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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,
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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
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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
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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.
Representative terms from entire chapter:
groundwater recharge
