Timing the last wheat irrigation may be a difficult decision. Applying an additional irrigation at the end of the season can waste water and cause unnecessary lodging. However, water stress at the end of the season may reduce quality and production.
Various conditions affect how growers will determine when to put on that last irrigation. Considerations include the growth stage of the crop, soil type, anticipated weather conditions, variety, and water availability (a 3-day carryover can make a difference under warm or hot conditions).
On the average, about 3-4 inches of water is needed to carry a crop from soft dough to maturity. A sandy loam soil will hold about 1.4 inches of water per foot. A root system of 2.5 feet would have enough soil water, assuming average weather conditions.
A heavy silty clay soil can hold 2.3 inches of water per foot and should more than adequately carry the crop to maturity with the last irrigation at soft dough. In no case should irrigation water be applied once the stems beneath the heads start to turn tan or brown.
Reduced irrigation interval data from wheat trials conducted at the UC Desert Research and Extension Center in Imperial Valley during the 2002-03 growing season indicated an average of 0.5 ton reduction in wheat yield with 25 percent less water use (6 inches less water for the season).
Wheat is generally irrigated when 50 percent of the available soil water is depleted. Irrigation data from a study conducted at the Maricopa Agricultural Center in Central Arizona showed an 837-pound increase in production by irrigating at 35 percent depletion rather than 50 percent depletion.
Irrigating at 35 percent depletions will definitely use more water. The economics of such a decision may not be beneficial depending on cost vs. revenue.
Irrigating at depletion levels below 50 increases crop water use due of increased evaporation.
Crop coefficients and CIMIS data can be used to predict water use on wheat.