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'Berner and Berner (1996); 2Van der Leeden (1975); 3Marcinek et al. (1996); 4Mook and de Vries (2000); 5Oki and Kanae. (2006); 6Seiler and Gat (2007); 7UNEP (2008); 8Lvovitch (1970); "Nace (1971); 10Jones (1997); "Schwartz and Zhang (2003); 12UNESCO (2003); 13Shiklomanov (1996); 14Gleick (1993); 15Shiklomanov and Rodda (2003)

'Berner and Berner (1996); 2Van der Leeden (1975); 3Marcinek et al. (1996); 4Mook and de Vries (2000); 5Oki and Kanae. (2006); 6Seiler and Gat (2007); 7UNEP (2008); 8Lvovitch (1970); "Nace (1971); 10Jones (1997); "Schwartz and Zhang (2003); 12UNESCO (2003); 13Shiklomanov (1996); 14Gleick (1993); 15Shiklomanov and Rodda (2003)

others. Also the values of precipitation and evaporation over the ocean provided by van der Leeden (1975) are more than 25% lower than the others. Such estimates of fluxes over the ocean vary considerably because they are often based on few measurements and have been constructed with the help of models. On the other hand, the numbers for subsurface runoff presented by Oki and Kanae (2006) are double when compared to Seiler and Gat (2007), and estimates of water stocks in the form of ice and snow by Berner and Berner (1996) are 1.8 times higher than most others. Differences may originate from difficulties to determine storage of water in the atmosphere due to its short residence times. Also note that surface runoff is mostly known from the largest rivers that have gauging stations. Most of these are summarized, for example, in the Global Runoff Data Centre (GRDC 2008). Smaller rivers, which in sum discharge considerable amounts of water to the oceans, are often not taken into account.

Estimates of global saline and fresh groundwater stocks differ greatly, with the lowest value of 4 million and the highest of 23.4 million km3. Thus, the total magnitude of variance reaches 585%. This is due to large uncertainties in volumetric evaluations of subsurface waters, which are only visible at piezometers, and wells or through high-resolution geophysical techniques, which often only apply to small scales. In addition, groundwater discharge to surface waters, particularly from coastal aquifers to the oceans, is only vaguely quantified, and on large scales the numbers are often calculated by difference between continental recharge via precipitation and discharge via rivers. While groundwater is likely present in Earth's crust to depths of several kilometers, it can be considered to be useable only to a few hundred meters below the surface. Beyond this depth, efforts for abstraction render groundwater use uneconomic in most cases, while salinities increase as well. Exact critical depths for groundwater use are often difficult to determine, and pore spaces of aquifers often remain poorly quantified. In addition, the depth to which groundwater is abstracted often depends on the urgency to supply water. For instance, in arid regions (e.g., Saudi Arabia and northern Africa), wells can reach depths of several thousand meters or more.

Most of the global water cycle is controlled by natural factors such as evaporation, transpiration, precipitation, and runoff. Nevertheless, human activities may cause yet unknown influences on global and regional water balances. Globally, humans may influence water stocks and fluxes through climate change; regionally their impact may be through reshaping waterways (e.g., damming, river channeling, subsurface constructions), groundwater abstraction, irrigation, or by re-injecting water to aquifers. The reduction of water quality through pollution may further influence useable water stocks. To date, these factors of the global water balance are diffi cult to quantify, but there are numerous examples for human influences on water quality and quantity (Diamond 2005). Such impacts are expected to grow with increasing world population and the growing water demands that are associated with technological and societal developments. Therefore the anthropogenic causes and effects on the water balance require much attention in the future. This is particularly important for groundwater with its smaller capabilities of re-naturation, due to slow flows and long memory effects, and its expected rapid increase of use.

Groundwater and Its Position in Global Water Balance

Next to food production, groundwater often serves as an important source for drinking water supply (Struckmeier et al. 2005). In some cases, groundwater is of limited use for human consumption and needs additional pretreatment measures and techniques, such as sanitation, filtration, and desalinization. Without such measures, untreated water may still be of use for industrial applications (Arad and Olshina 1984).

Groundwater represents 96-97% of easily accessible freshwater (Seiler and Gat 2007). About 53% of the entire continental surface (excluding Antarctica) is underlain by aquifers with major groundwater resources (Struckmeier and Richts 2008). The remaining 47% contain minor occurrences of groundwater, predominantly entrapped in the upper subsurface compartments (Struckmeier et al. 2005). Groundwater flow and recharge depends on hydrogeological characteristics of the surface and subsurface, climatic and atmospheric processes, and water regimes of lakes, streams, rivers, and wetlands (Freeze and Cherry 1979). The quantification of groundwater availability, flow, and recharge is possible with the help of direct measurements, which are carried out predominantly on local scales. These can lead to models on regional scales; however, the generation of accurate numbers of groundwater volumes on regional and continental scales remains challenging for several reasons (Balek 1989; Seiler and Gat 2007):

• Uncertainties in estimates of the volume of pore space in subsurface.

• Limited availability and reliability of data on recharge, runoff, and groundwater levels. This is especially true in countries that do not have sufficient resources for measurement networks.

• Difficulties in determining the groundwater table distance from the surface over larger areas and its long-term and annual variance.

• Unknown quantities and directions of flow of groundwater-surface water exchange, particularly for marine and coastal systems.

• Limited information on long-term effects of human impact on groundwater.

• Unknown volumes of groundwater with limited usability and/or access entrapped in deep aquifers and under ocean floors.

Although about 60% of global groundwater resources are stored at depths greater than 1 km (Arnell 2002), the useable part of the continental groundwa-ter is predominantly represented by water in more shallow aquifers. The closer to the surface, the more useable but more vulnerable the groundwater becomes.

The average depth of abstraction is less than 100 m below ground but can reach several thousand meters for deep and confined aquifers. Shallow groundwater usually has "modern meteoric origin"; it is formed as a result of rainfall with subsequent infiltration (i.e., groundwater recharge). Modeling studies show that today shallow continental groundwaters receive approximately 85% of the total recharge, whereas the remaining 15% of precipitation may reach deep groundwater (Seiler and Gat 2007). Due to the regular recharge by rainfall events, continental shallow groundwater has relatively short mean turnover times, from weeks to decades (Nace 1971). This residence time depends on the remoteness of the recharge area from the location of groundwater discharge (i.e., springs, rivers, lakes, wetlands, and the ocean). Estimates of the average velocity of shallow groundwater movement toward the discharge are on the order of several meters per day or slower, depending on the aquifer and type of solution (Seiler and Gat 2007). By contrast, mean transport times of deep groundwater vary between centuries and thousands of years, with average velocities of a few meters or even millimeters per year or less. For instance, large-scale numerical models by Lemieux et al. (2008) showed that deep groundwater dynamics in North America are still affected by Pleistocene glaciations (> 10,000 years ago).

Groundwater volumes and fluxes are also influenced by human activities such as abstraction, fundament measures, and channeling of rivers. During recent decades, humans have caused drastic changes in global water balances through the generation of considerable volumes of wastewater, heavy abstraction ofwater resources, and manipulations with soil and vegetation (Falkenmark et al. 1999). The consequences are often irreversible for centuries. Excessive abstraction of groundwater, for example, has caused depletion of aquifer systems in regions of Australia, India, China, Latin America, and Northern Africa (Diamond 2005) and has, in turn, often resulted in unpredictable inflows of groundwater from neighboring aquifers. Such growing anthropogenic pressures are likely to affect groundwater even more in the future. Areas of particularly high anthropogenic pressures are urbanized and agricultural regions. These areas deserve particular attention for sustainable water management.

In arid climates, nonrenewable groundwater constitutes a significant share of water resources and is often the only source of water. Increased demands of water for agricultural and industrial purposes in these areas render the issue of nonrenewable groundwater usage even more acute. Here, alternative principles of sustainable groundwater management need to be developed to avoid conflicts on the basis of the future water supply and to ensure preservation of nonrenewable groundwater from rapid depletion (Foster and Loucks 2006).

The regional distribution of groundwater and its recharge strongly depends on climatic factors. Precipitation provides water that further recharges ground-water through infi ltration, whereas evapotranspiration reduces the volumes of the recharge. High evapotranspiration rates in arid climates can prevent groundwater recharge or even withdraw some water from groundwater stocks via capillary forces or deep-rooted plants that reach the groundwater table. On the other hand, porosities and transmissivities of the unsaturated zone define how much water infi ltrates and how much becomes overland and interflow. Generally, regions with moderate and humid climates have higher rates of groundwater recharge and produce more overland flow. Conversely, areas with moderate precipitation volumes of less than 200 mm yr-1 occupy approximately 15% of Earth's continental surface. Often, evapotranspiration in such arid climates accounts for up to 97% of the precipitated water. In contrast, values for evapotranspiration in humid climates, such as Western Europe, return up to 62% of the overall precipitation to the atmosphere.

While discharge from open water toward groundwater is possible (Lerner et al. 1990; Tremolieres et al. 1994), one can assume that groundwater generally feeds rivers, lakes, streams, and wetlands via baseflow that moves toward these morphologically lower structures. Such baseflow is often the only source of river recharge in arid climates (Struckmeier et al. 2005). Note that the quantity of groundwater which discharges directly into the ocean has not been well quantified to date (Dragoni and Sukhija 2008; Moore et al. 2008).

Struckmeier et al. (2005) have estimated annual global groundwater abstraction between 600-700 km3 for 2001. Although this number corresponds only to about 15% of total global freshwater consumption, it makes groundwater the most mined environmental resource compared to other commodities such as gravel, coal, or oil (Zektser and Everett 2004). Often, excessive ground-water removal occurs in regions where replenishment is poor. For instance, Falkenmark (2007) found that up to 25% of India's harvests rely on aquifers from which annual groundwater abstraction exceeds recharge. Nevertheless, some of these abstracted volumes are not lost but are instead recycled to groundwater. Depending on the evapotranspirative capacity of plants, the infiltration capacity of the soil, and the irrigation method, up to about half of the water used for irrigation can be assumed to infiltrate back to the groundwater. In contrast, for drinking, household, and industrial water, one can assume that most of the used water is discharged to surface water systems.

Water Quality

Many urbanized areas in the world rely on groundwater abstraction to secure their water supply. As a result, the magnitude of groundwater withdrawals has often reached critical values in and around cities (Potter and Colman 2003). The largest cities of the world (e.g., Mexico City, Bangkok, Beijing, and Shanghai) have caused decreases of groundwater levels that reach 10-50 m (Foster et al. 1998). Often, this affects the quality of the water. For instance, leaking sewer systems and use of pesticides and fertilizers in green zones put additional pressures on subsurface waters in and around cities as well as in agricultural areas. This demonstrates that overexploitation of water often runs parallel with pollution. Shallow groundwater is particularly susceptible to contamination. Although soils, rocks, and minerals in the subsurface zone above and below the groundwater table may filter out, retain, or degrade many of the pollutants, more soluble compounds (e.g., pesticides, nitrate) may reach groundwater wells. In addition, floods may cause groundwater pollution through, for instance, the mobilization of the contents of septic tanks or contaminated areas near rivers. Groundwater contamination may, however, also occur naturally if aquifer materials have high concentrations of, for instance, arsenic, boron, or selenium. Table 12.2 lists the major sources of groundwater contamination.

The major components of groundwater quality deterioration are represented by a wide range of microbes, viruses, heavy metals, organo-metallic compounds, organic pollutants, and fertilizers (Fetter 1999). Furthermore, ground-water salinization is a widespread problem, particularly in arid regions and areas with intensive agriculture or downstream of dams. In addition, in many coastal zones, excessive groundwater abstraction has caused seawater intrusions that further reduce the quality of groundwater (Shannon et al. 2008). As a result, seawater that entered coastal aquifers compromises their use for drinking water. This situation has worsened after irrigation mobilized fertilizers and pesticides into the groundwater. Similar examples can be found elsewhere, especially in countries where few resources are available for planning sustainable water management and agriculture (Diamond 2005). To counteract such problems, artificial recharge is applied for restoring aquifers. However, care must be taken to ensure that groundwater levels do not approach the surface, as this enables excessive evaporation and may eventually render soils saline. Nonetheless, numerous examples of successful artificial groundwater recharge with pretreated rainwater exist, for instance, in India, Israel, Germany, Sweden, and the Netherlands (Fairless 2008).

Summary and Conclusions

To date, global freshwater resources and fluxes have not been clearly quantified, and large differences exist between various published estimates. This complicates future projections of natural water stocks and compromises local and large-scale planning of water resources. Such planning is crucial when accepting that global water demands will increase. Such increases can be expected due to higher rates of irrigation, industrial, and household uses of water commensurate with growing populations and societal developments. Therefore, factors such as climate change and growing anthropogenic influences on water quantity and quality need to be taken into account. These include large-scale reservoirs, re-channeling of streams, expansion of urban centers as well as chemical and microbial loading from landfills, sewerage, agriculture, industry, and households.

Table 12.2 Major anthropogenic sources of groundwater contamination, modified after Fetter (1988).

Origin Anthropogenic Groundwater Contamination Sources

Municipal Industrial Agricultural Individual

Table 12.2 Major anthropogenic sources of groundwater contamination, modified after Fetter (1988).

Origin Anthropogenic Groundwater Contamination Sources

Municipal Industrial Agricultural Individual

Air pollution

Air pollution

Air pollution

Air pollution

Landfills

Landfills

Fertilizers

Garbage

Surface or

Industrial sites

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