15-2 Supply, Renewal and Use of Water Resources
How Much Fresh Water Is Available? Natural Recycling to the Rescue
Only about 0.014% of the Earth’s total volume of water is easily available to us as soil moisture, usable groundwater, water vapour, and lakes and streams. Fortunately, the world’s fresh water supply is continuously collected, purified, recycled, and distributed in the solar-powered hydrologic cycle.
What Is Surface Water? Water on Top
One of our most precious resources is fresh water that flows across the Earth’s land surface and into the world’s rivers, streams, lakes, wetlands, and estuaries.
The precipitation that does not infiltrate the ground or return to the atmosphere by evaporation is called surface runoff; The region from which surface water drains into a river, lake, wetland, or other body of water is called its watershed or drainage basin.
About two-thirds of the world’s annual runoff is lost by seasonal floods and is not available for human use. The remaining one-third is reliable runoff: the amount of runoff we can count on as a stable source of water.
What Is Groundwater? Water Down Below
Some precipitation infiltrates the ground and percolates downward through voids in soil and rock. The water in these spaces is called groundwater—one of our most important sources of fresh water.
Close to the surface in the zone of aeration, the pores of the soil contain a mixture of air and some water. Lower layers of soil where the spaces are completely filled with water make up the zone of saturation.
Deeper down are geological layers called aquifers: porous, water-saturated layers of sand, gravel, or bed- rock through which groundwater flows.
An aquifer that is bounded only by the impermeable layer below it is called an unconfined aquifer; the water is not under pressure and a well drawing water from such an aquifer would have to lift or pump the water to the surface.
An aquifer that is bounded by both upper and lower layers of impermeable material is called a confined aquifer; the water is often under high pres- sure and drilling through the uppermost impermeable layer will result in a self-flowing artesian well.
How Much of the World’s Reliable Water Supply Are We Withdrawing? Taking Half Now and More Later
We are using more than half of the world’s reliable runoff of surface water and could be using 70–90% by 2025.
Withdrawal is the total amount of water we remove from a river, lake, or aquifer for any purpose.
For example, most water withdrawn from a river or lake to help cool power plants may be returned to its source. However, this input of heated water can disrupt aquatic life, a phenomenon known as thermal pollution.
Consumptive water use occurs when water withdrawn is not available for reuse in the basin from which it was removed—mostly because of losses such as evaporation, seepage into the ground, transport to another area, or contamination.
How Do We Use the World’s Fresh Water? Watering Crops Is Number One
Worldwide, irrigation is the biggest user of water (70%), followed by industries (19%), and cities and residences (11%).
Uses of withdrawn water vary from one region or country to another. Canada uses relatively little irrigation for agriculture, choosing instead to grow crops such as wheat and sunflowers that are well suited to the dry Prairies.
The United States, on the other hand, has elected to use extensive irrigation to grow crops such as corn and soybeans in the arid Midwest. China uses widespread irrigation for water-intensive crops such as rice.
Both Canada and the United States use large amounts of water for industry; more than three-quarters of this water is used for coding power plants.
Case Study: Water Resources in Canada— Abundant but Not Problem-Free
Canada has relatively abundant supplies of water. Our water supply begins as precipitation that is distributed unevenly across the country. Precipitation is highest in the western and eastern parts of the country, and lowest in the Arctic and in the southern Prairies.
Additionally, Canada has millions of lakes, including the myriad rocky lakes that dot the Canadian Shield, the icy green lakes of British Columbia, and the Great Lakes that carry runoff from Canada’s land surfaces to the surrounding oceans.
The management of specific bodies of water is carried out by groups such as the Lake Winnipeg Stewardship Board, the Fraser Basin Council, and Various Conservation authorities.
Despite its relatively abundant water supply, Canada faces problems related to distribution of water, water pollution, and impacts of water use.
Much of the best soil for agriculture is in the Prairies; yet there is relatively little water there and the region is subject to periodic droughts. Crops can be aided by irrigation, but overuse of groundwater can deplete aquifers, cause soil subsidence, and lead to soil salinization when the water evaporates leaving salts behind.