Page 252 - Environment: The Science Behind the Stories

Sustainable approaches to irrigation
maximize efficiency

One of the most effective ways to reduce water use in agricul-
ture is to better match crops and climate. Land in many arid
regions has been converted into productive farmland through
extensive irrigation, often with the support of government
subsidies that make irrigation water artificially inexpensive.
Some farmers in these areas cultivate crops that require large
amounts of water, such as rice and cotton (Figure 9.21). This
leads to extensive water loss from evaporation in the arid cli-
mate. Choosing crops that require far less water could enable
these areas to remain agriculturally productive while greatly
reducing water use.
(a) Flood-and-furrow irrigation of cotton in the southern Another approach is to embrace technologies that
California desert. improve efficiency in water use. Currently, irrigation efficiency
worldwide is low, as plants end up using only 43% of the water
that we apply. The rest evaporates or soaks into the soil away
from plant roots (Figure 9.22a). Drip irrigation systems that tar-
get water directly toward plant roots through hoses can increase
efficiencies to over 90% (Figure 9.22b). As such systems become
more affordable, more farmers are turning to them.

(b) Center-pivot irrigation circles in the Arizona desert.

Figure 9.21 Irrigating water-thirsty crops in arid regions
causes us to lose a great deal of water to evaporation.
Examples are (a) flood-and-furrow irrigation for cotton in southern
California and (b) center pivot irrigation (from the air) in Arizona.

Irrigation boosts productivity but (a) Conventional irrigation
can damage soil

The artificial provision of water beyond that which crops
receive from rainfall is known as irrigation. Some crops, such
as rice and cotton, use large amounts of water and generally
require irrigation, whereas others, such as beans and wheat,
use relatively little. The amount of water required also is influ-
enced by the rate of evaporation and the soil’s ability to hold
water and make it available to plant roots. If the climate is too
dry, or if too much water evaporates or runs off before it can CHAPTER 9 • So I l AN d A gr I culT ure
infiltrate the soil, crops may require irrigation. By irrigating
crops, people have managed to turn previously dry and unpro-
ductive regions into fertile farmland.
Fully 70% of all fresh water that people withdraw is
used for irrigation. Irrigated acreage has increased dramati-
cally worldwide, reaching almost 400 million ha (nearly 1 (b) Drip irrigation
billion acres), twice the area of Mexico. In some cases, with- Figure 9.22 Irrigation methods vary in their water use.
drawing water for irrigation has depleted aquifers and dried Conventional methods (a) are inefficient. In drip irrigation systems
up rivers and lakes. We will examine irrigation further in (b), hoses drip water directly onto the plants, so that much less
Chapter 15 (p. 415). is wasted. 251

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