This paper summarizes the treatment of the basic lines of
pursuit in solving the water problem for conditions of the future. The
task is an extremely complicated one, above all because of the diversity
of problems involved in rationalizing the use of water resources of the
entire world. These difficulties are compounded by the fact that the
types and specific methods of water-resource use and conservation have
evolved gradually over a long period of time (centuries in certain areas
of water management) and have become traditional. At the same time,
analysis demonstrates that some of these traditional approaches have
become outdated and need to be replaced by other methods that are in
line with the present cultural and economic levels. This is a defect in
some of the long-range forecasts of the development of water management,
e.g., the future water resources of the United States (Landsberg et al.,
1985). Another forecast for the United States for the year 2000 to 2020
(The Nation's Water Resources,” 1988) differs favorably from the
previous one. It is based on the dynamics of progress with respect to
standards and methods related to the use of water resources, so that the
increase in water consumption to meet the country's entire needs over
the half century up to the year 2020 will only double in approximate
terms and reach about 217 km3/yr. On one hand, it has been justified
that the earth is rich in fresh-water resources and that it can serve
humanity forever, meeting all of man's needs. On the other hand, fears
are expressed with increasing frequency concerning the threatened
exhaustion of water resources and the effect that there will be a limit
on population growth.
If the situation with fresh water resources is so favorable (though only in certain places), what can humanity expect when the earth's population almost doubles by the year 2000? And the economy multiplies many times over? These are the questions being asked by certain scientists. The answers they give are full of pessimism. There has been a theory of limits on development which believed this principal way to resolve the water problem by limiting human water needs, since it believed to be the only way of stopping the process of qualitative depletion of water resources and thereby extending over a longer period of time the use of existing water resources.
However, this theory is not well founded, mainly because it restricts human needs for food, clothing, heated and lighted buildings, paper, and so on. A regressive movement cannot provide the basis for solving the problem. The existence of municipal water supply systems and centralized sewer systems is the primary and very substantial source of pollution of natural water. The water resources are inexhaustible and are not lost to man regardless of the manner in which they are used.
The whole point is that it makes a great deal of difference whether the water is in the gaseous, liquid, or solid state and in what phase of the water cycle. The use of atmospheric vapor or natural ice is far more restricted than the use of water in the liquid state. It also matters from the standpoint of use what link in the water cycle it happe ns to be in, for example, the river link or the ocean link.
Depending on the purpose and technology of water resource use, man transfers water from one link in the water cycle to another. Sometimes man does this without taking into account the course of the natural processes of the water cycle. For instance, when water is taken from a river for irrigation, it spends mainly for evaporation and transpiration. Thus, man deliberately takes water from the river link in the water cycle and transfers it to the soil link. Consequently, the water taken from the river continues to be an element in the water cycle, but this does not mean at all that these consequences for water resources. If all the water that evaporates following irrigation is condensed and falls in the form of precipitation on the catchment area of the same river from which it was taken, the volume of flow of that river will still not be reestablished. The reason is that not all precipitation that falls becomes runoff. Part of it goes to infiltration of the soil and part goes to evapotranspiration. The runoff coefficients in a zone of variable wetting range between 0.25 and 0.40, while in an arid region they drop to as low as 0.1 or even lower. Streamflow then would be reestablished within these limits, but only if the water evaporates, condenses and falls in the form of precipitation in the catchment area of the same river from which it was taken for irrigation. However, given the nature and patterns of atmospheric circulation, the vapor may be carried to altogether different river basins where more water is not needed. Then again, it may fall as rain over the ocean or as snow over Antarctica. It follows that the human use of water, though it does not exclude the water from process of the water cycle, does bring about qualitative, quantitative, and geographic changes in the water exchanges. The consequences of those changes may not be favorable to the process of the water cycle.
These considerations are not by any means being expressed in order to convince that water resources should not be used in such a way as to violate the water cycle. Such an assertion would be unrealistic and erroneous. Water should be used fully and in the full amounts needed. However at the same time, it cannot but take into account the face that greater or lesser changes in the various links of the water cycle, which in one way or another affect water resources, almost always present a cost in water use.
Reference