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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?

Our water resources face a host of serious threats, all of which are caused primarily by human activity. They include sedimentation, pollution, climate change, deforestation, landscape changes, and urban growth.

One of the most serious threats to water resources is the degradation of ecosystems, which often takes place through changes to landscapes such as the clearance of forests, the conversion of natural landscapes to farmland, the growth of cities, the building of roads, and surface mining. Each type of change to a landscape will have its own specific impact, usually directly on natural ecosystems and directly or indirectly on water resources.

Although it is difficult to integrate the intricacies of ecosystems into traditional assessment and management processes a holistic ecosystem approach to water management is strongly recommended. More...

4.1 How are aquatic ecosystems threatened by sediment in water?

Sediments can occur in water bodies naturally, but they are also produced in large amounts as a result of land-use change and agriculture.

Activities such as farming, clearing forests, building roads, and mining can put too much soil and particulate matter in rivers. This sediment can harm plants and animals by carrying toxic chemicals into the water, smothering fish eggs and small organisms used by fish as food, raising water temperature, and reducing the amount of sunlight penetrating the water.

Sediment can also reduce the capacity of reservoirs and make it difficult for ships to navigate in waterways. It can also damage equipment used in water supply installations and hydroelectric plants, thus increasing their maintenance costs.

Table 4.4 Principal sources and impacts of sedimentation

Sedimentation should be factored into strategies to protect water resources More...

4.2 How can different kinds of pollution affect water resources?

Wastes that people dispose of can pollute the air, the land, and water resources. They affect the quality of rainwater and of water resources both above and below ground, and damage natural systems.

The causes of freshwater pollution are varied and include industrial wastes, sewage, runoff from farmland, cities, and factory effluents, and the build-up of sediment.

Emissions from factories and vehicles are released into the air. They can travel long distances before falling to the ground, for instance in the form of acid rain. The emissions create acidic conditions that damage ecosystems, including forests and lakes. The pollution that passes directly into water from factories and cities can be reduced through treatment at source before it is discharged. It is harder to reduce the varied forms of pollution that are carried indirectly, by runoff, from a number of widely spread non-point sources, into freshwater and the sea.

Only a small percentage of chemicals are regulated, and concern is growing about contamination by unregulated chemicals. A variety of pharmaceutical products, such as painkillers and antibiotics, are having an impact on water resources above and below ground. Conventional water treatment does not work for many of them.

In general, it takes much longer to clean up polluted water bodies than for pollution to occur in the first place, and there is thus a need to focus on protecting water resources. In many cases, clean-up takes more than 10 years. Although underground water is less easily polluted than water above ground, cleaning it once it is polluted takes longer and is more difficult and expensive. Ways are being found to assess where and how underground water is most vulnerable to pollution. The findings are important in cases where aquifers supply drinking water, and where natural ecosystems depend on them.

Sewage and runoff from farms, farmlands and gardens can contain nutrients, such as nitrogen and phosphorus, that cause excessive aquatic plant growth, and this in turn has a range of damaging ecological effects.

Complicating the problem of water pollution is the overall lack of adequate information about the quality of water around the world. Many countries do not collect enough data, and most of them are not prepared to sharing it. But this is changing because of growing awareness of the need for such information, and due to the availability of an international database, GEMSTAT, that went online in March 2005. More...

4.3 What are the consequences of excessive water withdrawal?

Around the world certain lakes, rivers and inland seas are in the process of drying up because too much water is being drawn from them or from their tributaries. Groundwater, too, is used faster than it is replenished, as is clear from a growing number of reports documenting sharp drops in aquifer levels. In many cases, drought periods have compounded this well-documented trend.

The Niger, the Nile, the Ganges, the Tigris, the Euphrates, the Yangtze, the Colorado, and the Rio Grande are just some of the major rivers suffering substantial reductions in flow. Numerous lakes and inlands seas are shrinking dramatically in many geographic regions. The Aral Sea and Lake Chad have decreased dramatically in size over the last few decades.

These problems persist though their causes have been evident for quite some time. Foremost are the very inefficient ways in which water is supplied to farms and cities, deforestation, and the failure to properly manage and control the withdrawal of water, and to think of more economic ways to use water.

Not enough consideration has been given to minimizing use and conserving water resources. Instead the supply has been further strained by the construction of new reservoirs and inappropriate diversions. While some towns and cities are taking action, only broad-based and fundamental change in national and regional practices can reverse the impact.

The threat to groundwater is not as obvious as that to lakes and rivers. There is less visual evidence and the effects of withdrawing too much groundwater take longer to recognise. In the last half-century, pumping from aquifers increased globally. But often the benefits—bigger harvests for example—were short-lived, ultimately resulting in lower water tables, drilling of deeper wells, and, sometimes, even the depletion of the groundwater source.

Cases from all climatic regions illustrate that excessive use of groundwater is relatively common practice. The consequences can be seen in reduced spring yields, diminished river flow, poorer water quality, damage to natural habitats such as wetlands, and the gradual sinking of land, known as subsidence. More...

4.4 How is climate change affecting water resources?

Exactly how global warming is affecting water resources is not altogether clear. New research suggests that climate change is increasing existing stress, for example by reducing runoff in areas already suffering from water shortages. Scientists agree that extreme weather events stemming from global warming, such as storms and floods, are likely to be more frequent in the future. However, based on current knowledge, scientists can only make general predictions about the impact of climate change on water resources. More information is needed, for instance, about impacts on water resources in specific regions and under different policy scenarios.

One type of water resource that has been clearly affected by climate change is glaciers. Scientists have long observed that land and mountain glaciers are shrinking, and this trend has accelerated considerably in recent years. For example, it has been predicted that most glaciers in Tibet could melt by 2100. And while it was initially thought that the water released could benefit China’s arid north and west, it now appears that the additional runoff evaporates long before it reaches drought-stricken farmers downstream. More...

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