Why is it that irrigated farming in some areas fails to achieve its potential benefits? The problem is not inherent in the principle of irrigation as such, but in the frequently inappropriate practice of it. More often than not the fault lies in the unmeasured and generally excessive application of water to land, with little regard either for the real cost of extracting the water from its source and delivering it to the farm, or for the cost of restoring the water resource after it has been depleted or polluted. By deliberately maintaining a low price for water, governments perpetuate the false notion that fresh water is a free good, rather than the scarce and expensive resource that it really is. It is the universal fallacy of humans to assume that if a little of something is good, then more must be better. In irrigation (as indeed in many other activities), just enough is best, and by that is meant a controlled quantity of water that is sufficient to meet the requirements of the crop and to prevent accumulation of salts in the soil, no less and certainly no more. The application of too little water is an obvious waste, as it fails to produce the desired benefit. Excessive flooding of the land is, however, likely to be still more harmful, as it tends to saturate the soil for too long, inhibit aeration, leach nutrients, induce greater evaporation and salin-ization, and ultimately raise the water-table to a level that suppresses normal root and microbial activity and that can only be drained and leached at great expense.
Apart from wasting water, therefore, excessive irrigation contributes to
its own demise by the twin scourges of water-logging and soil salinization. Instead of
achieving its full potential to increase and stabilize food production, irrigation in such
cases is in danger of becoming unsus-tainable. The ultimate economic and environmental
consequence of poorly managed irrigation is the destruction of an area's productive base.
The cost of rehabilitating the land after it has been degraded may be entirely
prohibitive.
From the point of view of water use, some large-scale irrigation projects operate in an
inherently inefficient way. Where water is delivered to farmers on a fixed schedule and
charges are set per delivery regardless of the actual amount used, irrigators tend to take
as much water as they can while they can. This typically results in overirrigation, which
not only wastes water but also contributes to project-wide problems associated with the
disposal of return flow and elevation of the water-table. Especially difficult to change
are management practices that lead to waste, not necessarily because of insurmountable
technical problems or lack of knowledge, but simply because it appears more convenient or
economical in the short term to waste water rather than to conserve it. Such situations
occur when the price of irrigation water is lower than the cost of the labour or of the
equipment needed to avoid overirrigation.
The classical method of irrigation, which evolved in the major river valleys of the Near
East, South Asia and East Asia, consists of flooding the land to some depth with a large
volume of water so as to saturate the soil completely, then waiting some days or weeks
until the moisture thus stored in the soil is nearly depleted before flooding the land
once again. In this low-frequency, high-volume, total-area pattern of irrigation, the
typical cycles consist of repeated periods of excess soil moisture alternating with
periods of likely insufficiency. Optimal conditions occur only briefly in transition from
one extreme condition to the other (Figure 3).
FIGURE 3
Flood irrigation wets the entire root zone to saturation
In contrast, the newer irrigation methods are designed to apply a small, measured volume of water at frequent intervals to where the roots are concentrated. The aim is to reduce fluctuations in the moisture content of the root zone by maintaining moist but unsaturated conditions continuously, without subjecting the crop either to oxygen stress (from excess moisture) or water stress (from lack of moisture). Moreover, applying the water at spatially discrete locations rather than over the entire area has the effect of keeping much of the soil surface dry, thus helping not only to reduce evaporation but also to suppress proliferation of weeds (Figures 4, 5, 6 and 7).
FIGURE 4
The pattern of wetting under furrow irrigation: if furrows are closely spaced the entire
root zone is wetted to near-saturation
FIGURE 5
The pattern of wetting under sprinkler irrigation: to compensate for the uneven
distribution of water around each sprinkler, adjacent sprinklers are spaced closely enough
to overlap (thus tending to equalize the spatial distribution of water)
FIGURE 6
A portable hand-move sprinkler irrigation system
FIGURE 7
Partial-area wetting around orchard trees under drip irrigation
This optimization of soil moisture is difficult to achieve with the
traditional flood irrigation methods still dominant in many river valleys. As a result,
the new approach to irrigation management has not yet been adopted very widely in
developing countries. Although it is gaining ground gradually, its progress should be
encouraged and accelerated wherever appropriate.
Ideally, the new irrigation systems should convey water to the field in concrete-lined
channels so as to avoid seepage losses, or preferably in closed conduits that avoid
pollution and allow pressurizing of the water thus delivered. In the field, the water can
be distributed via low-cost, weathering-resistant plastic tubes, and be applied to the
root zone by means of drip emitters, microsprayers or porous bodies placed at or below the
soil surface. Human labour and local materials may substitute for industrially produced
devices where such are unavailable or too expensive, while retaining the principles of
efficient irrigation.
As the frequency of irrigation increases, the infiltration period becomes a more important
part of the irrigation cycle. With small daily (rather than massive weekly or monthly)
applications of water, the pulses of added water are damped down within a few centimetres
or decimetres of the surface, so the flow below that depth is essentially steady. A
skilled irrigator can control the moisture content of the root zone as well as the rate of
internal drainage by adjusting the rate and quantity of application according to the
soil's infiltrability, the soil solution's concentration, and the climate-imposed
evaporative demand. Thus the irrigator can manage the system optimally so as to both
increase yields and conserve water (Figures 6 and 7).
The long-accepted notion that the entire volume of the root zone must be wetted to
capacity at each irrigation has been contradicted by recent experience proving that a crop
can fare well when the wetted zone is restricted to a fraction of the soil volume - 50
percent, or even less. This is on condition, of course, that the supply of moisture and
nutrients in that partial volume is sufficient to satisfy full crop needs.
Since a high-frequency irrigation system can be adjusted to supply water at very nearly
the exact rate required by the crop, the irrigator no longer needs to depend on the soil's
ability to store water during long intervals between irrigations. Hence water storage
properties, once considered essential, are no longer decisive in determining whether a
soil is irrigable. New lands, until recently believed to be unsuited for irrigation, can
now be brought into production. Examples are coarse sands and gravels, where moisture
storage capacity is very low and where the conveyance and spreading of water by surface
flooding would cause too much seepage. Such soils can now be irrigated even on sloping
ground by means of drip, trickle, microsprayer or soil-embedded porous emitters that apply
the water frequently or continuously to the root zone at a controlled rate.
Though they offer many advantages, high-frequency partial-volume systems have shortcomings
too. With only a fraction of the potential root zone wetted, there is less moisture
storage in the soil, so the crop depends vitally on the continuous operation of the
system. Any short-term interruption of the irrigation (whether caused by neglect,
mechanical failure or water shortage) can quickly result in severe distress to the crop.
The imperative to maintain continuous operation is difficult to meet if the system depends
on costly and vulnerable equipment imported from abroad. The system must therefore be
simplified so as to make the local farmers self-reliant.
In general, it is difficult to change a pre-existing pattern of human behaviour and
institutional norms. An infrastructure designed for one mode of operation cannot readily
be converted to another. Habits and traditions, once established, acquire an inertia, with
vested interests in maintaining the status quo and a resistance to reforming it. That is
why it is considered so important to start new irrigation projects appropriately by
instituting efficient practices from the outset.