In the United Nations Millennium Declaration, adopted in September 2000, and during the Johannesburg Earth Summit held in 2002, an initiative, known as the "Water for Life" decade, was announced (Gardiner 2002). The primary goal of this was to contribute to a more sustainable use of global water resources, with particular emphasis on the access to safe drinking water. The initiative is justified by the fact that today close to 1 billion people do not have access to drinking water of a reliable quality (Diamond 2005). In addition, the number of people with poor access to proper sanitation was recently unveiled at the World Water Week 2008 in Stockholm to be over 2.5 billion. Despite the announcement of ambitious plans to halve the number of people with poor access to safe drinking water by 2015, it has become increasingly evident that these goals will not be reached. One reason is the absence of clear incentives for different nations to implement such plans. This may be further restricted by lack of wealth, rapid population growth, urbanization, climate change, and economic development, among other factors. Above all, however, better quantification and understanding of the natural water cycle is needed on local, regional, and global scales as a basis for action.

Water is by far the most abundant substance on our planet's surface (Berner and Berner 1996). It is stored on the continents, in the marine environment, and in the atmosphere. The key compartments for storage on the continents are ice and snow, aquifers, surface waters, soil moisture, the biosphere, water bound in non-aquiferous rock formations, and juvenile water released through rock-forming processes. Today's global water resources are thought to have formed from meteorites, vaporization, and subsequent condensation during Earth's early stages of formation (Berner and Berner 1996; Shiklomanov and Rodda 2003). The total estimated volume of available global water ranges between 1300-1500 million km3 (Berner and Berner 1996; Shiklomanov 1996; Shiklomanov and Rodda 2003; UNEP 2008; UNESCO 2003). However, the quantifi cation of water resources needs to focus on freshwater for potential human use. For this, one commonly cited estimate describes the stocks of freshwater with an estimated volume of 35-40 million km3 (UNESCO 2003). Among these, almost 70% are stored in the form of ice and snow, with the remaining part attributed predominantly to groundwater resources. Foster and Chilton (2003) outline that globally groundwater provides 50% of drinking water, 40% of industrial water, and 20% of the water used for irrigation. Other figures produced by Struckmeier (2008) and UNEP indicate that today more than 25% of the world population (i.e., 1.5-2 billion people) relies on ground-water with expected future rapid growth (UNEP 1996).

Struckmeier et al. (2005) estimated renewable volumes of useable water to be about 43,000 km3 yr-1. This number seems plausible as it roughly matches the outcome of global water balance models by Doll and Fiedler (2008). Compared to this, current and future estimates of total global water use differ greatly. For instance, the annual global withdrawal of all types of freshwater is estimated at 4000 km3, approximately 17% of the volume of Lake Baikal, which has a volume of 23,600 km3 and is globally the largest open freshwater reservoir. Many experts predict an increase in annual global freshwater withdrawals to about 5300 km3 by 2025 (Seiler and Gat 2007; Shiklomanov and Rodda 2003). The latter number corresponds roughly to 22% of the volumes of Lake Baikal. Other studies estimate the annual need of freshwater withdrawals to be 12,000 km3 by 2050 (Nature 2008), approximately the volume of Lake Superior or about 51% of Lake Baikal (Figure 12.1).

One other promising avenue to stabilize the freshwater supply is through water desalination. It is considered to be an increasingly attractive option to compensate for excessive large dams, pipelines, or canals but needs careful consideration in terms of energy efficiency (Schiermeier 2008). Several promising technological solutions for producing freshwater include forward osmosis, aligned carbon nanotubes, and polymer membranes. Nonetheless,

Withdrawal t; 12,000 km3

Withdrawal t; 12,000 km3

Figure 12.1 Scenarios of annual global freshwater withdrawals compared with the volume of Lake Baikal. Red shows the approximate magnitude of freshwater withdrawal.

on average these techniques are still more expensive than freshwater supply through groundwater extraction (Schiermeier 2008; Shannon et al. 2008). For instance, Diamond (2005) stated that such techniques remain 3.5 times more expensive than pumping groundwater from aquifers. Moreover, the problem of brine and other residues formed during the process of desalination needs to be taken into account. Residual brines and salts need to be stored, or else their concentrations need to be diluted to make them harmless to ecosystems. Finally, even though the highest population density exists in coastal areas, saltwater is often not available in remote areas within the continents, thus rendering local desalination impossible in these regions.

Although globally, humankind seems to consume less water than is being renewed by global continental precipitation and large reservoirs of freshwater exist, projected shortages of available and good quality water are expected to worsen in the future. This is rooted in the fact that water renewal and storage is unevenly distributed on Earth. For instance, arid regions suffer most from water shortages, but even areas with good water renewal rates are increasingly affected by water shortages if hygienic and chemical qualities decrease. In addition, the need for freshwater increases steadily with future population growth, agricultural as well as industrial development, and generally higher living standards. Our principal aim in this chapter is to provide an overview of global water resources. Among the various storage compartments, ground-water currently represents the most plausible water resource for human use, as it is comparatively accessible at low costs and often requires only little or no further treatment. Thus, focus will be put on its quantification and expected future pressures on this valuable resource, including protection and water management. Methods of detection and quantification are outlined in Appendix 2.

Comparison of Global Water Estimates

The variety of methodological approaches and the diversity of input parameters allow only rough estimates, such as the above-mentioned 1300-1500 million km3 of globally available water. Ocean water represents about 96-97.5% of this total water volume. The remaining 2.5-4%, or 35-50 million km3, is attributable to freshwater resources that can be subdivided into ice and snow (~ 2-3% of globally available water) as well as surface water and groundwater (~ 0.5-1%). The volumes of water stored in the atmosphere amount to only 0.013 km3, thus making up only 0.033% of the global freshwater stocks. The estimates of volumes attributed to each of the freshwater storage compartments differ considerably, depending on the source of data and methods of estimation.

Figure 16.1 (Kanae, this volume) presents a merged annual global water balance based on UNEP (2002a) and Seiler and Gat (2007). The figure also shows an annual global precipitation of 0.5 million km3 yr-1, with almost 80% attributed to precipitation that occurs over the oceans (0.39 million km3 yr-1). Evapotranspiration from the land surface is set to 0.07 million km3 yr-1. Assuming a steady state system, the difference between continental precipitation and continental evapotranspiration (0.11-0.07 million km3 yr-1) makes up the global runoff from continents (0.04 million km3 yr-1). It can be subdivided into surface runoff (estimated as 55% or 0.022 million km3 yr-1) (Seiler and Gat 2007) and subsurface or groundwater discharge into the oceans (estimated as 45% or 0.018 million km3 yr-1). The sum of groundwater and surface water discharge to the oceans (0.04 million km3 yr-1) makes up the difference between evaporation from the oceans (0.43 million km3 yr-1) and precipitation over the oceans (0.39 million km3 yr-1). The latter closes the cycle of the global water balance. These numbers roughly match the figures by Kanae (this volume)

Table 12.1 provides various estimates for fluxes such as global precipitation, evapotranspiration, surface and subsurface runoff, and available stocks. It shows that numbers of precipitation over the oceans generated by UNEP (2008) and Seiler and Gat (2007) differ by 15-25% from those provided by

Table 12.1 Selected estimates of the fluxes and stocks within the global water balance. Numbers are given in million km3 yr 1 for fluxes and in million km3 for stocks.
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