater processes in climate mod- els, including more realistic storage parameters, landscape partitioning into recharge and discharge ar- eas, groundwater uptake by vegetation, and fluxes to wetlands, lakes and streams. Data from the benchmark sites discussed under Recommendation 1-1 above could be utilized to test the improved parameterizations. Recommendation 2-3 A better understanding of the effects of human use of groundwater for water supply on climate s needed. This would require comprehensive tabulation of regional, continental and global groundwa- ter withdrawals and the extent of the area of wetlands drained during the past century accompanied by evaluation of the effects of the withdrawals and drainage on climate

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58 Finding 3 Groundwater Fluxes Across Interfaces Groundwater measurements are needed across a range of temporal and spatial scales; measure- ments at one scale are often needed to address questions at another scale. For example, remote sensing techniques provide extensive, spatially complete data sets that hold promise for addressing many of the unresolved questions identified in this report. However, these data often provide information at large regional scales, like the information soon to be available from the NASA-supported micro-gravity mis- sion GRACE (Gravity Recovery and Climate Experiment), and must be integrated with information generated at smaller scales. This will require an understanding on how groundwater processes scale spatially and temporally. But it is unclear how the variability measured at small scales will change as we move up in scale, and whether there are thresholds of continuity or uniformity that correlate with practical scales of measurements. Recommendation 3-1 A broad and coherent strategy for the observation of groundwater recharge and discharge across scales is needed. This would involve the development of sensors that measure recharge and discharge at "point" scales, research to increase our understanding of the scaling of these measurements and un- derlying processes, the development of procedures for integrating measurements and observations across scales, and generation of mathematical tools to assimilate and synthesize observations at all scales into groundwater process models. Such a strategy could be tested on both the benchmark sites (Recommendation 1-~) and on aquifers of regional extent. It is hoped that this report will lead to progress in understanding the spatial and temporal vari- ability in diffuse and focused groundwater recharge and discharge, the interaction of groundwater with the climate system, and the spatial and temporal scales of recharge and discharge fluxes. Improved un- derstanding is needed for sustainable utilization of groundwater resources, ecologically sound man- agement of wetlands, lakes and watersheds, and to understand, predict and cope with the effects of po- tential climate change.