General Aspects

Irrigation has been practiced for millennia. Until the 20th century, all irrigation depended on gravity to deliver water to the fields and to spread water across the surface of the land. Development of efficient engines, pumps, and impact sprinklers in the first half of this century allowed farmers to mimic rainfall with sprinkler irrigation. In the past 20 years, microirrigation has become more common where water is scarce, crop values are high, and the technology is practical. Surface irrigation remains the dominant system in use worldwide, although sprinkler irrigation has become widely used in some areas.

Total irrigated areas in the World are given in Table 5.3. An estimate of the percentages of the three primary methods of irrigation in four regions is presented in Table 5.4. Data in the table cannot be extrapolated to world figures because the data are not available for most regions, particularly Asia, which represents 69% of the world's irrigated area and where surface irrigation is predominant.

The many types of irrigation systems usually fall into one of three categories. Surface irrigation systems are those that depend on gravity to spread the water across the surface of the land. These systems also are referred to as gravity or flood irrigation systems. The shape of the soil surface and how the water is directed across the surface determine the types of surface systems (i.e., furrow, border, or basin). Sprinkler systems attempt to mimic rainfall by spraying the water evenly across the soil surface. The water is

5.4 Irrigation Methods

Table 5.3. Estimated irrigated areas in the world

Regi ion

Irrigated Area (ha)

Asia

Europe

Africa

South/Central America United States and Canada Australia World

174,300,000 25,150,000 11,480,000 17,650,000 22,100,000 2,300,000 252,990,000

Source: Food and Agriculture Organization AQUASTAT survey.

Table 5.4. Estimated use of the three primary irrigation methods

Use (%)

Region

Surface

Sprinkle

Micro

Africa*

85.0

12.5

2.5

Near Eastb

87.6

11.0

1.4

United Statesc

51.6

44.1

4.3

Former Soviet Uniond

58

42

0.1

For 20 countries representing 44.7% of the total irrigated area in the region [1].

For 18 countries representing 45.8% of the total irrigated area in the region [2].

For 20 countries representing 44.7% of the total irrigated area in the region [1].

For 18 countries representing 45.8% of the total irrigated area in the region [2].

b d pressurized with a pump, distributed to areas of the fields through pipes or hoses, and sprayed across the soil surface with rotating nozzles or sprayers. Types of sprinkler systems depend on the layout of the distribution pipelines and the way they are moved (i.e., solid set, hand move, center pivot, or rain gun). Microirrigation systems, also called drip or trickle systems, use small tubing to deliver water to individual plants or groups of plants. These systems use regularly spaced emitters on or in the tubing to drip or spray water onto or into the soil. Microirrigation systems are categorized by the type of emitters (i.e., drip or microspray).

Some systems do not clearly fit into these three categories. For example, subirrigation uses gravity to distribute water below the soil surface; it is uncommon and is not discussed here. Low-energy precision application (LEPA) systems use center-pivot machines to spray or dribble water onto small areas. They are discussed with sprinkler systems. Overhead or undertree impact sprinklers on fruit trees are considered sprinkler systems, whereas undertree sprays are considered microsprays.

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