Water Cycle And Water Resources

Less water trapped in ice means that more water winds up in the other reservoirs, such as the atmosphere, streams and lakes, and the oceans. Generally, the water cycle is becoming more extreme: Wet regions are becoming wetter, and dry regions are becoming drier. Europe is wetter and is experiencing increased runoff and stream flow. The United States has weathered a 20% increase in blizzards and heavy rainstorms since 1900; the total amount of winter precipitation is up 10%.

By contrast, dry areas have more than doubled in size since the 1970s. Arid and semiarid regions, such as Africa's Sahel, are experiencing increased drought. Reduced rainfall in the southwestern United States has lowered Colorado River flow to less than it was in the Dust Bowl years of the mid-1930s. For five millennia, the Hamoun wetlands, covering 1,500 square miles (4,000 sq. km) and containing ample water, fish, and game, were a place of refuge for the people of Central Asia. The removal of water for irrigation before it could enter the wetlands, coupled with intense droughts, turned the area into a region of salt flats in 2002.

Warmer air has increased the temperature of surface water in the Northern Hemisphere's lakes and rivers by about 0.3 to 3.6°F (0.2 to 2°C) since the 1960s. The ice on large lakes and rivers in the mid and high latitudes now freezes nine days later, breaks up 10 days earlier, and is thinner and less extensive than in the past. In some East African lakes, deep water has also warmed, which can affect deep aquatic life. This trend will likely be seen in other lakes.

Warmer temperatures change the thermal structure of lakes. A warm surface layer is not dense enough to sink, so its ability to mix with the colder deeper layers is reduced. This keeps oxygen out of the deep layers of the lake and causes aquatic life to suffer. Water quality also decreases in the lake surface (where most organisms live) as solids, salts, and pollutants collect and are no longer mixed throughout the lake.

Rivers are also experiencing changes due to rising temperatures. Due to shorter winters, snow melts earlier in spring, and river flow peaks earlier in the year. Because communities typically need more water in summer, when there is less rainfall, this shift puts a strain on water supply systems.

Water systems will soon be strained by shrinking glaciers. The people of the Andes Mountains of South America rely on snow and ice melt for their water during the dry summers. Runoff is currently high because the glaciers are melting back at about 328 feet (100 m) per decade. By the end of this decade, however, some glaciers will be gone or too small to provide much meltwater. Himalayan glaciers are also melting. These glaciers feed seven rivers that provide more than half the drinking water for 40% of the world's people.

Continue reading here: Oceans

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