Managing Scarce Water Resources in Agriculture in West Asia and North Africa - Restorative Redevelopment of Devastated Ecocultural Landscapes - page 457

7

Rainfed
Supp. irrigation

6

5

4

3

2

1

0

Dry year Average Wet year

FIGURE 22.5 Impact of supplemental irrigation on wheat grain yield for a dry year (234
mm rainfall with SI of 183 mm); average year (316 mm rainfall with SI of 120 mm); and
wet year (504 mm rainfall with SI of 75 mm). (Adapted from Oweis T. 1997. Supplemental
Irrigation: A Highly Efficient Water-Use Practice. ICARDA, Aleppo, Syria. 16 pp.)

variation in rainfed production in Syria was reduced from 100 percent to 10 percent
when SI was practiced.
Average WP of rain in producing wheat in the dry areas of WANA ranges from
about 0.35 to 1.00 kg grain/m 3 . However, water used in supplemental irrigation can
be much more efficient. ICARDA found that a cubic meter of water applied at the
right time (when crops suffer from moisture stress) and good management could
produce more than 2.5 kg of grain over the rainfed production. This extremely high
WUE is mainly attributed to the effectiveness of a small amount of water in alle-
viating severe moisture stress during the most sensitive stage of crop growth. The
stress usually causes a collapse in the crop development and seed filling and reduces
the yields substantially. When SI irrigation water is applied before such conditions
occur, the plant may reach its high potential.
In comparison to the productivity of water in fully irrigated areas (rainfall effect
is negligible) we find greater advantage with SI. In fully irrigated areas with good
management, wheat grain yield is about 6 t/ha using a total amount of 800 mm of
water. This makes WP about 0.75 kg/m 3 , one-third of that under SI with similar man-
agement (Figure 22.6). This suggests that water resources may be better allocated to
SI when other physical and economic conditions are favorable.
Deficit irrigation is an optimizing strategy under which crops are deliberately
allowed to sustain some degree of water deficit and yield reduction. One important
merit of deficit supplemental irrigation is the greater potential for benefiting from
unexpected rainfall during the growing season due to the higher availability of stor-
age space in the crop root zone. Results on wheat, obtained from field trials conducted
in a Mediterranean climate in northern Syria, reported significant improvement in
SI water productivity at lower application rates than at full irrigation. Highest water
productivity of applied water was obtained at rates between one-third and two-thirds
of full SI requirements, in addition to rainfall.
Research in the WANA region has shown that applying only 50 percent of full
supplemental irrigation requirements causes a yield reduction of only 10 to 15 per-
cent. Farmer-managed field plots were established to demonstrate this finding. It was