560
Usually, however, one to three supplemental irrigation applications of not more than
100 mm each growing season (totaling 100–300 mm) is sufficient, depending on the
rainfall amount and distribution. In the WANA region, these irrigations are best
given between late March and early May. This recommendation is based on the
availability of labor and water and the cost involved, but agronomically-speaking is
best given in multiple small doses so that the supply of water in each dose does not
exceed the instantaneous demand of the crop (Oweis and Hachum 2012 ).
3.2 Supplemental Irrigation for Early Sowing
In the lowlands, farmers usually sow their land when sufficient rain has fallen, i.e.
at the ‘onset rainfall’, meaning the beginning of the rainy season. It is implicitly
assumed that there will be little risk of an early dry spell for the crop. Nevertheless,
there is always the risk of a false start to the rainy season or a late start. It is in these
situations that SI plays a role. Farmers can decide at the beginning of the growing
season whether to help their crop combat terminal drought during its later stages.
November–December is the optimal sowing period in WANA countries. In the
winter-rainfall environment of the WANA region, delaying sowing retards crop ger-
mination and seedling establishment due to the rapid drop in air temperature around
November. In the lowlands of the WANA region, where continuous cropping pre-
vails as pure cereal or cereal–legume rotations, mid-November has been established
as the optimum sowing time for cereals. Weekly delays after this time reduce yields
by 200–250 kg ha−^1 (Oweis and Hachum 2012 ). If the onset of seasonal rain is
delayed, early sowing can be realized with the help of an SI system. With SI, it is
possible to sow predominantly rainfed crops without needing to wait for the onset
of seasonal rain. This results in a longer growing season, better yields and an earlier
maturity that helps crops to escape terminal drought.
In the highlands, frost conditions occur between December and March and field
crops remain dormant. Usually, the first rainfall sufficient to germinate seeds (the
onset rain) comes late (November) and results in a short crop stand when frost
occurs in December. As a result, rainfed yields are much lower than when the pre-
frost crop stand is good. Ensuring a good crop stand before frost sets in can be
achieved by early sowing and application of 50–70 mm of SI. Supplemental irriga-
tion at early sowing dramatically increases wheat yield and water productivity. In
the highlands of Turkey, applying 50 mm of SI to wheat sown early increased grain
yield by more than 60 %, adding more than 2.0 t ha−^1 to the average rainfed yield of
3.2 t ha−^1 (Ilbeyi et al. 2006 ). Water productivity reached 4.4 kg m−^3 of consumed
water compared to WP values for wheat of 1–2 kg m−^3 under traditional practices.
Similar results were found in the Iran Highlands for wheat and barley (Tavakoli
et al. 2010 ).
V. Nangia and T. Oweis