Topic 4.1 Irrigation

People have used irrigation to provide water for crops almost as long as they have practiced agriculture. In the harsh, hot deserts of southern Arizona, the prehistoric Hohokam Native American communities dug extensive networks of irrigation canals to divert water from the nearby Salt River to their crops. Thus their society thrived where it would otherwise have perished. Aerial photography reveals the location of these ancient canals, which follow many of the same routes as today's concrete-lined waterways that carry water throughout the area of Phoenix, Arizona.

In the Tigris and Euphrates Valleys of Mesopotamia (modern day Iraq and Syria) which have been called the "cradle of civilization," the irrigation of crops yielded similar results. Civilization may have declined when the irrigated fields were "salted out"—made useless by the continuing addition of salts to the soil from poor-quality water. The Incas of ancient Peru also practiced extensive irrigation, including some innovative methods involving raised beds and provision of water from below.

Irrigation as practiced today in world agriculture is governed by the same general principles as in ancient times. Water is expensive and therefore must be used as efficiently as possible. If surface water is used, dams and waterways must be constructed and maintained. If water is pumped from the ground, energy must be expended to raise it to the surface.

Perhaps surprisingly, most of the inefficiency in the use of water is encountered before the water ever reaches the plant in the field. Evaporation from lakes, seepage from canals, transpiration from aquatic weeds that grow in the canals, and uneven application in the fields are just some of the problems faced by irrigation specialists. A system is considered efficient if 50% of the water available is delivered to the root zone of the crop it is supporting.

Various methods have been developed for delivering water to fields. If the land can be graded and sloped appropriately, furrows with a very slight downhill slope can be placed between rows of plants. Water is then applied at the uphill end and allowed to flow through the furrows. Some water is wasted, because the soil at the uphill end becomes saturated before enough has been delivered in the middle of the field. Water may also "puddle" at the downhill end.

Level-basin irrigation is being used increasingly to overcome these problems. Large basins are leveled to within 10 to 20 mm by laser-directed machinery. If the soil is uniformly permeable throughout the basin, this leveling allows water to be applied evenly. If the land cannot be leveled or if the amounts of water applied are small, sprinklers are often used. However, sprinklers not only require energy to pump and pressurize the water, but they also produce small droplets from which evaporation is excessive. In addition, if the water is of poor quality, sprinkling will deposit salts directly on the leaves, where they can injure the plants. In some cases, sprinkling at night can prevent much of the injury because the salt-laden droplets do not evaporate as fast.

Some soils with very high clay content expand as they are wetted and shrink and crack as they dry. When they are wet they tend to become sealed, and water enters very slowly. For these cracking clay soils, a technique known as surge irrigation has been developed. Water is supplied to the field extremely rapidly so that it can flow down the cracks and enter the root zone before the soil swells and the cracks disappear.

In recent years a technique known as drip irrigation (also called trickle irrigation or microirrigation) has come into use in some areas of the world. Water is pumped directly to the base of a plant by plastic tubing and bled through an emitter at a slow rate that just meets the plant's needs. This approach is very efficient, but it is also very expensive and requires diligent maintenance of the hardware to keep the system working. For instance, the emitters tend to become plugged by both mineral deposits and slime produced by microorganisms. Periodically, the system must be flushed out with acid or with disinfectant. So far, drip irrigation has been used mostly for high-value crops for which quality (and price) depends strongly on a reliable supply of water. Under these conditions the profit from drip irrigation will pay for the extra costs. Fresh fruits such as blueberries and strawberries are irrigated extensively by this method in the United States.

With the availability of extremely efficient irrigation systems such as drip irrigation, more attention can be shifted to inefficient use of water by the crops themselves and to possible improvements in plant water use efficiency. The physiology of water in plants is an exciting area of research because many problems are waiting to be solved. For example, grain crops are most sensitive to drought when they are flowering, when abortion of the very young embryos can result in a barren plant. The solving of these problems will allow agronomists to make grain yields reliable even without resorting to irrigation.