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Global Water Supply
Canadians expect to get clean water when we turn on the tap. Many countries don’t have enough water, they are “water-stressed”. Water stress results when there is less than 1700 m3 per year, of water per person.
Water stress also depends on the ratio of the volume of water withdrawn to the volume of water potentially available. Using more than 20% of the total renewable water resources causes water stress. Withdrawals of 40% or more result in high stress.
The consequences of water stress depend on water management. For example, water may be drawn for irrigation from an underground aquifer faster than the water cycle can replace it.
Climate change will affect water supply by affecting precipitation patterns across the globe.
In 1990, approximately 30% of the world’s population lived in countries using more than 20% of their water resources. Scientists predict that by 2025 the percentage of the world’s population in this situation will grow to 60%. This reflects an increase in population and does not take into account climate change.
By the 2020’s about half a billion people could see increased water stress as a result of climate change.
Figure 1 National water resources per capita (m3 per year), in 1990.
Blue diamonds: 1990 water resources.
Pink bars: 2050 resources with no climate change.
Black bars: 2050 resources under different climate change scenarios.
ACTIVITY 1 Questions
1. Which countries are already stressed according to the definition of having less than 1700 m3 per year of water, per person?
2. What may happen to the level of water stress in these countries by 2050 without climate change?
3. By 2050, which countries not currently stressed, will become stressed, simply because of population growth?
4. Which countries will have reduced water supplies in 2050 under all climate change scenarios (all black bars are below the pink bar)?
5. Name any countries which will have increased water resources in 2050 with climate change.
6. Convert 1700 m3 to litres.
7. In 1996 Canadians used 343 litres per person per day. Convert that to an annual volume. Are we under water stress?
8. Investigate your own family’s usage. Calculate your family’s annual volume and determine your per capita usage. Are you above or below the Canadian average?
Global Water Quality
Water quality in a lake, river or aquifer depends on several characteristics: chemical, biological and physical.
Chemical characteristics are determined by dissolved materials that come from the air, from the river or lakebed, and from the receiving body of water.
Biological characteristics are defined by the living plants and animals in the water.
Physical characteristics include temperature, sediment, and colour.
Pollution can be broadly defined as deterioration of some aspect of the chemical, physical, or biological characteristics of water (its ‘quality’) to such an extent that it affects our use of that water, or affects ecosystems within the water.
Major water pollutants include:
1. Organic material, such as sewage. This causes oxygen deficiency in water bodies. Bacteria consume oxygen as they decompose the organic matter.
2. Nutrients, such as nitrates and phosphates. They ‘fertilize’ the water, causing excessive growth of algae (algal blooms). The algae block light from submerged plants, and cause oxygen depletion when they die and decompose. Some algal blooms can also be toxic to other organisms.
3. Heavy metals, such as lead and mercury, and organic compounds such as insecticides that are toxic to living organisms.
The severity of water pollution depends on two factors: the concentration of the pollutants and the ability of the receiving body of water to absorb or degrade the pollutant. Not all pollutants can be degraded. For example, DDT (dichloro-diphenyl-trichloroethane) persists for a long time. It is banned in most countries.
Chemical river water quality depends on three factors: the amount of chemicals (chemical load), water temperature, and the volume of flow.
1. Chemical Load: The chemical load is affected by the geology of the surrounding land, how the land is used, and other human activities. Agriculture, industry, and public water use may be pollution agents.
Effect of climate change: A changing climate may change agricultural practices. It also may change chemical processes in the soil, including chemical weathering. Temperature and precipitation increase can cause an increase in streamwater alkalinity. Load also is influenced by the processes by which water reaches the river channel. Nitrates, for example, frequently are flushed into rivers in intense storms following prolonged dry periods.
2. Temperature: River water temperature depends not only on atmospheric temperature but also on wind and solar radiation. Biological and chemical processes in river water depend on water temperature.
Effect of climate change: Dissolved oxygen concentrations are lower in warmer water. Higher temperatures encourage the growth of algal blooms, which consume oxygen on decomposition. Cold or cool water fish species like pike and trout cannot tolerate low levels of oxygen so they are replaced by species like carp and catfish.
3. Volume of Flow: Streamwater quality is affected by streamflow volumes. This affects both concentration of substances and the total loads.
Effect of climate change: For a given level of inputs, a reduction in streamflow might lead to increases in peak concentrations of certain chemical compounds. In other cases, a reduction in runoff into a river results in reduced nitrate and phosphate amounts. Nutrient loadings to receiving coastal zones vary primarily with streamflow volume. Increased heavy rainfall decreases water quality by increasing pollutant loads flushed into rivers and possibly by causing overflows of sewers and waste storage facilities. Polluting material also may be washed into rivers and lakes following flooding of waste sites and other facilities located on floodplains.
Lakes form layers of different temperatures in the summer. In colder regions, this layering also occurs in winter. Spring and fall turnovers distribute nutrients and gases.
Effect of climate change: In simulations, increases in lake water temperature are slightly below the increase in air temperature. Increases in temperature speed up oxygen-consuming biological activities and decrease the saturation concentration of dissolved oxygen. This affects the species of fish that can live in the lake. Fish that tolerate low levels of oxygen, like carp, replace species like trout that need high oxygen concentrations.
Water quality in many rivers, lakes, and aquifers, however, depends on direct and indirect human activities. Land-use and agricultural practices greatly affect water quality. Water management of point and non-point sources of pollution and treatment of wastewaters are also critical. In such water bodies, future water quality will depend on human activities, including water management policies. The direct effect of climate change may be small compared to the effect of human activity.
Pollutants that dissolve in fat and that do not degrade easily, can enter the food chain. This is how it works:
1. An insecticide is sprayed on a marsh to control mosquitoes. Trace amounts of the compound accumulate in the cells of the plankton in the marsh.
2. Filter-feeders, like clams and some fish, take in the insecticide with their food. The concentration of insecticide is now 10 times greater than the concentration in the plankton because the filter-feeders take in more than their own mass in food.
3. The process of concentration goes up the food chain from one trophic level to the next. Gulls, which feed on clams, may accumulate insecticide 40 or more times the concentration in their prey. There is now a 400-fold increase in concentration along the length of this short food chain.
This is exactly how DDT affected the food chain and why it was banned in Canada and the U.S.A. (See Figure 1.) Some carnivores at the ends of longer food chains (ospreys, pelicans, falcons, and eagles) suffered serious declines in reproduction because of biomagnification.
Figure 1. Biomagnification of DDT with concentrations in parts per million (ppm)
ACTIVITY 2 Extension Activity:
Interview someone who has a pond on their property a neighbour, farmer, garden centre owner, or parks and recreation department employee. Discuss the requirements for maintaining the health of the pond and balancing the populations living in this habitat. What species inhabit the pond? How difficult is it to maintain this ecosystem? What does the balance depend on? Report your findings in a poster or brochure or as suggested by your teacher.