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Dec. Jan. Feb. March April May June

Wheat growing season

Figure 1 Typical soil moisture pattern over the growing season of a Mediterranean-type wheat. (From Oweis, 1997.)

demonstrated substantial increases in crop yield in response to the application of relatively small amounts of irrigation water. Table 1 shows increases in wheat grain yields under low, average, and high rainfall in northern Syria, with application of limited amounts of SI. By definition, rainfall is the major source of water for crop growth and production; thus the amount of water added by SI cannot by itself support economical crop production. In addition to yield increases, SI also stabilized wheat production over years (i.e., reduced the interannual variability of yields).

The impact of SI goes beyond yield increase to substantially improving water productivity. The productivity of irrigation water and rainwater is improved when they are used conjunctively (Oweis et al., 1998, 2000). Average rainwater productivity of wheat ranges from 0.35 to 1.0 kg/m3. It was found that 1 m3 of water applied as SI at the proper time could produce more than 2.0 kg of wheat.

Using irrigation water conjunctively with rain was found to produce more wheat per unit of water than if used alone in fully irrigated areas where rainfall is negligible. In fully irrigated areas, water productivity for wheat ranges from 0.5 to

Dec. Jan. Feb. March April May June

Wheat growing season

Figure 1 Typical soil moisture pattern over the growing season of a Mediterranean-type wheat. (From Oweis, 1997.)

Table 1 Yield and Water Productivity (WP) for Wheat under Rain fed and Supplemental Irrigation (SI) in Dry, Average, and Wet Seasons in Tel Hadya, North Syria

Rainfed

Rainfall

Irrigation

Total

Irrigation

Season/Annual

Yield

WP

Amount

Yield

Yield Increase

WP

Rainfall (mm)

(t/ha)

(kg/m3)

(mm)

(t/ha)

due to SI (t/ha)

(kg/m3)

Dry (234 mm)

0.74

0.32

212

3.38

3.10

1.46

Average (316 mm)

2.30

0.73

150

5.60

3.30

2.20

Wet (504 mm)

5.00

0.99

75

6.44

1.44

1.92

Source: Adapted from Oweis (1997).

Source: Adapted from Oweis (1997).

about 0.75 kg/m3, one-third of that achieved with SI. This difference suggests that allocation of limited water resources should be shifted to more efficient practices (Oweis, 1997). Food legumes, which are important for providing low-cost protein for people of low income and for improving soil fertility, have shown similar responses to SI in terms of yield and water productivity.

In the highlands of the temperate dry areas in the Northern Hemisphere, frost occurs between December and March. Field crops go into dormancy during this period. In most years, the first rainfall sufficient to germinate seeds comes late, resulting in a poor crop stand when the crop goes into dormancy. Rain-fed yields can be significantly increased if the crop achieves good early growth before dormancy. This can be achieved by early sowing with application of a small amount of SI. A 4-year trial, conducted at the central Anatolia plateau of Turkey, showed that applying 50 mm of SI to wheat sown early increased grain yield by more than 60%, adding more than 2 t/ha to the average rain-fed yield of 3.2 t/ha (ICARDA, 2003). Water productivity reached 5.25 kg grain/m3 of consumed water, with an average of 4.4 kg/m3. These are extraordinary values for water productivity with regard to the irrigation of wheat.

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