Page 292 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

256 part II The Water, Weather, and Climate Systems
electricity generation in the Pacific Northwest. Dams have already been built on the best multipurpose dam sites, with numerous detrimental consequences for river environments. Many of the largest hydropower projects are old, and overall production of hydropower is declin- ing. Chapter 15 examines the environmental effects of dams and reservoirs on river ecosystems and reports on recent dam removals. Worldwide, however, hydropower is increasing; several large projects are proposed and under construction in Brazil alone.
Water Transfer Projects The transfer of water over long distances in pipelines and aqueducts is especially im- portant in dry regions where the most dependable water resources are far from population centres. The California Water Project is a system of storage reservoirs, aqueducts, and pumping plants that rearranges the water budget in the state: Water distribution over time is altered by hold- ing back winter runoff for release in summer, and water distribution over space is altered by pumping water from the northern to the southern parts of the state. Completed in 1971, the 1207-km-long California Aqueduct is a “river” flowing from the Sacramento River delta to the Los Angeles region, servicing irrigated agriculture in the San Joaquin Valley along the way. The Central Arizona Project, another system of aqueducts, moves water from the Colorado River to the cities of Phoenix and Tucson (Figure 9.15).
▲Figure 9.15 The Central Arizona Project. A Central Arizona Project canal transports water through the desert west of Phoenix. Given the declining discharge in the Colorado River, the CAP an- nounced the probability that they will cut back on water deliveries beginning as early as January 2016. [Central Arizona Project.]
China is currently constructing a massive infrastruc- ture of water transfer canals to bring water to the north- ern, industrialized regions of the country. With a goal of completion by 2050, the project will construct three main diversions that link four major rivers and will displace hundreds of thousands of people.
Wetlands
A wetland is an area that is permanently or seasonally sat- urated with water and characterised by vegetation adapted to gleysolic soils (soils saturated long enough to develop anaerobic, or “oxygen-free,” conditions). The water found in wetlands can be freshwater or saltwater (Chapter 16 discusses saltwater wetlands). Marshes, swamps, bogs, and peatlands (bog areas composed of peat, or partly de- cayed vegetation) are types of freshwater wetlands that occur worldwide along river channels and lakeshores, in surface depressions such as prairie potholes in the U.S. Great Plains region, and in the cool, lowland, high-latitude regions of Canada, Alaska, and Siberia. Figure 9.13 shows the global distribution of some major wetlands.
Large wetlands are important sources of freshwater and recharge groundwater supplies. When rivers flow over their banks, wetlands absorb and distribute the floodwaters. For example, the Amazon River floodplain is a major wetland that stores water and mitigates flood- ing within the river system—the river and its associated wetlands provide about one-fifth of the freshwater flow- ing into the world’s oceans. Wetlands are also significant for improving water quality by trapping sediment and removing nutrients and pollutants. In fact, constructed wetlands are increasingly used globally for water purifi- cation. (See Chapter 19 for more on freshwater wetlands.)
Groundwater Resources
Although groundwater lies beneath the surface, beyond the soil-moisture zone and the reach of most plant roots, it is an important part of the hydrologic cycle. In fact, it is the largest potential freshwater source on Earth— larger than all surface lakes and streams combined. In the region from the soil-moisture zone to a depth of 4 km worldwide is an amount of water totaling some 8340000 km3, a volume comparable to 70 times all the freshwater lakes in the world. Groundwater is not an in- dependent source of water; it is tied to surface supplies for recharge through pores in soil and rock. An important
Georeport 9.3 Satellite GRACE Enables Groundwater Measurements
Groundwater is a difficult resource to study and measure since it lies hidden beneath Earth’s surface. In 2002, NASA launched its Gravity Recovery and Climate Experiment (GRACE) satellites to measure Earth’s gravity field by noting the tini-
est changes associated with changes in Earth’s mass. Scientists are now using GRACE data to study changes in groundwater storage on land. Recent analyses reveal water table declines in northern India as large as 33 cm between 2002 and 2008, almost entirely caused by human use, which is depleting the resource more quickly than it can recharge. In the Middle East, GRACE data show that reservoir volumes have declined precipitously, and scientists attribute the cause to groundwater pumping by humans. GRACE has given scientists a “scale in the sky” with which to track groundwater changes—yielding information that may prove critical for initiating action on water conservation in these regions.