Water Resources

water that is suitable for use; in practice, all the waters of the hydrosphere—that is, the water of rivers, lakes, canals, reservoirs, and seas and oceans; subterranean water; soil moisture; the water (ice) of mountain and polar glaciers; and the water vapor of the atmosphere. The

concept of water resources also includes bodies of water— rivers, lakes, and seas—insofar as they are used for certain purposes (navigation, hydroelectric power, fishing, recreation, and tourism) without the withdrawal of water from them.

A notion of the quantity of water resources may be formed by adding the stationary supplies in various parts of the hydrosphere (see Table 1) and the supplies that are continuously renewed in the process of water circulation.

The continuously renewed resources include river discharge, whose distribution in various parts of the world is given in Table 2.

The following system of equations for the water balance of dry land serves to provide a comprehensive balance estimate of water resources: R — U + S, P = U + S — E, andW = P — S = U + E, where R is the total river discharge, U is subterranean discharge into rivers, S is surface discharge (flood-waters), P is atmospheric precipitation, E is evaporation, and W is gross moistening of land.

By means of these equations it is possible to evaluate the various sources of water resources interdependently, according to the intrinsic unity of waters, which is determined by water circulation (see Table 3).

River water resources consist of two unequal parts with different origins: the subterranean and surface parts. The former is stable; thus, as a rule, it does not require regulation. In addition, it characterizes in general the renewable supply of subterranean waters of the zone of active water exchange. Deep subterranean waters (below the drainage level of rivers) do not participate extensively in present-day water circulation; they are stagnant in nature and thus are most often highly mineralized. The surface (floodwater) discharge is highly variable and, as a rule, requires regulation for its utilization.

The total gross moistening of land characterizes the annual renewable store of soil moisture.

Fresh waters make up less than 2 percent of the stationary supplies of the hydrosphere, but if the water (ice) of polar glaciers is excluded for being presently unavailable for use, accessible fresh water accounts for only 0.3 percent of the stationary volume of the hydrosphere. As a rule, river water resources, under the influence of a high level of activity, are fresh. (On the average, they are replaced every 11 days; see Table 1.) Circulating lakes and a large part of the subterranean water of the zone of active water exchange are also fresh. These water resources are used the most extensively and for diverse purposes (water supply, irrigation, recreation and tourism, fishing and fish farming, hydroelectric power, and internal navigation). In terms of absolute quantities, the USSR is richest in water resources, but in terms of specific water supply per unit area, it is below the world average, particularly for subterranean discharge (55 percent) and soil moisture (63 percent).

Water resources are theoretically inexhaustible, since with proper use they are continuously renewed in the process of circulation. Even in the recent past it was believed that the waters of the earth existed in such quantity that, with the exception of individual arid regions, it was not necessary to be concerned about possible water shortages. However, water consumption is growing at such a rate that mankind is more and more often encountering the problem of how to supply future needs. A shortage of water resources, increasing with every year, is already being felt in many countries and regions of Europe and America. The rapidly increasing pollution of the waters of rivers, lakes, and, to a considerable degree, seas, produced by the dumping of sewage, presents a great danger of depletion of water resources.

The yearly expenditure of water for all forms of water supply on the earth is 150 cu km, but at the same time about 450 cu km of sewage is discharged into rivers and lakes; more than 5,500 cu km of pure river water—one-seventh of the world’s resources of river discharge—is required to render this water harmless. If the discharge of sewage into rivers continues, by the year 2000 it will be necessary to expend the world’s entire river-discharge resources for this purpose, even if the quality of the preliminary purification of sewage is substantially improved.

To avoid the qualitative exhaustion of water resources, it is essential to carry out a complex of purposeful measures, prominent among which is a worldwide reduction and subsequent total cessation of the use of rivers, lakes, and reservoirs for the disposal and purification of sewage. It is possible to achieve this by reusing sewage (for irrigation of agricultural fields or in certain enterprises after purification) and by the reduction of the water requirements of industry in every way possible—that is, the reduction of the expenditure of water per unit of production and the shifting of certain water-consuming enterprises to dry technology.

The expanded production of water resources—that is, in-creasing the resources most accessible to use at the expense of inaccessible and potential water resources—takes place extensively in the practice of aquiculture and agriculture. This is achieved by the transformation of water resources— for example, multiplying the resources of soil moisture through land-reclamation and agrotechnical means or increasing the stable river discharge at the expense of the surface (floodwater) flow by means of regulation by reservoirs. Artificial storage of underground waters (the establishment of large, continually replenished underground reservoirs with great water transit) is becoming important.