|Diversity, Globalization, and the Ways of Nature (IDRC, 1995, 234 p.)|
|7. Managing planetary thirst|
Most of the worlds water is stored in the oceans (97.39%) and in glaciers and ice sheets (2.01%). A large part of the remainder is contained in geological formations (0.54%). Only about 0.06% occurs as surface water, of which more than half is salty, making it unpotable. Therefore, available fresh water constitutes less than 0.02% of the hydrosphere. Of surface fresh water, 95% is stored in lakes. Flowing water represents only about 0.001% of the water on the planet (Bethemont 1980). However, this volume of flowing water is more than enough to satisfy all human needs now and in the near future.
Every year, 496 thousand cubic kilometres of water falls as precipitation - that is about 100 thousand cubic metres per person per year. If the annual precipitation was spread evenly over the planet, it would amount to about 973 millimetres. However, only 25% of this total falls on the continents. Asia receives the most (28%), despite its low average precipitation of 696 millimetres per year. South America, with less than half the area, receives almost as much (25%) because of its higher average precipitation (I 564 millimetres per year). Africas average precipitation is similar to Asias; North Americas is slightly lower (645 millimetres per year). Assuming that the volume of stored groundwater is unchanged, the volume of water lost to evaporation from the land masses is as high as 84% of the total precipitation in Africa, 67% in Australia, and 62% in North America. In Asia and South America, evaporation loss accounts for 60% of the fallen water; in Europe, 57%. Only in Antarctica is the rate considerably smaller at 17%.
If we restrict our calculations to precipitation falling on the continents and subtract the amount lost to evaporation (about 60%), over 80 thousand cubic metres would be available for each person annually. Per-capita need varies from place to place, but generally does not exceed I cubic metre per day. These figures show that availability of water for human use does not relate to its volume. Rather, it depends on many other factors that we identify and characterize in this chapter.
Hydrographic basins are natural units made up of the various terrestrial environments through which water moves toward a given outlet. In that sense, hydrographic basins can be defined as the upstream territories of a lake or stream. Basins are complex; they include both surface and underground water. These two categories of water are closely interrelated and must be considered together. The main components of a typical basin are watersheds, a hydrographic network, and groundwater systems.
The three parts of a hydrographic basin are interconnected: watersheds receive precipitation, which infiltrates groundwater systems or flows toward valleys, forming streams. Part of the groundwater can go back into the streams, and water from the streambeds often recharges the underlying aquifers. Some water may reenter the atmosphere through evaporation and fall again on the watershed, closing the cycle. By and large, however, the system is open because most basins exit toward the sea or other major water body. The outlet of the basin is also an outlet for sediments, dissolved salts, and contaminants.
The geomorphic water cycle
Surface waters may occur in a complex array of hydrologic features and systems, including streams, lakes, swamps, and other flowing or lentic water bodies. Surface water bodies are fed from three main sources: instantaneously from rainstorms and subsequent runoff; from springs (groundwater discharge); and from the melting of ice and snow.
In tropical and temperate arid climates, streams are mainly fed through runoff. Precipitation falls on bare soils, little or no infiltration occurs, and the water flows downhill into river valleys. Rivers in arid areas have irregular flow patterns and may suffer catastrophic floods and droughts. In humid climates, the opposite occurs. Soils are covered by vegetation, and rainwater is intercepted by leaves and branches. Most of the water evaporates or infiltrates the soil and only a small fraction remains as surface runoff. Underground, the water moves through geological formations, reappearing as springs next to streams, lakes, or swamps.
Thus, in humid climates most of the water comes from springs, whereas in arid areas the supply of water to the natural surface systems is related to runoff processes. In addition, as a result of higher evaporation rates and the presence of salts in the soil, water in arid climates tends to contain a higher concentration of dissolved solids; in humid environ meets the opposite is true.
Difficulty of managing international hydrographic basins
Hydrographic basins, both on the surface and underground, do not respect national boundaries; nor are national borders arranged around water systems. The sharing of water resources is common throughout the world. In some cases, conflicts may develop, and water issues may become important factors in international politics.
Some hydrographic basins, even a few large ones, are mainly or entirely within a single country; for example, the Yangtze River in China and the Mississippi River in the United States. Hydrographic basins are more frequently shared by two or more countries, however, making agreement on management strategies difficult.