Taking into account ETm, for non-stress conditions about 50 to 60 percent of available soil water can be utilized by the crop before the next irrigation, with somewhat higher depletion levels during the.ripening period (4). There is a distinct advantage for both winter and spring wheat in having the entire root zone filled to field capacity prior to or soon after sowing to attain optimum root development. Over watering during the vegetative period (1) produces luxurious growth that can cause lodging, which may also occur after a too heavy irrigation in the late yield formation period (late 3), particularly with sprinkler irrigation.
Where rainfall is low and irrigation water supply is limited, in addition to preirrigation, applications should be scheduled to avoid water deficits during the flowering period (2) and at about 50 to 70 days after sowing of spring wheat. Irrigation and rainfall during the late yield formation period (late 3) show little effect on yields when sufficient soil water in the root zone is carried over from the previous period to see the crop into the ripening period.
Selected Recommendations for Different Regions
Kansas, USA - Winter Wheat
To irrigate a large area most efficiently with limited water supply: one heavy autumn irrigation to wet the full root zone during September-November, one spring irrigation and one irrigation in the flowering period (2).
Texas, USA - Winter Wheat
1. high production level: preplanting irrigation to wet soil profile to 1.5 rn, one irrigation when about 120 mm of soil water has been depleted which with normal rainfall is about March, with next two irrigations at the end of April and late May. When rainfall is below normal during spring, irrigation may be required starting early March.
2. medium production level: preplanting irrigation with first spring irrigation when some 150 mm has been depleted at the beginning of April, the second at beginning of May. In dry areas three spring irrigations may be required.
3. low production level: preplanting irrigation or during germination, with one spring irrigation early May to ensure sufficient soil water available during peak water requirement period.
USSR - Winter Wheat
High yield with one full irrigation and one to four spring irrigations with soil water depletion-in the first 1 m soil depth not exceeding 70 percent of the total available water.
Canada - Winter Wheat
One irrigation during the establishment period (0) but still beneficial when applied as late as the flowering period.(2). With two irrigations, highest yields when applied during early vegetative (la) and flowering periods. With three irrigations the additional application is given at the late vegetative period (lc) but the effect on yield between two and three spring irrigations may be small.
Israel (approximately 250 mm winter rain) - Winter Wheat
Sowing in dry soil with application of 150 min after sowing; in the case of substantial rain, irrigation should bring the upper 0.6 m to field capacity. The second irrigation is applied when water depletion in the upper 1 m has reached 100 to 120 mm. In average years no further irrigation is required until heading. If at flowering (heading) soil water depletion is less than 30 mm, irrigation can be delayed until early yield formation period (early 3) when the first 0.6 m is brought to field capacity. If at heading soil water depletion is greater than 50 mm irrigation should be applied at that time with no further irrigation afterwards.
Israel - Spring Wheat
During early growth adequate water should be available in the soil profile and one application of 150 mm is recommended. Winter rains of 250 mm are usually adequate to bring the crop to maturity. At flowering 150 mm of available soil water should be stored in the root zone; when smaller, an additional irrigation is required.
Northern India - Spring Wheat
In addition to pre-irrigation, one irrigation during early vegetative period (la); two irrigations - one during early vegetative period (1a) and one just prior to head emergence through flowering period (2); three irrigations - during early vegetative period (1a), just prior to head development through flowering period (2) and early yield formation period (early 3); four irrigations - during early vegetative (1a), late vegetative (1c), flowering (2) and early yield formation (early 3) periods.