106 / Basics of Environmental Science
Some of the rain falling on the ground sinks vertically through the soil, as ‘gravitational water’, until
it reaches the ground water, the region where the soil is saturated, the upper boundary of the saturated
region being the water table. Above the water table, particles comprising the unsaturated soil are
coated by a very thin film of ‘adhesion water’ held by attraction between water molecules and the
electrically charged surfaces of soil particles. Even the driest dust is usually coated with adhesion
water. This film is covered by an outer film of ‘cohesion water’, held by the attraction of hydrogen
bonds between water molecules themselves.
Water molecules at the bottom of a pot of water, or adjacent to the impermeable material underlying
the ground water, are subject to a pressure equal to the weight of water above them. The higher they
are in the pot, the less pressure bears down on them, until, at the surface, the pressure is zero. Any
water in tiny but connected spaces above the surface will be under even less (i.e. negative) pressure:
it will be under tension, a force pulling it upward rather than downward. Molecules will be easily
attracted by the adhesive charge on soil particles and further molecules will join them because of the
cohesive attraction of the molecules already in place. This is capillary attraction. It has little effect
on adhesion water, which moves very little, but cohesion water is less tightly bound and can move.
Under soil moisture tension, it moves to coat dry soil particles (becoming adhesion water) and to
equalize the thickness of the layer of cohesion water throughout the soil. Very slowly, the water will
rise through the unsaturated layer, and a very small suction by a plant root hair will be sufficient to
dislodge cohesion water and move it into the plant (FOTH AND TURK, 1972, pp. 64–74).
Water rises through both plants and soil, evaporates, and is replaced by more water rising through
the capillary pore spaces in the soil. Water vapour is almost pure H
2
O, and any substances dissolved
in the liquid are precipitated as it evaporates. Soil water is far from pure. Salts dissolve into it as it
moves through the soil and some soils contain quite large amounts of soluble salts. Irrigation water
itself is seldom pure; farmers do not irrigate their land with water fit for human consumption. The
water they use commonly contains between 750 g m-3 and 1.5 kg m-3 of dissolved salts (FOTH AND
TURK, 1972, p. 407). These may be left as evaporates near the soil surface, deposited from water
that evaporated before soaking into the soil or from water that descended gravitationally and then
rose again by capillary attraction. Gradually, the salinity of the upper soil increases until plants begin
to suffer, the most salt-intolerant first, but eventually most crop species.
Salinization most commonly occurs in arid or semi-arid climates, where the rate of evaporation is
high, but it is under these conditions that irrigation is most urgently needed and where it may bring
its greatest benefits. The risk may be avoided by installing adequate drainage to remove surplus
water before it can evaporate and by controlling the dissolved-salt content of irrigation water, especially
on saline soils.
Should it occur, the remedy is slow, difficult, and expensive. The first area ever to have been irrigated,
in the Tigris and Euphrates valleys, suffered from salinization and to this day much of it remains
barren because its reclamation would be too costly. Fresh water, containing little or no dissolved
salts, must be used to flush the salts from the soil and into a drainage system that will remove them,
and it may be necessary to take care in disposing of the salt-laden water. If salinization was caused
by salt-water intrusion, the freshwater aquifer must also be recharged. The old adage still applies, of
course: the water used to clean saline land must come from and go to somewhere.
Over-zealous irrigation on poorly drained land can lead to a quite different problem. If more water is
added to the soil than can evaporate or be transpired by plants, the water table will rise. It may do so
for some time before the consequences become apparent, but eventually soil around the roots of crop
plants will be saturated and, being saturated, airless. No water may be visible lying on the surface,
but nevertheless the land is waterlogged and crop yields will fall dramatically. In this case the remedy