Physical Resources / 103required makes reverse osmosis difficult to apply on a large scale, but advances made in recent years
have reduced the energy needed to below that required for distillation and the technique is becoming
commercially attractive.
In years to come, rising demand for fresh water will lead to greater reliance on desalination. At
present, the high energy requirement makes all industrial-scale desalination technologies too expensive
for many of the less developed countries, where the increased demand will be felt most acutely, but
this situation could change. More efficient techniques for exploiting solar energy might reduce costs
in low latitudes, and in high latitudes waste heat from coastal industrial plants, especially nuclear
power stations, might be used to the same end.
As the production of fresh water by desalination grows, however, so will the amount of highly
concentrated brine for which no economic use can be found. It would be as well to develop satisfactory
means for its disposal before proceeding rapidly along this path.
25. Irrigation, waterlogging, and salinization
Deprived of water, before long any plant other than a cactus or other succulent will begin to look
very sick indeed. Its leaves will become flaccid and if it lacks a woody stem the entire plant will
grow limp and collapse. It will wilt. The condition may be temporary, the plant recovering when its
access to water is restored, but if it continues for too long the wilt will be permanent and the plant
will die.
Plants need water to give rigidity to their cells, but water stress also produces other, more subtle
effects. The stressed plant will spend more time with its stomata closed. These are the pores, each
opened and closed by the expansion and contraction of a pair of guard cells, through which gases are
exchanged and from which water evaporates. Keeping stomata closed reduces water loss, but a
reduction in the rate of gas exchange implies a reduction in the rate of photosynthesis. The plant will
grow more slowly and will be smaller than it would otherwise be, and growth is inhibited before the
plant is so short of water that it wilts visibly. When an adequate amount of water becomes available
to a formerly stressed plant it will increase its production of foliage, but in the case of a crop plant its
final weight will never be greater than that of an unstressed plant and usually it will be smaller.
Water shortage is an obvious problem facing farmers in semi-arid climates, or in climate types with
pronounced wet and dry seasons, such as that of the Mediterranean. Less obviously, it can also
reduce agricultural production where rainfall is distributed fairly evenly through the year. The monthly
extent of water surplus or deficit can be calculated by comparing the amount of rainfall with the
amount of water lost by evaporation and transpiration from grass supplied with abundant water.
Such calculations show that in central England a water deficit may occur during the summer and
autumn, from June to October, when evaporation exceeds precipitation (WINTER, 1974, p. 7). If
water is provided in addition to that received as rainfall, field experiments at the National Vegetable
Research Station in England have shown that crop yields increase dramatically: those of maincrop
potatoes rose from 37 t ha-1 to 50 t ha-1, an increase of 13 t ha-1, and those of cabbage from 41 t ha-1
to 59 t ha-1, an increase of 18 t ha-1. For every 25 mm of irrigation per hectare, yields of main-crop
potato increased by 3 t and those of cabbage by 18 t (WINTER, 1974, p. 117).
Irrigation is clearly beneficial, even in much of Britain, but this is hardly news. Farmers were irri-
gating their crops seven thousand years ago in Mesopotamia and irrigation techniques were developed
independently in China, Mexico, and Peru. In some countries unirrigated agriculture would be
impossible; all farm land is irrigated in Egypt, for example. In the world as a whole, about 15 per