Environmental Engineering Reference
In-Depth Information
half a century earlier. New agricultural
techniques—involving dry farming as well as
irrigation—coupled with a more appropriate use
of the land, help to offset the worst effects of the
arid environment, but some problems will always
remain.
a channel much larger than the present volume
requires (Turk 1980).
Agriculture has resorted to the use of
groundwater in those areas with no fluvial water
supply available. Groundwater has several
advantages over surface water, such as reliability
of supply and uniform quality, but many of the
aquifers beneath the plains have been so
extensively used that the recharge rate has fallen
far behind demand, and they are becoming
depleted. This, in turn, leads to the need for
deeper wells, and increased pumping time, with
a consequent rise in costs.
Whatever the source of the additional
moisture, irrigation has one major problem
associated with it worldwide, that is the growth
of salinization or the build up of salts in the upper
layers of the soil. It is a problem as old as
irrigation itself (see e.g. Jacobsen and Adams
1971). In areas of high temperature, evaporation
rates are generally high, and this causes the
salinity of surface water in these areas to be high.
Groundwater also tends to have a high salt level,
particularly if it has been in the system for a long
time. When the water is applied to the crops, the
evaporation loss is high, and the salts remain
behind in the soil after the water has evaporated.
The salt build-up may be so great over time that
it interferes with the growth processes in the
plants, and they die or provide only much reduced
yields. In some cases, the salt may be flushed out
of the soil, but the process is costly, and not
always completely successful. As a result, land
which has been affected by salinization may have
to be abandoned.
There are also economic factors which must
receive consideration along with these physical
problems. Although modern irrigation techniques
are remarkably successful, they are also costly; a
modern central pivot system, for example, will
require an investment well in excess of $1000
per hectare. The capital costs of providing
equipment to combat drought, in areas of
contingent drought, such as the Great Plains,
where major drought may occur perhaps only
every 15 to 20 years, have to be weighed against
the losses that would occur if no action is taken.
Dry farming and irrigation
Dry farming is based on the preservation of
several years of precipitation to be used for the
production of one crop. The land is deep
ploughed and allowed to lie fallow for several
years. Deep ploughing provides a reservoir for
the rain that falls, and various techniques are used
to reduce losses by evapotranspiration. Perhaps
only a quarter of the total precipitation is made
available to the plants in this way, but in Kansas
and Nebraska grain yields may double after a
three- to four-year fallow (More 1969).
Obviously, this is only possible if rain falls in the
first place. It might be necessary to counter the
lack of precipitation by introducing water from
elsewhere in the hydrologic cycle, and making it
available through direct irrigation. Most of the
major rivers on the plains have been dammed
and the underlying aquifers tapped to provide
the moisture required. Grandiose schemes, such
as the North American Water and Power Alliance
(NAWAPA), which would bring water to the
plains from the Hudson Bay and Arctic
watersheds in northern Canada, have also been
suggested (Schindler and Bayley 1990). While this
may be ideal for agriculture, it is not without its
environmental problems. The larger projects have
been roundly criticized for their ability to cause
continental scale environmental disruption, but
even local or regional schemes can be harmful.
The rivers downstream from dams experience
reduced flow, which produces physical changes
in the stream channel and disrupts the balance
in the aquatic environment. Return flow from
irrigated fields contains fertilizer and pesticide
residues, which alter the chemical composition
of the water. The classic example of such changes
is the Colorado River, which at its mouth is a
mere trickle of highly salinated water, flowing in
