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Executive Summary Atmospheric, surface and subsurface portions of the hydrologic system are three dynamically linked water reservoirs having distinctly different time and space scales. Many challenges remain in understanding and measuring the dynamic interchange among these reservoirs, especially for interchanges with the subsur- face. Most subsurface storage of water is in the groundwater reservoir, with a small amount of water stored as soil moisture in the overlying unsaturated zone. Fluxes to and from the groundwater zone are called, respectively, recharge and discharge. Natural groundwater recharge has several origins. The most important of these are the flux of water across the water table from precipitation that percolates through the unsaturated zone, and the influx of water from a bound- ing or overlying surface water body including rivers, lakes, wetlands and the ocean. Natural groundwater discharge is the efflux of water from the groundwater reservoir to surface water, or to the land surface itself where, for example, it may return to the atmosphere through evaporation and transpiration. PROBLEM DEFINITION AND MOTIVATION Much attention has been given to both the theory and measurement of groundwater fluxes to and from surface water bodies, yet there are still many difficulties in obtaining accurate estimates of the spatial and temporal distribution of these fluxes, including fluxes to and from rivers and streams, reservoirs and lakes, wetlands, and the ocean. Similarly, there are no standard procedures for measuring recharge to the groundwater system from precipitation. The problems in measurement arise in part because of the diffuse nature and spatially large extent of most groundwater discharge and recharge areas. Challenges in addressing issues related to groundwater fluxes also arise because researchers are based in many different disciplines including soil science, hydrol- ogy, oceanography, geochemistry, geophysics, and wetland ecology. Estimates of recharge/discharge fluxes are needed at many different scales and for many different purposes, including evaluating local risks of landslides, basin-scale sustainable use and management of groundwater resources, management of nuclear waste, and global budgets of water and greenhouse gasses for climate studies. Various scientific committees and federal agencies have identified groundwater fluxes at interfaces as a priority area for research. To respond to this need, the Committee on Hydrologic Science (COHS) convened a workshop on groundwater fluxes across interfaces in Egg Harbor, Wisconsin, in May 2002. Among other tasks, the participants were asked to assess the state of knowledge and science needs concerning three general issues related to groundwater fluxes: diffuse vs. focused recharge/discharge fluxes,
Groundwater Fluxes Across Interfaces climate feedback functions, and spatial and temporal scales. The outcome of the workshop formed the fiame- work for this report. STATEMENT OF TASK The purpose of this report is to call attention to the importance of groundwater fluxes, to explore the potential of new technologies to measure or estimate these fluxes, and to identify research gaps and the po- tential for interdisciplinary collaboration. The report is not meant to be a comprehensive analysis of all is- sues related to groundwater fluxes but instead focuses on the following questions, drawn from a broader set of issues (see preface) that workshop participants were asked to consider: I. Diffuse vs. focused recharge and discharge . What is the relative importance of diffuse versus focused recharge/discharge in various hydro- geologic settings? · Is fresh groundwater discharge a significant source of fresh water recycling to estuaries and the oceans? climate? 2. Interactions of groundwater with climate Do groundwater recharge and discharge processes provide feedback mechanisms that affect What are the important time scales for groundwater reservoirs affecting continental and global water balances, and how are they controlled by fluxes and storage? What is the magnitude of the effect of fluctuations of sea level and levels of large lakes (e.g., the Great Lakes) on groundwater recharge/discharge? 3. Spatial and temporal scales of recharge and discharge . How do estimates of groundwater recharge/discharge aggregate when averaged over different scales and what implications does this have for measurement scale? · How accurately can recharge/discharge patterns/rates be estimated at a regional or national scale, and how might uncertainty in these patterns/rates vary with spatial and temporal scale and geographic location? FINDINGS AND RECOMMENDATIONS Finding I 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 ground- water 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 aquifers to contamination
Executive Summary and the discharge of groundwater into wetlands influences associated ecological and biogeochem~cal proc- esses. 3 A key science question is how landscape heterogeneity controls spatial and temporal variability of recharge and discharge. Addressing this question will require consideration of the geology, biology and cTi- mate 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 understanding of dishibu- tion and process is limited. Recommendation 1-1 Experimental benchmark sites should be established with the goal of improving both measurement 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 pro- gram should also work cooperatively with field programs connected with large-scale hydrocTimatic stud- ies for example, the WCRP Global Energy and Water Experiment (GEWEX) and with studies conducted under the NSF-supported CUAH:SI (Consortium of Universities for the Advancement of Hydrologic Sci- ence, Tnc.) initiative. Recommendation I-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 CUAHST. Such an activity would deter- mine 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 modeling that would be undertaken and an evaluation of the historical data available for designing experiments. Finding 2 The roles of groundwater storage, and recharge and discharge fluxes in the climate system are un- der-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 processes, and therefore the variability and fluctuation In groundwater levels can influence climate. For ex- ample, the net accumulation (or depletion) of peat (and the sequestration or release of its stored carbon) de- pends 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 paTeocTimatic conditions would be helpful in understanding past climate and its variability and would supplement information provided by study of tree rings and ice cores. Recommendation 2-1 Research should address the relationship between long-te~ 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 lev- els, in areas unaffected by direct human influence. These efforts should include a broad range of techniques
4 Groundwater Fluxes Across Interfaces including paleoclimatic research such as reconstruction of paleolake levels and isotope 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 models, including more realistic storage parameters, landscape partitioning into recharge and discharge areas, 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 is needed. This would require comprehensive tabulation of regional, continental and global groundwater withdrawals and the extent of the area of wetlands drained during the past century accompanied by evalua- tion of the effects of the withdrawals and drainage on climate Finding 3 Groundwater measurements are needed across a range of temporal and spatial scales; measurements 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 mission 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-l 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 underlying 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 initially be tested on both the benchmark sites (Recommendation I- 1) and on aquifers of regional extent. It is hoped that this report wit] lead to progress in understanding the spatial and temporal variability 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 understanding is needed for sustainable utilization of groundwater resources, ecologically sound management of wetlands, lakes and watersheds, and to understand, predict and cope with the effects of potential climate change.
