550
often occurs during the most sensitive stages of crop growth (flowering and grain
filling). As a result, rainfed crop growth is poor and yield is consequently low.
Supplemental irrigation, especially during critical crop growth stages, can improve
crop yield and water productivity. Substantial increases in rainfed crop yields, in
response to the application of relatively small amounts of water, have been observed
(Oweis and Hachum 2003 ). When rainfall is low, more water is needed, but the
response is greater, and yield increases are remarkable even when rainfall is as high
as 500 mm (Oweis and Hachum 2012 ).
Unlike full irrigation, the timing and amount of SI cannot be determined in
advance given the rainfall variability. SI in rainfed areas is based on the following
three basic aspects (Oweis and Hachum 2012 ):
- Water is applied to a rainfed crop that would normally produce some yield with-
out irrigation. - Since rainfall is the principal source of water for rainfed crops, SI is only applied
when the rainfall fails to provide essential moisture for improved and stable
production. - The amount and timing of SI are optimally scheduled not to provide moisture
stress-free conditions throughout the growing season, but rather to ensure that a
minimum amount of water is available during the critical stages of crop growth
that would permit optimal yield. SI can be applied to field crops, fruit trees and
even landscape areas.
In this chapter, we discuss the case studies from different dryland countries in the
WANA region where supplemental irrigation successfully increased land and water
productivity.
2 Case Studies
2.1 Syria
In the Dara’a area of Syria, the available rainfall of 300–400 mm is sufficient to
support marginal rainfed olive (Olea europaea L.) production. A three-year study
by ICARDA, in collaboration with GCSAR (General Commission for Scientific and
Agricultural Research) of Syria, investigated the effects of adopting supplemental
irrigation for olive productivity (Nangia et al. 2014 ; Sikaoui et al. 2014a, b, c, d;
Nangia et al. 2016 ). The research experiment was carried out at Jillin Agricultural
Research Station, located 25 km north of Dara’a City with an average annual rain-
fall of 400 mm. The experiment included three water treatments: irrigation to 100 %
of crop water requirement (CWR), irrigation to 50 % CWR, and rainfed treatment
(control). The Sorani and Jlout cultivars of olive were, on average, 30 years old and
spaced 8 × 8 m apart. Soils have a high content of clay (between 67 and 71 %), field
capacity (volumetric) ranged between 45 and 46 %, and bulk density between 1.2
and 1.4 g cm−^3.
V. Nangia and T. Oweis