Supplemental Irrigation

Precipitation in the rainfed areas is low in amount and suboptimal in distribution, with great year-to-year fluctuation. In a Mediterranean climate, rainfall occurs mainly during the winter months. Crops must rely on stored soil moisture when they grow rapidly in the spring. In the wet months, stored water is ample, plants sown at the beginning of the season are in early growth stages, and the water extraction rate from the root zone is limited. Usually little or no moisture stress occurs during this period (Figure 14.1). However, during spring, plants grow faster, with a high evapotranspiration

FIGURE 14.1

Typical soil moisture pattern over the growing season of a Mediterranean-type wheat. (From Oweis, T. and Hachum, A. 2012. Supplemental irrigation, a highly efficient water-use practice. 2nd edition. ICARDA, Aleppo, Syria. iv + 28 pp. With permission.)

rate and rapid soil moisture depletion due to higher evaporative demand. Thus, a stage of increasing moisture stress starts in the spring and continues until the end of the season. As a result, rainfed crop growth is poor and yield is low. The mean grain yield of rainfed wheat in the dry areas is about 1 t/ha, far below the yield potential of wheat (more than 5-6 t/ha).

Supplemental irrigation aims to overcome the effects of drought periods as soil moisture drops and halts crop growth and development. Limited amounts of water, if applied during critical times, can result in substantial increases in yield and water productivity.

Research results from the International Center of Agricultural Research in the Dry Areas (ICARDA) and other organizations, as well as harvests from farmers' fields, have demonstrated substantial increases in crop yield in response to the application of relatively small amounts of irrigation water. Table 14.1 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. 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 (Oweis et al. 1998, 2000).

TABLE 14.1

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

Season/Annual Rainfall (mm)

Rainfed

Yield

(t/ha)

Rainfall

WP

(kg/m3)

Irrigation

Amount

(mm)

Total

Yield

(t/ha)

Yield Increase Due to SI (t/ha)

Irrigation

WP

(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, T. and Hachum, A. 2012. Supplemental irrigation, a highly efficient water-use practice. 2nd. edition. ICARDA, Aleppo, Syria. iv + 28 pp.

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 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 and Hachum 2012.). 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. Rainfed yields can be significantly increased if the crop achieves good early growth before dormancy. This can be achieved by early sowing with the 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 percent, adding more than 2 t/ha to the average rainfed yield of 3.2 t/ha (Ilbeyi et al. 2006). 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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