3.3.2 Non-climatic drivers
Many non-climatic drivers affect freshwater resources at the global scale (United Nations, 2003). Water resources, both in quantity and quality, are influenced by land-use change, the construction and management of reservoirs, pollutant emissions, and water and wastewater treatment. Water use is driven by changes in population, food consumption, economic policy (including water pricing), technology, lifestyle, and society’s views of the value of freshwater ecosystems. Vulnerability of freshwater systems to climate change also depends on water management. It can be expected that the paradigm of Integrated Water Resources Management will be increasingly followed around the world (United Nations, 2002; World Bank, 2003; World Water Council, 2006), which will move water, as a resource and a habitat, into the centre of policy making. This is likely to decrease the vulnerability of freshwater systems to climate change.
Chapter 2 (this volume) provides an overview of the future development of non-climatic drivers, including: population, economic activity, land cover, land use, and sea level, and focuses on the SRES scenarios. In this section, assumptions about key freshwater-specific drivers for the 21st century are discussed: reservoir construction and decommissioning, wastewater reuse, desalination, pollutant emissions, wastewater treatment, irrigation, and other water-use drivers.
In developing countries, new reservoirs will be built in the future, even though their number is likely to be small compared with the existing 45,000 large dams (World Commission on Dams, 2000; Scudder, 2005). In developed countries, the number of dams is very likely to remain stable. Furthermore, the issue of dam decommissioning is being discussed in a few developed countries, and some dams have already been removed in France and the USA (Gleick, 2000; Howard, 2000). Consideration of environmental flow requirements may lead to modified reservoir operations so that the human use of the water resources might be restricted.
Increased future wastewater use and desalination are likely mechanisms for increasing water supply in semi-arid and arid regions (Ragab and Prudhomme, 2002; Abufayed et al., 2003). The cost of desalination has been declining, and desalination has been considered as a water supply option for inland towns (Zhou and Tol, 2005). However, there are unresolved concerns about the environmental impacts of impingement and entrainment of marine organisms, the safe disposal of highly concentrated brines that can also contain other chemicals used in the desalination process, and high energy consumption. These have negative impacts on costs and the carbon footprint, and may hamper the expansion of desalination (Cooley et al., 2006).
Wastewater treatment is an important driver of water quality, and an increase in wastewater treatment in both developed and developing countries could improve water quality in the future. In the EU, for example, more efficient wastewater treatment, as required by the Urban Wastewater Directive and the European Water Framework Directive, should lead to a reduction in point-source nutrient inputs to rivers. However, organic micro-pollutants (e.g., endocrine substances) are expected to occur in increasing concentrations in surface waters and groundwater. This is because the production and consumption of chemicals are likely to increase in the future in both developed and developing countries (Daughton, 2004), and several of these pollutants are not removed by current wastewater treatment technology. In developing countries, increases in point emissions of nutrients, heavy metals, and organic micro-pollutants are expected. With heavier rainfall, non-point pollution could increase in all countries.
Global-scale quantitative scenarios of pollutant emissions tend to focus on nitrogen, and the range of plausible futures is large. The scenarios of the Millennium Ecosystem Assessment expect global nitrogen fertiliser use to reach 110 to 140 Mt by 2050 as compared to 90 Mt in 2000 (Millennium Ecosystem Assessment, 2005a). In three of the four scenarios, total nitrogen load increases at the global scale, while in the fourth, TechnoGarden, scenario (similar to the SRES B1 scenario), there is a reduction of atmospheric nitrogen deposition as compared to today, so that the total nitrogen load to the freshwater system would decrease. Diffuse emissions of nutrients and pesticides from agriculture are likely to continue to be an important water quality issue in developed countries, and are very likely to increase in developing countries, thus critically affecting water quality.
The most important drivers of water use are population and economic development, and also changing societal views on the value of water. The latter refers to such issues as the prioritisation of domestic and industrial water supply over irrigation water supply, and the extent to which water-saving technologies and water pricing are adopted. In all four Millennium Ecosystems Assessment scenarios, per capita domestic water use in 2050 is rather similar in all world regions, around 100 m3/yr, i.e., the European average in 2000 (Millennium Ecosystem Assessment, 2005b). This assumes a very strong increase in usage in Sub-Saharan Africa (by a factor of five) and smaller increases elsewhere, except for developed countries (OECD), where per capita domestic water use is expected to decline further (Gleick, 2003). In addition to these scenarios, many other plausible scenarios of future domestic and industrial water use exist which can differ strongly (Seckler et al., 1998; Alcamo et al., 2000, 2003b; Vörösmarty et al., 2000).
The future extent of irrigated areas is the dominant driver of future irrigation water use, together with cropping intensity and irrigation water-use efficiency. According to the Food and Agriculture Organization (FAO) agriculture projections, developing countries (with 75% of the global irrigated area) are likely to expand their irrigated area until 2030 by 0.6%/yr, while the cropping intensity of irrigated land will increase from 1.27 to 1.41 crops/yr, and irrigation water-use efficiency will increase slightly (Bruinsma, 2003). These estimates do not take into account climate change. Most of this expansion is projected to occur in already water-stressed areas, such as southern Asia, northern China, the Near East, and North Africa. A much smaller expansion of irrigated areas, however, is assumed in all four scenarios of the Millennium Ecosystem Assessment, with global growth rates of only 0 to 0.18%/yr until 2050. After 2050, the irrigated area is assumed to stabilise or to slightly decline in all scenarios except Global Orchestration (similar to the SRES A1 scenario) (Millennium Ecosystem Assessment, 2005a).