Ice Caps and Glaciers About 215 percent of the worlds total surface water but

Ice caps and glaciers about 215 percent of the worlds

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detail later in this chapter. Ice Caps and Glaciers About 2.15 percent of the world's total surface water, but more than 70 percent of the freshwater, is held as ice in ice caps and glaciers. This water is mostly contained within the ice cap and glaciers of Antarctica and for all practical purposes is unavailable for human use (Figure 11.10). Proposals to tow large icebergs to water-deficient areas such as the Middle East have been proposed, but have not yet resulted in any significant financial backing or serious efforts to carry out the proposals. In the short term, the amount of water held in glaciers and ice caps may be considered constant, but during the Pleistocene Age about 25,000 years ago, at the height of the most recent Ice Age, the amount of water held as ice in these regions was as much as 50 percent greater than at present. During the major glacial advances, more of the snowfall over polar and cold temperate landmasses built up and persisted, with the result that glaciers advanced and sea level dropped as much as 330 feet below its present level. In contrast, during warmer interglacial periods, sea level rose significantly above present levels. One of the potential consequences of the rising level of atmospheric carbon dioxide and subsequent global warming is a drastic reduction of ice in the Arctic and Antarctic regions. The resulting runoff would contribute to a substantial rise in sea level. Furthermore, general FIGURE 11.10 The world's ice caps and glaciers such as these in Victoria Land, Antarctica, contain most of the world's nonsaline water, but this water is inaccessible for human use. 396 Part 6 Water and Soil for Life Support worldwide warming, even by very small amounts, would also result in volume expansion of ocean waters and further contribute to sea level rise. Biologists have noted that there would be numerous secondary effects of such changes, such as shifting biological habitats and a drastic impact on animals such as polar bears. The bears rely on the presence of floating summer pack ice for their survival. All of this further emphasizes the interconnected ness and complexity of Earth's systems. Surface Runoff, Floods, and Flood Control Most rainfall produces some surface runoff. The amount of this runoff is a function of the amount of rainfall, the slope and length of the drainage basin, the rock and soil type of the drainage basin, the vegetation cover, and the extent of any impermeable areas in the basin. The runoff can range from zero to more than 90 percent of total rainfall in a given basin; the remainder evapotranspi res back into the atmosphere, percolates into the groundwater system, or is held back in storage facilities. Activities such as mining, timbering, farming, and construction frequently promote an
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increase in the amount and rate of surface runoff. Removal of the trees and other vegetation reduces the retention of water in the biologic materials (leaves, roots, grasses, trees) and soil, and the amount of evapotranspi ration (see Figure 11.36). At the same time, there is a great increase in the surface runoff and erosion. Surface runoff can be defined by a hydrograph or lag-time diagram (Figure 11.11). This
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