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Interactions of Groundwater with Climate Water and energy are freely exchanged among the continents, the atmosphere, and the oceans, and these exchanges are, in many ways, the defining characteristics of climate. Groundwater is by far the largest unfrozen stock of fresh water (Maidment, 1993~. In some cases, the physical processes of interaction be- tween groundwater systems and climate are well understood qualitatively, but quantitative and practical consequences are unknown. In other cases, interactions may be speculative or yet undiscovered. Climate and the hydrogeologic environment are the joint controls of groundwater recharge, dis- charge and storage. Amounts and pathways of aquifer recharge differ greatly from humid to arid climatic regions, with consequent influences on groundwater storage and discharge. Temporal changes in climate are reflected in groundwater fluxes, albeit with dampening of high-frequency variations Not so obvious is that groundwater fluxes may have significant reciprocal impacts on climate and cTimate-related aspects of the Earth system. Groundwater fluxes are a component of the surface water bal- ance, which is tightly coupled to atmospheric processes and, thus, to climate. Because groundwater is one of the major reservoirs in the hydrosphere, changes in its volume would also be reflected ultimately by changes in sea level. Groundwater may also play an important, indirect role in determining climate by affecting atmos- pheric composition. Atmospheric concentrations of radiatively active gases such as carbon dioxide and methane are determined partially by exchanges with the continents. Such exchanges may be affected by water fluxes between the atmosphere and land and by biogeochemical processes near the land surface. The latter can be strongly influenced by soil moisture, which may be influenced by groundwater (e.g., high water tables), with consequences for soil aeration, microbiological activity, and greenhouse gas emissions. This is particularly true for northern peatiands. The location of the water table determines whether the peat is sub- jected to aerobic or anaerobic decomposition rates, which are substantially different, and the subsequent re- lease of methane and carbon dioxide." This chapter focuses on three topics: how climate affects groundwater fluxes, how groundwater fluxes affect climate, and how groundwater storage may have potential to indirectly affect the composition of the atmosphere, especially greenhouse gasses. Many kinds of studies within these areas could be done at the benchmark sites recommended in Chapter 2. 32

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interactions of Groundwater with Climate INFLUENCE OF CLIMATE ON GROUNDWATER 33 Groundwater recharge is determined to a large extent as an imbalance at the land surface between precipitation and evaporative demand; the latter depends primarily on the surface radiation balance and also on atmospheric temperature, humidity and windspeed. When precipitation exceeds evaporative demand by an amount sufficient to replenish soil-water storage, any further excess flows deeper into the ground, arriv- ing at the water table as recharge. Groundwater systems, therefore, respond to temporal variations in cli- mate. Because of the relatively slow response of many groundwater systems to changes in forcing (TownIey, 1995; Hait~ema~ 19951~ how. ever ~eY tend to reflect much more the Tow-~eauencY "climate" signal than the _ ~ 7 7 ,~ — — · — ~—————— ~ ~ high-Dequency "weather" fluctuations (e.g., Box 3-~. This tendency contributes to the value of groundwater as a resource by imparting to it a high degree of temporal stability. It also can cause significant lags between climate changes and the resultant changes in groundwater characteristics. Given the relatively long response times of groundwater systems, it is the climatic variations at longer time scales that most strongly influence changes in groundwater discharge. At shorter time scales, groundwater recharge will be affected by the short term variations of precipitation and evapotranspiration, and their relative magnitudes. Long term trends in the balance between precipitation and evapotranspiration (decades to centuries), caused by either long term variability or by anthropogenic global change, can be ex- pected to cause fundamental changes in distributions of groundwater recharge (Vaccaro, 1992) and avail- ability. As an example of the impact of climate warning on recharge, the melting ice sheets may have con- tributed significantly on recharge to confined aquifer systems (e.g. Breemer et al., 2002~. Geochem~cal evi- dence exists suggesting many confined aquifer systems of North America and Europe have experienced high recharge rates via sub-ice sheet recharge during the last glacial maximum (e.g., Siegel, 1991; Siegel and Mandle, ~ 9941. Evidence for this paradigm comes from observations of isotopically light, Tow-saTinity groundwater within the discharge area of the Williston Basin. Grasby et al. (2000) presents compelling ev~- dence that the recharge beneath the Laurentide ice sheet reversed groundwater flow directions dunng the last glacial maximum. Since ice sheets were the dominant land cover of northern latitudes for the last 2 m~- lion years (Pettier, 1998), ice sheet topography may have controlled recharge to deep aquifer systems in Canada and northern North America. In more recent times and at shorter time scales, climate variability and change also affect rates of grounowater recharge and discharge. For example, a rising water table may increase the near-surface soil moisture and streamflow, causing major changes in surface vegetation and ecosystems, in the extreme by creating or expanding wetlands and thereby affecting surface processes that influence recharge/discharge rates. Conversely, a decline in the water table may significantly reduce evapotranspiration and surface water discharges, also affecting vegetation and ecosystems. Changes in groundwater discharge may influence flu- v~al and estuarine habitats. Pumpage from developed aquifers also depend on evapotranspiration rates, growing season lengths, temperatures, and other climatic variations. Groundwater hydrologists recognize these strong controls on croundwater recharge and discharge by climate. It is recognized, for example, that in arid regions much of the water being exploited today comes from aquifers that were recharged at higher rates during wetter or cooler conditions in the past (e.g., Zhu et al., 1998~. PaleocTimate descriptions and data offer an important too] for providing {ong-term de- scriptors between climate variability and groundwater, especially as it affects lake levels. For example, Smith, A. et al. (2002) showed that about 5000 years ago, Elk Lake (Grant County, western Minnesota) dropped over ~ 5 meters in response to the mid-Holocene warm period when the prairie-forest border shifted eastward from the Minnesota-North Dakota into Wisconsin. An extensive quantitative database exists for North America characterizing changes in climate during the Holocene and late Pleistocene using climatic

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34 Groundwater Fluxes Across Interfaces BOX 3-1 Amplification of Seasonal to Century Scale Oscillations in Closed Basins: The Role of Groundwater Fluxes in a Mountain-Front Setting Mountain-front recharge and surface-groundwater interaction have a central role in closed-basin-lake re- sponse, forced by climate fluctuations at seasonal, interdecadal and century time scales. The hydrologically closed mountain and basin systems of the Great Basin Physiographic region of the western United States, and in particular the Great Salt Lake basin, in north-central Utah, creates an ideal environment to study the impact of climate variability on a terrestrial system where subsurface flow and discharge to mountain-front streams are forced by orographic precipitation and basin evaporation. Shun and Duffy (1999) hypothesized that the water cycle within topographically and hydrologically closed landforms of the Great Basin represent a multis- cale averaging of the climate signal. They further hypothesized that dominant time scales of streamflow and lake level are determined by the space-time scales of storage within the system. A conceptual model of the hydrogeology of the Wasatch-Front (Shun and Duffy, 1999) suggests that groundwater may play an important role in the interannual and decadal-scale dynamics of lake levels through stream-aquifer interaction and groundwater discharge. Deep groundwater upwells and discharges to the low end of streams before entering the Great Salt Lake (GSL). The ultimate source of this water is recharge at high elevations through the mountain block and mountain-front, and channel losses across the alluvial depos- its adjacent to the mountain-front. The GSL historical record is one of the longest measured climatic records in North America. The lake volume spectrum for the period 1847-1997 is shown in Figure 3-1. The labeled, filled circles on the graph at the edge of, or beyond, the noise spectrum, represent probable signals amid the noise. While the annual signal shows up strongly, lower-frequency (long period) interdecadal oscillations (i.e., the points labeled 11 and 14 years on Figure 3-1) are also prominent. The rivers along the Wasatch Front have the same spectral signature (not shown) as the low-frequency modes in the Great Salt Lake, suggesting that areal recharge and losing- gaining streams crossing the mountain-front contribute substantial amounts of ungaged flow to deep ground- water in the upper parts of alluvial fans that ultimately returns as baseflow (feedback) to the lower reaches of streams before entering the lake, or as underflow from fractured bedrock to basin sediments. The relation of the time scale of runoff to altitude is further demonstrated with the "noise-removed" pre- cipitation-temperature-runoff phase-plane plots at three elevations in the Wasatch Range (Figure 3-2~. Note that at high elevation, monthly runoff is closely correlated with monthly precipitation and temperature, while at low elevation the dampening effect of upwelling groundwater smears this correlation. Research is neces- sary to explore what this behavior can tell us about unsaturated moisture flow, feedback and coupling of sur- face and groundwater at multiple elevations, and resonance-like effects that may arise from stochastic and pe- riodic forcing in such systems.

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Interactions of Groundwater with Climate Eigenspectrum: Great Salt Lake Volume .~ ~ . ~ ~ :: :::: . ~ :: :~ ~ i.. ::: I: · ~ ~ ~~ ~ . <~' 1 0 CO A= a' it_ Lot . _ CO lo it Frequency(cyclesimonth) FIGURE 3-1 The spectrum for the historical record of the Great Salt Lake produced from bimonthly volume time series 1847-1997. The labeled, filled circles on the graph represent probable signals above or at the upper range of the noise envelope. Aside from the annual and semi-annual signals (indicating the obvious effect of yearly and seasonal changes in temperature and precipitation on runoff), interdecadal oscillations (i.e., the points labeled 11 and 14 years on Figure 3-1) are also prominent. This suggests that the groundwater basin may be amplifying these lower-frequency components of the climate signal. SOURCE: Reprinted, win permission, from Shun and Duffy (1999~. ~1999by American GeophysicalUnion. 3' is Spectral Slope=2.0 ~ . . - ~ . ~ ~ : ~ ~~: ~ ~ . ~ . : : : : : : :: .. 1 0 by: : : :~ :: : : :: :: : :: : : ::: . .~ ~ ~ ~ : :~ ~ :~ :; ~ : ~ . oyr . . I: ~ ~ ,. ~ ~ -I , ~ ~ ~ .i, . ~ ~ i--: . .~ I. ~ . ~ ~ . ~ ~ ~ ~-~ it': ~ :::\: -I: ::::--: ,~ ~ ~ ~4~.8 ~~ :: I. `~ ~ ..* . ~ ~ ~ ~ `~ ~ ~ mica,`_ ~ ~— ~~ i: : ~ ~ ~ ~ ~ ~ : ~ . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~ ~ ~ .~ ~ ~1 ~ ~ ~~ ~~ -a ~ ~ -all ~ : ~ ~ ~~ 1~12yr ~ ~~ ~ ~ : L:1 ::: ~ >,,` ; ,,, ~ ~ ~ —~ — . . . ; _ . `` ~ ~ . ~ . F . . all. ~ ~ . ~ : ~ ~~ ·wi ~ ~ ~ ~ () I' W~ ~~ ~~ ~—,~+ 41~` ' ~ ~ (~ - at!,,, ~ ~ : `` . ~ ~ , ~+ - _ ~ : ~ : I ~~~ ~ He ~ ~ A =$ t ~ it_ ~ ; ~ , ~ 4: ~ ~—A ~~ !_ . a:: : i: . ~ "] A. ~ : ~ ~ ~ ' ~ ~ ~ ~ ~ ~ ~ ~ ~ : ~ - ~~ : ~ : : : -:: : : : a: ~ :: :: :: , . ::: ':::::: ~ ::::: ¢ : · ~ : ~ :~I:~nl~erc ecac aft ~ ~ ~ Ir,- ;~ra~n~n~ue ~ annua 1Q~

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36 Groundwater Fluxes Across Interfaces 25 20 15 10 ~ 0~ -5 150 Boar 1 00 r45. \ :1341 hi a. it. i~ . ~ \ ,f~ W~ 30 ~~ - FIGURE 3-2 The 40yr "Noise-removed" precipitation-temperature-runoff (P_T_Q) phase- plane plot for three elevations along the Wasatch Front. These three variables represent the mountain-front water cycle, with each loop representing one year. At high elevation, oscillation is tightly bound (i.e., monthly runoff is closely correlated win monthly precipitation and tem- perature), reflecting the influence of seasonal conditions, while the low-elevation oscillation for P_T_Q is smeared out, representing the dampening effect of upwelling groundwater. This low- frequency contribution to runoff is derived from the climate but is amplified by the long-time scales of mountain-front recharge and valley discharge. SOURCE: Reprinted, with permission, from Shun and Duffy (1999~. ~ 1999 by American Geophysical Union.

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Interactions of Groundwater with Climate 37 reconstructions based on lake-sediment cores. Quantitative reconstructions of paleoclimate have been de- veloped using spatial and temporal changes in the assemblages of aquatic plant macrofossiTs, ostracodes, diatoms, pollen and chironom~ds, as well as sediment geochemistry and sediment facies information to infer changes in lake levels, paleo lake salinity, precipitation, and temperature (e.g. Bartlein and Whiltiock, 1993; Bernabo and Webb, 1977; Webb et al., 1998; Fritz et al., 2000~. Filby et al. (2002) demonstrated paTeocTi- matic reconstructions based on pollen-transfer functions could reproduce m~-Holocene lake levels (four meters below modem) using a hydrologic model of groundwater-surface calibrated using a modern histori- cal records of hydrologic stresses within the Shingobee watershed in Northern Minnesota. For influence of climate variability on groundwater more recently, lake shore reconstruction through historical data bases can provide important information when linked to historical climate variability. For example, Donovan et al. (2002) used air photo analysis of lake shore lines across a glaciated watershed in Grant County, western Minnesota to document historical declines in lake and aquifer levels by over 5 m in response to drought conditions during the dust bowl (1923-1938~. All but the deepest, lowest elevation lakes completely dried up during this time period. With respect of the influence of potential elects of climate change on major aquifer systems, the above paleo and historical analyses can help in providing a setting for anticipated change, and for the model- ing studies of anticipated change on groundwater systems. For example, Loaiciga et al. (2000) developed alternative scenarios of future temperature and rainfall in south-central Texas based upon projections from seven general circulation models (GCMs) of a doubling of carbon dioxide in the atmosphere. The scenarios of temperature and rainfall were used to calculate recharge of the aquifer, which occurs primarily through seepage of water from streams that cross the aquifer. The impacts of these scenarios on water levels and natural springflows were assessed for different rates of groundwater pumping. Most of the scenarios showed that water resource problems that already exist in the area would be exacerbated. Rosenberg et al. (1999) analyzed the prospects for recharge rates of several regions of the OgalIaTa aquifer under climate change by analyzing climate predictions for 30 different scenarios of three GCMs, including four levels of temperature and three levels of CO2 concentration. A hydrologic model, HUMUS, was coupled in daily time-steps to a vegetation-crop model to estimate irrigation needs and evapotranspira- tion. Other studies include Bouraoui et al.'s (1999) simulations of reduced groundwater recharge near Grenoble, France, mostly due to increases in evaporation during the recharge season, and Vaccaro's (1992) study of the climate sensitivity of groundwater recharge for the Ellensburg basin of the Columbia Plateau in Washington. Another usefi~l approach is to evaluate the response of a groundwater system to climatic condi- tions in the past through an analysis of paleowaters (e.g., Remenda et al., 19941. Despite these and a few other studies, the vulnerability of groundwater systems to climate changes and the implications for society and ecosystems remain poorly explored. Climate variation and change will affect groundwater systems primarily by changing the rates and distributions of recharge of water to aquifers, the discharge of water from aquifers, and the removal of groundwater from aquifers by plants and human activities as near-surface water availability and energy in- puts change. At long time scales, the salinity conditions and discharge at the freshwater/salt water interface of coastal aquifers may (or may not) be in equilibrium with modern ocean levels and conditions (Kohout, et al., 1977; Essaid, 1990; Voss and Andersson, 1993), and a better understanding these conditions and the un- derlying causes would enhance the understanding of climate-scale variability of coastal aquifers. Therefore, a better understanding of how groundwater recharge, discharge and demands vary with climatic fluctuations and change will be the first and most essential requirement for addressing the broader question of climate impacts on both the surface and subsurface hydrology. For changes in the last century, preservation and analysis of groundwater level records in areas un- affected by direct human influences will be particularly useful. Streamflow records also contain information on spatially integrated, historical variations in groundwater discharge and these records should be fully ex-

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38 Groundwater Fluxes Across Interfaces plotted. Baseflow separation techniques can be combined with other data, such as that from chemical trac- ers, to provide increased confidence in physical interpretations. For information over a range of time scales, additional isotopic and geochem~cal studies of groundwater would be useful. The isotopic signature of wa- ter is a Unction of climatic factors such as temperature and humidity, and could inform investigations of natural climatic variations and of anthropogenic climate change (e.g., Shanley et al. 1998~. For regional to continental scales, satellite gravimetry (e.g., NASA's Gravity Recovery and Climate Experiment, or GRACE; see Box 4-~) shows promise for identifying changes in the water table by periodi- cally measuring the Earth's gravity field (Wahr et al., 1998; Rodell and Fam~glietti, 19994. Altimetry tech- niques, such as Interferometnc Synthetic Aperture Radar (InSAR) (Amelung et al., 1999), laser and m~cro- wave altimetry, and global positioning systems (van Dam et al., 2001) permit measurement of vertical dis- placements of the land surface that can be directly indicative of subsurface pressure changes and/or chang- ing groundwater loads. Altimetry may also prove useful for synoptic monitoring of surface-water levels that can be indicative of changes in either discharge or groundwater storage, depending on hydrogeologic framework. Once recharge, discharge, and demand responses to climate variations can be projected with some level of confidence, any number of existing methods and models of groundwater resources and responses to change can be employed to assess the eventual vuinerabilities of groundwater systems (as parts of the natu- ral world and human resource-supply systems) to Tong-term climate change. INFLUENCE OF GROUNDWATER ON CLIMATE Areas of groundwater discharge moisten and coo! the atmosphere, with desert oases providing one example of this phenomenon. Similarly, persistent temporal anomalies in groundwater storage could condi- tion local anomalies in climate, as climate is sensitive to water and energy balances (Yeh et al., 1984; Milly and Dunne, 1994~. The partitioning of precipitation into runoff and evapotranspiration, and the partitioning of available radiation into sensible and latent heat fluxes (directly related to evapotranspiration) into the at- mosphere establish boundary conditions that help drive both the atmospheric and oceanic circulations. The surface-water-balance partitioning is thus at the crux of these controls. To the extent that groundwater systems influence evapotranspiration and runoff, they wall influence climate. From a surface perspective, groundwater flow systems collect recharge from certain parts of the landscape, store it, and redistribute it to other parts of the landscape as discharge. These processes provide non-Iocal and temporally stable sources of water for evapotranspiration and induce runoff of precipitation in groundwater-discharge areas. Thus, spatial and temporal characteristics of groundwater flow systems have the potential to alter the spatial and temporal patterns of water-balance partitioning (Salvucci and Entekhabi, 1995), and ultimately climate. For the same reasons, groundwater fluxes may influence sensitivities of water balance and climate to external forcing, such as land-cover change. Influences on Climate Projections The welI-recognized and central importance of conditions very near the land surface for water- balance partitioning (Milly, 1 994a,b) has perhaps obscured the potential importance of groundwater systems to affect atmospheric interactions. Most atmospheric models used for climate simulation have a one- dimensional representation of "soil moisture" that conceptually represents water in the plant root zone. Of- ten, effects of groundwater must be parameterized in terms of, or "aTiased" into, the "soil moisture" variable. While models may be tuned to account for some resulting biases, the confidence in the ability of the model

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Interactions of Groundwater with Climate 39 to represent sensitivities to climatic variations must be questioned. A few investigators have recently incor- porated the effects of landscape and lateral soil-water flow (i.e., interflow) on water balances (Fam~glietti and Wood, 1994; Stieglitz et al., 1997; Koster et al., 2000~. Continued progress may result from considera- tion of greater time and space scales, beginning perhaps with representation of the permanent water table within a landscape-based framework (Salvucci and Entethabi, 1995; Winter, 2001~. Tn current weather-prediction and climate models, the partitioning of water and heat at all land sur- faces is largely dependent on the availability of water in soil layers that extend down through the root zone (or slightly below). In the past decade, the weather-prediction community has learned that (short term) weather variations and predictions are sensitive to imposed or parameterized soil-moisture conditions (e.g., Atias et al., 1993; Bazaars et al., 1996; Paegle et al., 1996~. Climate models also are sensitive at both local and continental scales, to the parameter~zation and behavior of soil moisture (e.g., Charney, 1975; Xue and Shukla, 1993; Zeng and Neelin, 2000~. Soil moisture, through its influence on the partitioning of heat and moisture fluxes at the land surface, provides potentially important feedbacks affecting continental precipita- tion and temperatures by modulating the Bowen ratio and (on climate time scales) vegetation land cover with its effects on albedo and roughness of land surface, which in turn modulate the forms and intensities of heat and momentum fluxes into the atmosphere on a variety of time scales. The response time of many groundwater systems will greatly exceed that of some types of climate change (e.g., century-scare "greenhouse" global warming). Such systems will provide a stabilizing influ- ence, continuing to deliver discharge that has already been "in the pipeline" for hundreds or thousands of years. These discharges are probably not large enough to be a major influence on global climate. However, groundwater influences at regional and, especially, local scales, where society interacts with groundwater systems, are more uncertain and variable, and therefore are an issue deserving careful evaluation, especially as our climate simulations telescope to increasingly small scales in the near future. Furthermore, given the nonlinear nature of hydrologic and Earth system processes, groundwater may alter the perceptible time scales of those processes (e.g., Duffy, 1996~. Influence of Changes in Groundwater Storage on Sea Level Rise Sea-level rise during recent decades has been estimated to be I.0-2.0 mm/y (IPCC, 2001), and the true Value may be near the upper end of this range (Douglas and Peltier, 2002~. Such a large rise is not fully explained by expansion due to ocean warming and glacier- and icecap-storage changes. Because groundwa- ter is one of the major reservoirs in the hydrosphere, changes in its volume are likely to be reflected in the ocean volume. For example, a comprehensive tabulation of groundwater from the Mississippi River basin (Milly and Dunne, 2001), including much of the High Plains aquifer, suggests a contribution to sea-level rise on the order of 0.02 mm/y from groundwater-leve] changes in that basin. Groundwater mining thus could be a contributor to changes in ocean mass. Crude estimates of the global scope of mining (Gornitz, 2000) support this hypothesis, but leave a wide range of uncertainty. Extensive drainage of wetlands during the past century also may have contributed to sea-level rise, as may natural fluctuations in groundwater storage driven by climatic variability (P. C. D. Milly, USGS, written common., 20034. The following are needed in order to examine the consequences and importance of feedback mecha- nisms between groundwater and climate: (1) Monitoring of groundwater-storage changes at large scales and over long observational periods as an indicator of climate and water cycle change on the largest scales. This can be done by detecting small changes in land-surface elevation (InSAR) and gravitational potential (Rodell and Famiglietti, 1999; 2001; 2002), properly validated with ground-based physical or geophysical measurements of groundwater level.

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40 Groundwater Fluxes Across Interfaces (2) Parameterizations of the groundwater function in global climate models for numerical experi- ments and climate projections. Key ingredients would include realistic storage parameters and landscape partitioning into recharge and discharge areas. The degree of availability of groundwater for evaporation and uptake by plant roots should also be represented in climate models. Ideally the "nonlocal" (from a sur- face perspective) distribution of recharge from one area to discharge in another would be included in some models. Development of such parameterizations and models requires regional to global scale maps of groundwater recharge and discharge areas, depths to water table, and estimates of evapotranspiration and groundwater discharge. (3) Large-scaTe experimental watersheds with simultaneous surface and groundwater monitoring in varying climates and with varying aquifer characteristics, as described in chapter 2. (4) Identification of areas that have undergone significant land cover/land use changes and analysis of resulting ground-water/surface-water/atmospheric water-balance changes. Comparisons should be made between responses by stand-alone surface water/groundwater models driven by prescribed climate varia- tions and responses by coupled atmosphere/ surface water/groundwater models. A promising beginning to this - a coupled aquifer-land surface-atmosphere model to study aquifer-atmosphere interactions on decadal timescales was documented by York et al. (2002) and Gutowski et al. (2002~. In the watershed studied by York et al. (2002), during periods of persistent drought much of the evapokanspiration that occurred in re- gional discharge areas was apparently supported by groundwater. (5) Comprehensive regional, continental, and global tabulations of groundwater withdrawals, in or- der to lead to improved estimates of the contribution of groundwater mining to sea level rise. Such research would also have intrinsic value to water-resource managers. Areas that have undergone significant altera- tions in water balance through groundwater pumping (e.g., for irrigation) should be identified and the changes in water balance and meteorological (e.g., precipitation) impacts assessed. EFFECTS ON ATMOSPHERIC COMPOSITION Atmospheric composition determines the atmospheric radiation balance, which determines climate. To the extent that groundwater systems affect atmospheric composition, they will also influence global cli- mate. As humans change global cycling of water, energy, carbon and other chemicals, chemical fluxes at the groundwater interface will change; as humans change groundwater systems, they also may inadvertently change the global geochem~cal balances directly or indirectly (through the influence of groundwater change on vegetation, riverine, and coastal-ocean systems). Groundwater may influence atmospheric composition in two ways: (1) by modifying the "natural" atmospheric composition, and (2) by driving and modulating contemporary anthropogenic changes in atmospheric composition. Of importance, then, are the generally long residence times in groundwater systems, which can range from decades to centuries and more. Changes in chemical composition of groundwater only start to show up in groundwater discharge after many years, and continue for many decades and centuries (Weiss- mann et al., 2002) after atmospheric inputs may have changed. Exchange of radiatively active gases, such as carbon dioxide and methane, between the atmosphere and land is sensitive to water fluxes and to the "wetness" and thermal state of the land (which depends in many settings upon groundwater discharge) and is important in global geochem~cal balances (Crill et al., 1992; Moore and RouTet, 1993; Romanowicz et al., 1993; Siegel et al., 1995~. One of the major problems related to annual global carbon fluxes is an apparent deficit of ~2 giga- tons (Gt) C/yr. in some budgets (e.g., Sun~quist, 1993~. Dissolved carbon in precipitation that recharges groundwater systems dissolves minerals by hydrolysis (Equation 1~. In this process, the dissolved CO2 is sequestered as dissolved inorganic carbon, the amount depending on the mineralogical composition, the ex-

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Multiplying Me above terms out Carbon sequestered as dissolved carbon (lug) Carbon sequestered as dissolved carbon (kg) Carbon sequestered as dissolved carbon (Gt) (1 G~FlOE+12 kg) Percent of budget deficit SOURCE: D. Siegel, Syracuse University, written communication, 2003. Interactions of Groundwc~ter with Climate TABLE 3-1 Estimation of Carbon Sequestration in Groundwater Per Meter Rise in Water Table Area of Continents (m ) 7.5E+13 Liters/m3 Estimated HC03 concentration (mgC/L) Porosity (assumed) Increase in water table (m) 4 1 1,000 37.3 0.2 5.6E+17 5.6E+1 1 0.56 ~30. tent to which the groundwater system is open or closed to soil CO2 replenishment, reaction rates, and resi- dence time. Equation I. Carbonate and silicate minerals +n CO2 + H2O = dissolved inorganic carbon (DTC) + base cations + silica ~ clay minerals. A simple calculation (Table 3-~) shows that the amount of soil CO2 that can be temporarily seques- tered in ground water is very large. Assume that 1) the water table rises ~ meter over half of terrestrial earth, 2) the concentration of dissolved inorganic carbon is 37.3 mgC/L of ground water (the arithmetic average of data in White et al., 1963), and 3) the porosity of shallow unconsolidated aquifers is ~20 percent. Multiplying the terms together produces ~30 percent of the carbon deficit per meter rise of the wa- ter table. Obviously, most of the terms in this crude estimate are unknown. More carbon would be seques- tered in groundwater if the water table on average rises more, or if dissolved inorganic carbon concentra- tions were closer to equilibrium with carbonate rn~nerals at typical open conditions. Groundwater at a typi- cal soil PCO2 of 0.01 amino spheres, under open conditions and equilibrated with calcite, contains about four times as much dissolved carbon as is found in the same volume of soil gas (based on calculations made with the geochemical reaction model PHREEQC, http://wWwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/; D. Siegel, Syracuse University, written commun., 2003~. Of course, if the average porosity were less, less car- bon would be sequestered In ground water where the water table rises. In dry places where the water table drops because of the lack of recharge and evaporation, dissolved carbon can also be sequestered as precipi- tates in the soil zone. The simple mass estimates above suggest that carbon sequestering in groundwater recharge may play an important role in the global cycling of carbon at a time scale important to climate change. It is pos- sible that an assumption of steady-state with respect to carbon storage in ground water systems is not appro- priate. Anthropogenically sequestering carbon in deep groundwater systems by injection is a major current topic of research (McPherson and Lichtner, 2003~. However, little has been done with respect to evaluating how carbon is naturally sequestered and lost in shallow groundwater flow systems. Research is needed to address the role of groundwater in (1) determining the "natural" atmospheric composition, and (2) driving and modulating contemporary anthropogenic changes in atmospheric composi- tion. Global (or, at least, large scale) quantification of the historical and projected volumes of groundwater mined, changes in water table elevations, and estimates of water- and geochem~cal throughputs in ground- water systems wall be building blocks for studies to address these issues.