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Thirst: Ecosystems and the

Worlds Water Supply

Water is the best of all things, said Pindar, the Greek lyric poet some twenty -five hundredyears ago. How right he was! We cannot do without it, and few pleasures are more satisfying

than a cool drink from a mountain spring when we are truly thirsty. But perhaps Pindar

should have said that fresh water is the best of all things, since only about 2.5 percent of

Earths water is fresh. The rest is salt water, which will hardly slake ones thirst, as the

Ancient Mariner noted. Moreover, of that small fraction of our water that is fresh, some two-

thirds is totally inaccessible in ice caps and glaciers. Only about 0.77 percent is in lakes,

swamps, rivers, aquifers, pores in soils, the atmosphere, and the bodies of living organisms.

But Earth has correctly been called the water planet, and as a result that 0.77 percent is a huge

amount of watermore than 10 million cubic kilometers.

The renewable freshwater supply, that flowing through the solar powered hydrologic cycle, ismuch less. The cycle lifts about 430,000 cubic kilometers of water by evaporation from the

surface of the oceans. Roughly 390,000 cubic kilometres of that water falls right back into the

oceans as precipitation. But some 40,000 cubic kilometres are carried by winds from the

oceans to the land surface, where they add to the rain, snow, sleet, and hail that fall there.

That water with its source in oceanic evaporation, accounts for a little over a third of

terrestrial precipitation; a little less than two-thirds has as its source water evaporating from

the land surface and pumped out of the ground by plants.

Of the renewable 110,000 cubic kilometers, only a third or so runs back to the sea in rivers,

streams, and groundwater flows. And only about 10 percentsome 12,500 cubic

kilometersis reasonably accessible runoff. That much sloshing liquid may seem like a vast

supply, except that humankind is already appropriating more than half for its own use.

Furthermore, both the human population and its per capita demand for freshwater are still

growing. Two of the key roles wild organisms play in the hydrologic cycle (from the point of

view of humanity) are helping to maintain quality and supply. Plants and micro organisms

play a major part in this process, as they do in other cycles of the natural internet.

Vegetation slows the flow of water over the land surface toward stream systems and thus

provides time for it to soak into the ground. Plants break the force of heavy rains and protect

the soil. This is particularly obvious in tropical rain forests where torrential downpours are

intercepted by the tree canopy so that, although the ground is very wet, the water soaks gentlyinto the leaf litter and soil. In addition, the roots of plants in a well-vegetated watershed both

hold the soil in place and help to retain nutrients in the system. Soil secured in this way

absorbs rainfall and then gradually releases the water into streams and rivers. The result is an

even flow that is generally beneficial to humanitymaking it easier to use water for

transport, power generation, irrigation, recreation, and the like. When soil is eroded away,

this water-metering function is impaired and the results downstream can be alternating

droughts and floods.

The loss of the flood-control service supplied by plants in natural ecosystems is demonstrated

dramatically in southern California every year or so. Much of the vegetation in the coastal

region is chaparral. It consists of many species of shrubs that are highly flammable so that,after a season of dry weather, it tends to burn fiercely over large areas. In the winter, when

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the rain comes, the burnt-out areas are inevitably plagued with mudslides and floods, for the

vegetation has not yet recovered sufficiently to control the flow of water. In the Midwestern

and south eastern portions of the United States, disastrous spring floods are frequent. One

reason is vast upstream areas where agriculture has removed much of the native vegetation

that once broke the fall of rain and held soils in place. It appears that the most famous

flood/mudslide disaster of the late 1990s, caused by Hurricane Mitch in Central America, wasmade worse by the removal of natural plant growth from hillsides needed for cultivation.

Similarly, areas of India and Bangladesh are more subject to inundation than previously,

owing in part to deforestation in the Himalayas and the resulting diminution of the flood-

control ecosystem service.