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Unit 8: Water Resources // Section 2: The Global Water Cycle

Water covers about three-quarters of Earth's surface and is a necessary element for life. During their constant cycling between land, the oceans, and the atmosphere, water molecules pass repeatedly through solid, liquid, and gaseous phases (ice, liquid water, and water vapor), but the total supply remains fairly constant. A water molecule can travel to many parts of the globe as it cycles.

As discussed in Unit 2, "Atmosphere," and Unit 3, "Oceans," water vapor redistributes energy from the sun around the globe through atmospheric circulation. This happens because water absorbs a lot of energy when it changes its state from liquid to gas. Even though the temperature of the water vapor may not increase when it evaporates from liquid water, this vapor now contains more energy, which is referred to as latent heat. Atmospheric circulation moves this latent heat around Earth, and when water vapor condenses and produces rain, the latent heat is released.

Very little water is consumed in the sense of actually taking it out of the water cycle permanently, and unlike energy resources such as oil, water is not lost as a consequence of being used. However, human intervention often increases the flux of water out of one store of water into another, so it can deplete the stores of water that are most usable. For example, pumping groundwater for irrigation depletes aquifers by transferring the water to evaporation or river flow. Our activities also pollute water so that it is no longer suitable for human use and is harmful to ecosystems.

There are three basic steps in the global water cycle: water precipitates from the atmosphere, travels on the surface and through groundwater to the oceans, and evaporates or transpires back to the atmosphere from land or evaporates from the oceans. Figure 2 illustrates yearly flow volumes in thousands of cubic kilometers.

The water cycle

Figure 2. The water cycle
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Source: MIT OpenCourse Ware.

Supplies of freshwater (water without a significant salt content) exist because precipitation is greater than evaporation on land. Most of the precipitation that is not transpired by plants or evaporated, infiltrates through soils and becomes groundwater, which flows through rocks and sediments and discharges into rivers. Rivers are primarily supplied by groundwater, and in turn provide most of the freshwater discharge to the sea. Over the oceans evaporation is greater than precipitation, so the net effect is a transfer of water back to the atmosphere. In this way freshwater resources are continually renewed by counterbalancing differences between evaporation and precipitation on land and at sea, and the transport of water vapor in the atmosphere from the sea to the land.

Nearly 97 percent of the world's water supply by volume is held in the oceans. The other large reserves are groundwater (4 percent) and icecaps and glaciers (2 percent), with all other water bodies together accounting for a fraction of 1 percent. Residence times vary from several thousand years in the oceans to a few days in the atmosphere (Table 1).

Table 1. Estimate of the world water balance. Source: MIT OpenCourseWare.
Surface area (million km2) Volume (million km 3) Volume (%) Equivalent depth (m) Residence time
Oceans and seas 361 1,370 94 2,500 ~4,000 years
Lakes and reservoirs 1.55 0.13 <0.01 0.25 ~10 years
Swamps <0.1 <0.01 <0.01 0.007 1-10 years
River channels <0.1 <0.01 <0.01 0.003 ~2 weeks
Soil moisture 130 0.07 <0.01 0.13 2 weeks to 50 years
Groundwater 130 60 4 120 2 weeks to 100,000 years
Icecaps and glaciers 17.8 30 2 60 10 to 1,000 years
Atmospheric water 504 0.01 <0.01 0.025 ~10 days
Biospheric water <0.1 <0.01 <0.01 0.001 ~1 week

Solar radiation drives evaporation by heating water so that it changes to water vapor at a faster rate. This process consumes an enormous amount of energy—nearly one-third of the incoming solar energy that reaches Earth's surface. On land, most evaporation occurs as transpiration through plants: water is taken up through roots and evaporates through stomata in the leaves as the plant takes in CO2. A single large oak tree can transpire up to 40,000 gallons per year (footnote 1). Much of the water moving through the hydrologic cycle thus is involved with plant growth.

Since evaporation is driven by heat, it rises and falls with seasonal temperatures. In temperate regions, water stores rise and fall with seasonal evaporation rates, so that net atmospheric input (precipitation minus evaporation) can vary from positive to negative. Temperatures are more constant in tropical regions where large seasonal differences in precipitation, such as monsoon cycles, are the main cause of variations in the availability of water. In an effort to reduce these seasonal swings, many countries have built reservoirs to capture water during periods of high flow or flooding and release water during periods of low flow or drought. These projects have increased agricultural production and mitigated floods and droughts in some regions, but as we will see, they have also had major unintended impacts on water supplies and water quality.

The hydrologic cycle is also coupled with material cycles because rainfall erodes and weathers rock. Weathering breaks down rocks into gravel, sand, and sediments, and is an important source of key nutrients such as calcium and sulfur. Estimates from river outflows indicate that some 17 billion tons of material are transported into the oceans each year, of which about 80 percent is particulate and 20 percent is dissolved. On average, Earth's surface weathers at a rate of about 0.5 millimeter per year. Actual rates may be much higher at specific locations and may have been accelerated by human activities, such as emissions from fossil fuel combustion that make rain and snowfall more acidic.

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