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4. How can human actions seriously affect water resources?

  • 4.1 How are aquatic ecosystems threatened by sediment in water?
  • 4.2 How can different kinds of pollution affect water resources?
  • 4.3 What are the consequences of excessive water withdrawal?
  • 4.4 How is climate change affecting water resources?

The source document for this Digest states:

A number of forces continue to seriously affect our natural water resources. Many of these are primarily the result of human actions and include ecosystem and landscape changes, sedimentation, pollution, over-abstraction and climate change.

The removal, destruction or impairment of natural ecosystems are among the greatest causes of critical impacts on the sustainability of our natural water resources. This issue is dealt with more broadly in Chapter 5. However, it should be emphasized that the ecosystems with which we interact are directly linked to the well-being of our natural water resources. Although it is difficult to integrate the intricacies of ecosystems into traditional and more hydrologically-based water assessment and management processes, this approach is being strongly advocated in some sectors and scientific domains (e.g. Falkenmark and Rockström, 2004; Figueras et al., 2003; Bergkamp et al., 2003). The basis of this approach is the recognition that each type of landscape change will have its own specific impact, usually directly on ecosystems and directly or indirectly on water resources. The magnitude of the impacts will vary according to the setting’s conditions with a wide range of possible landscape changes. Changes that can occur to landscapes include: forest clearance, crop- or grazing lands replacing grasslands or other natural terrestrial ecosystems, urbanization (leading to changes in infiltration and runoff patterns as well to pollution), wetlands removal or reduction, new roadwork for transportation, and mining in quarries or large-scale open pits.

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 3. Human Impacts, p.136

4.1 How are aquatic ecosystems threatened by sediment in water?

The source document for this Digest states:

Sediments occur in water bodies both naturally and as a result of various human actions. When they occur excessively, they can dramatically change our water resources. Sediments occur in water mainly as a direct response to land-use changes and agricultural practices, although sediment loads can occur naturally in poorly vegetated terrains and most commonly in arid and semi-arid climates following high intensity rainfall. Table 4.4 summarizes the principal sources of excessive sediment loads and identifies the major impacts that this degree of sediment loading can have on aquatic systems and the services that water resources can provide. A recently documented and increasing source of high sediment loads is the construction of new roads in developing countries where little consideration is given to the impacts of such actions on aquatic systems and downstream water supplies. Globally, the effects of excessive sedimentation commonly extend beyond our freshwater systems and threaten coastal habitats, wetlands, fish and coral reefs in marine environments (see Chapter 5). The importance of sediment control should be an integral consideration in any water resources development and protection strategy. UNESCO’s International Sediment Initiative (ISI) project will attempt to improve the understanding of sediment phenomena, and provide better protection of the aquatic and terrestrial environments.

Table 4.4 Major principal sources and impacts of sedimentation

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 3. Human Impacts, 3.a Sedimentation, p.136-137

4.2 How can different kinds of pollution affect water resources?

The source document for this Digest states:

Humans have long used air, land and water resources as ‘sinks’ into which we dispose of the wastes we generate. These disposal practices leave most wastes inadequately treated, thereby causing pollution. This in turn affects precipitation (Box 4.2), surface waters (Box 4.3), and groundwater (Box 4.4), as well as degrading ecosystems (see Chapter 5). The sources of pollution that impact our water resources can develop at different scales (local, regional and global) but can generally be categorized (Table 4.5) according to nine types. Identification of source types and level of pollution is a prerequisite to assessing the risk of the pollution being created to both the aquatic systems and, through that system, to humans and the environment. With the knowledge of the principal sources of the pollution, the appropriate mitigation strategy can be identified to reduce the impact on the water resources.


Atmospheric contamination from industrial plants and vehicle emissions leads to dry and wet deposition. This causes acidic conditions to develop in surface water and groundwater sources and at the same time leads to the destruction of ecosystems. Acid deposition impairs the water quality of lakes and streams by lowering pH levels (i.e. increasing acidity), decreasing acid-neutralizing capacity, and increasing aluminum concentrations. High concentrations of aluminium and increased acidity reduce species diversity and the abundance of aquatic life in many lakes and streams. While fish have received most attention to date, entire food webs are often negatively affected. Despite improvements, it still remains a critical situation that impacts water resources and ecosystems in some developed regions of Europe and in North America. The situation remains an important issue in several developing countries (for example in China, India, Korea, Mexico, South Africa and Viet Nam) where there are typically lower emission controls and inadequate monitoring and evaluation (Bashkin and Radojevic, 2001). In recognition of this, UNEP and the Stockholm Environmental Institute are sponsoring programmes such as RAPIDC (Rapid Air Pollution in Developing Countries) with the aim of identifying sources and sensitive areas and measuring levels of acid rain. Extensive funding from ADB is now being used to source reductions in several Asian nations. The problem has broad transboundary implications as acid rain can get carried over long distances from polluting areas to other countries. For example, Japan is impacted by Korean and Chinese emissions, while Canada, in addition to its own sources, receives substantive emissions from the US.

As reported by Driscoll et al. (2001), there are still impacts to water quality in northeastern US and eastern Canada, even though improved conditions developed after the introduction of the Clean Air Act and its amendments (1992).

41 percent of lakes in the Adirondacks of New York and 15 percent of all lakes in New England exhibit signs of chronic and/or episodic acidification. Only modest improvements in acid-neutralizing capacity have occurred in New England with none in the Adirondacks or Catskills of New York. Elevated concentrations of aluminum have been measured in acid-impacted surface waters throughout the Northeast.

Figure 4.6: Acid rain and its deposition processes


The challenge of how to improve water quality by rehabilitation and protection of lakes, streams, reservoirs, wetlands and related surface water bodies is a growing global concern, typified by the recent European Commission Water Framework Directive (EC, 2000). However, surface water pollution risks, particularly in developing nations, remain relatively widespread. A valuable initial step in identifying the nature and extent of water quality impacts linked to pollution is to distinguish their point (PS) and non-point sources (NPS). PS pollution is commonly linked directly to end-of-pipe releases from industry and municipal wastes. Its control is more direct and quantifiable and in many developed countries its mitigation has been linked to treatment achieving lower contaminant concentrations before discharge. NPS pollution occurs when contaminants from diverse and widely spread sources are transported by runoff into rivers, lakes, wetlands, groundwater and coastal areas. This type of pollution is more difficult to address as there are a large number of sources, for example, varied agricultural areas all of which are using pesticides and nutrients. Today, however, NPS pollution is receiving more attention as its impacts are becoming evident over large areas in lakes, streams and groundwater and can also be linked to the degradation of aquatic freshwater and marine ecosystems.

Further detail on pollution impacts are found in the chapters on human settlements (Chapter 3), agriculture (Chapter 7) and industry (Chapter 8).

Emerging Issues

Only a small percentage of chemicals are regulated locally, nationally or internationally (Daughton 2004). An emerging concern is contaminants in high population settings that are neither traditionally measured nor regulated, for example pharmaceuticals (Wiegel et al. 2004). Reynolds (2003) reports:

Scientists are becoming increasingly concerned about the potential public health impact of environmental contaminants originating from industrial, agricultural, medical and common household practices, i.e., cosmetics, detergents and toiletries. A variety of pharmaceuticals including painkillers, tranquilizers, anti-depressants, antibiotics, birth control pills, estrogen replacement therapies, chemotherapy agents, anti-seizure medications, etc., are finding their way into the environment via human and animal excreta from disposal into the sewage system and from landfill leachate that may impact groundwater supplies. Agricultural practices are a major source and 40 percent of antibiotics manufactured are fed to livestock as growth enhancers. Manure, containing traces of pharmaceuticals, is often spread on land as fertilizer from which it can leach into local streams and rivers.

Reynolds further notes that conventional wastewater treatment is not effective in eliminating the majority of pharmaceutical compounds. Since various contaminants do not always have coincident pollution patterns, single indicators for all contaminants are not effective. Reynolds (2003) suggests that ‘pharmaceutical contamination in the environment will involve both advanced waste and water treatment technologies and source control at the point of entry into the environment … all of which are issues of ongoing scientific research’.]


Protection of groundwater sources is becoming a more widespread global concern as typified by the recent European Commission directive which focuses on preventing rather than cleaning up pollution (EC 2003). Incidents of groundwater pollution arising from human actions, particularly in developing nations, remain relatively widespread and its impacts in terms of degraded water quality are summarized in Zektser and Everett (2004). Throughout the world, most countries’ practices of urbanization, industrial development, agricultural activities and mining enterprises have caused groundwater contamination and its most typical sources are illustrated in Figure 4.8. A 2002 joint World Bank, GWP, WHO and UNESCO online guidance document (Foster et al. 2002) states ‘There is growing evidence of increasing pollution threats to groundwater and some well documented cases of irreversible damage to important aquifers, following many years of widespread public policy neglect’. This guide is supplemented by recommendations in a 2003 joint FAO, UNDESA, IAEA and UNESCO report directly addressing the universal changes needed in groundwater management practice (FAO 2003b) to arrive at more sustainable water development and use.

Groundwater pollution contrasts markedly in terms of the activities and compounds that most commonly cause surface water pollution. In addition, there are completely different controls that govern the contaminant mobility and persistence in the two water systems’ settings. Foster and Kemper (2004), UNEP (2003), FAO (2003b) and Burke and Moench (2000) point out that groundwater management commonly involves a wide range of instruments and measures (technical, process, incentive, legal and enforcement actions/sanctions and awareness raising) to deal with resources that are less visible than those in our surface water bodies.

Mapping groundwater vulnerability Groundwater is less vulnerable to human impacts than surface water. However, once polluted, cleaning it up (remediation) takes a relatively long time (years), is more technically demanding, and can be much more costly. While this has been recognized for several decades (Vrba 1985), this important message has not been adequately or consistently conveyed to the policy-makers or the public. To address this gap, groundwater vulnerability assessment methods are being developed. These emerging ‘vulnerability maps’ have historically been applied to other risks such as flooding and landslides and they can now be used as direct input to water resources and land planning (Vrba and Zaporozek 1994). Results of such studies are absolutely critical where aquifers are used for water supplies and have sensitive ecosystem dependencies. In conjunction with other environmental input, they have become effective instruments used to regulate, manage and take decisions related to impacts from existing and proposed changes in land use, ecosystems and sources of water supplies. Large-scale groundwater vulnerability maps (e.g. France, Germany, Spain, Italy, The Czech Republic, Poland, Russia and Australia) serve as guidelines for land use zoning at national or regional levels.

Table 4.5: Freshwater pollution sources, effects and constituents of concern

The potential impacts from the different pollution types based on the area (scale) affected, the time it takes to contaminate, the time needed to clean up (remediate) a contaminated area, and the links to the major controlling factors are illustrated in Table 4.6 (Peters and Meybeck, 2000). With the exception of pathogenic contaminants, all other forms of pollution can extend to a regional scale. The fact that it takes considerably longer to remediate a contaminated area than to pollute it clearly highlights the need for adopting the precautionary principle and prioritizing protection strategies rather than costly ad-hoc restoration measures.

Table 4.6: Spatial and time scales within which pollution occurs and can be remediated

Developed countries have historically experienced a succession of water quality problems relating to pathogens, eutrophication, heavy metals, acidification, organic compounds and micro-pollutants and sediments from municipal, industrial and agricultural waste sources (Webb, 1999; Meybeck et al., 1989; Revenga and Mock, 2000). In rapidly developing countries – such as Brazil, China and India – similar sequences of water problems have emerged over the last few decades. In other developing countries, water pollution still remains problematic and is one of the single leading causes of poor livelihood and bad health (Lenton, 2004; and see Chapter 6).

Global water quality and pollution information

Assessing water quality enables the natural characteristics of the water to be documented and the extent of the pollution to be determined; however, today monitoring is a more holistic process relating to health and other socio-economic issues. The international compilation of surface water and groundwater quality data sets at a global scale is still in its relative infancy as compared to precipitation or surface water runoff data. Although some facilities have existed for several decades to collect and disseminate this type of data, it has been historically difficult to collect. This is attributable to several reasons. National centres have not always been linked to institutional networks. Most nations are simply not used to providing this information to anyone other than their immediate institutions and users for either national or specific project purposes. In addition, data in many developing countries is not extensive and even where it has been collected, making it publicly available as a data set is frequently not a priority for the already overloaded and meagrely resourced national and subnational water resource institutions. However, progress has been made in the past three years in this area. The GEMS/Water international water quality database4 went online in March 2005 and now has begun to work with a broad range of agencies, NGOs and data quality groups to harmonize the reporting of water data and information. They have established a QA/QC (quality assurance/quality control) programme that includes laboratory evaluations based on a freely available published set of methods that are used by most of the laboratories that report their data to GEMS/Water. GEMS/Water (2005) reports that data is now received from about 1,500 stations globally, including about 100 for lakes and groundwater.

Increased awareness of the need for water quality data to evaluate impacts and design improved water use and reuse strategies in order to meet quality and quantity demands is emerging at national and river-basin levels. Moreover, there is increasing use and future development of shared aquifers and river basins – many of which are being supported extensively by programmes of the GEF (Global Environment Facility) and UNESCO.

[4 See www.gemstat.org  for more information]

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 3. Human Impacts, 3b. Pollution, p.137

Sources of Contemporary Nitrogen Loading (from intro to Section 2 p.117)

Nitrogen actively cycles through the atmosphere, the continental land mass and the world's oceans, and represents a critical nutrient upon which plant, microbial, and animal life depend. Nitrogen, the most abundant gas in the atmosphere, is delivered to watersheds through natural processes including chemical transformation and washout from precipitation as well as biological fixation. The pathways that nitrogen follows as it travels through the environment are complex. Contemporary human activities have greatly accelerated the transport of reactive nitrogen through river basins that ultimately deliver this nutrient into coastal receiving waters (Galloway et al., 2004). Globally there has been a two-fold increase in the delivery of this nutrient to the oceans, with more than ten-fold increases in some rivers draining industrialized regions (Green et al., 2004). These increases arise from the widespread application of fertilizer, animal husbandry and point source sewage inputs.

These human induced changes to the nitrogen cycle have far reaching impacts on water quality and public health, protein supply for humans, and even the planetary heat balance through the emission on nitrogen-based greenhouse gases. The map below shows the predominant source of nitrogen within each grid cell. Fixation is the primary source throughout South America, Africa, Australia, and the northernmost reaches of Asia and North America. Atmospheric pollution and subsequent nitrogen deposition plays a dominant role throughout the industrialized northern temperate zones of Europe, Asia and North America. Fertilizers are the predominant source across major food producing regions. Livestock constitutes the most important source in Eastern Europe and India. Urban sewage loads create localized ‘hotspots’ for pollution. Understanding the patterns of such loadings is critical to the design of management interventions to protect society and well-functioning ecosystems.

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Sources of Contemporary Nitrogen Loading, p.117

4.3 What are the consequences of excessive water withdrawal?

The source document for this Digest states:

The problems of over-abstraction in surface water bodies and groundwater, sometimes tied directly to upstream diversions, reservoirs and deforestation, are well documented. The problems commonly become exacerbated when combined with extended natural dry periods. Notable examples of substantive reductions in large major river flows can be found around the world. Some of the basins suffering from this reduction are: Niger, Nile, Rwizi, Zayandeh-Rud (Africa); Amu Darya, Ganges, Jordan, Lijiang, Syr Darya, Tigris and Euphrates, Yangtze and Yellow (Asia); Murray-Darling (Australia); and Columbia, Colorado, Rio Grande and San Pedro (North America). Examples of lakes and inland sea areas decreasing dramatically in size and volume include: Lakes Balkhash, Drigh, Hamoun, Manchar, and the Aral and Dead Seas (Asia); Lakes Chad, Nakivale and in the Eastern Rift Valley Area, e.g. Nakuru (Africa); Lake Chapala (North America); and Mono Lake and the Salton Sea (North America). Dramatically lowered water levels in aquifers are increasingly reported, for example in the Mexico City and the Floridian and Ogallala aquifers (North America), as well as in China, India, Iran, Pakistan and Yemen (Asia).

Despite years of clear over-use with evident changes in both water and related ecosystem conditions, many of the same causes persist. Among the most prominent are the highly inefficient water supply provisioning practices for agriculture and municipal use, deforestation, and the basic lack of control over exploitation of the actual surface and groundwater sources. Inappropriate development of reservoirs and diversions combined with inadequate considerations of alternatives in conservation and use minimization (demand management) have further complicated and increased the impacts on existing water resources. While there are some hopeful signs of change emerging in selected local actions (see Chapters 5 and 7), these are few in comparison to the broad-based and fundamental modifications needed in national, regional and subnational practices to reverse and counteract these ongoing substantive impacts.

Groundwater over-abstraction represents a special situation as the visual evidence is typically less obvious and the effects are more difficult to recognize and react to. Increased pumping from aquifers has increased globally, particularly during the second half of the twentieth century. While this has produced a number of important benefits, some have been sustainable over only relatively short periods and have had significant negative side-effects (UNEP, 2003; FAO, 2003b; Burke and Moench, 2000). We see, for example, that an initially impressive benefit was experienced in India where shallow groundwater development allowed irrigated land area to be essentially doubled, thereby dramatically increasing food production. However, it also caused momentous changes to local water regimes that resulted in a variety of impacts, including lowered water tables and entirely depleted groundwater resources in some areas. Similar cases from all climatic regions of the world illustrate that over-abstracting groundwater is relatively common. The results of groundwater over-abstraction can be seen in: reduced spring yields; rivers drying up and having poorer water quality because of lowered base-flow contributions; intrusion of saline waters or other poor quality water into the freshwater zones of aquifers; lowered or abandoned productivity as water levels decline in wells; higher production costs from wells or the need to extend underground aqueducts (qanats) as inflow rates decrease; and diminished groundwater-dependent ecosystems, including wetlands, as they become stressed or lose resilience from inadequate water sources. Subsidence is another particularly widespread impact that occurs from excessive over-pumping, with some notable examples in a number of major cities in China, Japan, Mexico and the US. However, this type of impact can be stopped when the over-pumping of the aquifer is discontinued, although the effects are not usually reversible. Llamas and Custodio (2003) provide a recently updated compilation of papers that illustrate the wide-ranging impacts of intensive groundwater exploitation by identifying examples of criteria that have led to over-abstraction actions and by explaining how these criteria can be part of sustainable development strategies.

Map 4.3 introduces a groundwater development indicator that compares the degree of groundwater use in each nation to the volume of estimated recharge. Exploitation, for example of more than 50 percent of recharge, will likely result in particular stress on the aquifer sustainability of groundwater systems. High levels of exploitation are currently taking place in many countries in the Middle East, Southern and Northern Africa, Asia, selected countries in Europe, and in Cuba. In addition, as noted above, parts of China, India, Mexico, Pakistan and the US are also being overexploited in selected regions where there is high aridity and population density. Tracking groundwater use as compared to recharge volumes at national and subnational levels – and particularly for individual aquifers – should be practised and implemented to identify and take corrective action as needed to maintain groundwater development sustainability.

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 3. Human Impacts, 3c. Over-abstraction, p.143

4.4 How is climate change affecting water resources?

The source document for this Digest states:

As noted above, there is empirical evidence of impacts on water resources from global warming. The IPCC, in cooperation with new partners, has begun to address this issue in addition to their more traditional focus on greenhouse gases and temperature changes. A recent IPCC expert meeting (IPCC, 2004, p. 27) identified two issues related to water and the impacts from global warming: one related to impacts and the other to knowledge gaps. These two issues, as taken from the IPCC report, are as follows:

  • ‘The extreme event frequency and magnitude will increase even with a small increase in temperature and will become greater at higher temperatures. The impacts of such events are often large locally and could strongly affect specific sectors and regions. Increased extreme events can cause critical design values or natural thresholds to be exceeded, beyond which the impacts’ magnitudes increase rapidly.’
  • Knowledge gaps related to the water sector were identified as:
    1. Insufficient knowledge of impacts in different parts of the world (especially in developing countries),
    2. Almost complete lack of information on impacts under different development pathways and under different amounts of mitigation,
    3. No clear relationship between climate change and impacts on water resources,
    4. Little analysis of the capacity and cost of adaptation, and
    5. Lack of understanding of how changes in variability affect the water environment.

Arnell (2004) also assessed predicted impacts of both population and climate on water-stressed regions, based on population growth scenarios and climate change models. He concludes:

  • Climate change increases water resources stresses … where runoff decreases, including around the Mediterranean, in parts of Europe, central and southern America, and southern Africa. In other water-stressed parts of the world – particularly in southern and eastern Asia – climate change increases runoff, but this may not be very beneficial in practice because increases tend to come during the wet season and extra water may not be available during the dry season.

However, he further points out that model results differ by up to four times in terms of persons impacted according to different population and climate scenarios.

Shiklamanov and Rodda (2003) conclude that only general predictions and observations have been developed based on the assessments of global warming impacts on water resources to date. They agree with Arnell (2004) that assessments of future water resources can only be obtained by using estimates of possible regional (rather than global) changes in climate (primarily precipitation and temperature by seasons and months). They specify that the existing climate change estimates are extremely unreliable even for the largest regions and river basins. Furthermore, they suggest that the gap in knowledge related to the specific impacts of global warming on water resources is one of the largest scientific challenges in hydrology today.


Land-based and mountain glaciers have generally experienced a worldwide retreat and thinning during the last century. Notably, glacier decline has considerably accelerated on a global basis during recent years (Arendt et al. 2002; Dyurgerov 2003). The mean mass balance decrease that took place during the period 1990–99 was three times greater than that of the previous decade (Frauenfelder et al. 2005). Data for this figure are based on measured changes in glacier mass balance made at thirty glaciers located in nine high mountain regions of Asia, Europe and North and South America.

As a specific country example we can look to China. In 2004, AFP (L’Agence France-Presse) cites renewed concerns of disappearing glaciers being broadcast in Asia, notably in China and Nepal. Yao Tangdong, China’s foremost glaciologist, was quoted in state media as saying, ‘An ecological catastrophe is developing in Tibet because of global warming and that most glaciers in the region could melt away by 2100’. His conclusion was based on the results of a forty-month study by a group of twenty Sino-American scientists which showed separated ice islands that used to be connected with the glaciers at levels above 7,500 m. While Tibet’s glaciers have been receding for the past four decades due to global warming, the rate of decline has increased dramatically since the early 1990s. It was initially thought that the water from the melting glaciers could provide additional water for China’s arid north and west.

However, this hope has not been realized as much of the glacier runoff evaporates long before it reaches the country’s drought-stricken farmers. ‘The human cost could be immense’ states AFP (2004), as 300 million Chinese live in the country’s arid west and depend on the water flowing from the glaciers for their livelihoods.

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 3. Human Impacts, 3d. Global warming and climate change, p.144

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