Abstract
A computer simulation model is developed to study the effect of adding one supplemental irrigation of 75mm on the relative yield of wheat . The growing season of wheat is divided into 26 weeks and a single irrigation is added to any one of the weeks . The model is run for 28 years through the growing season of wheat with two different scenarios for initial water contents in the root zone. The results show that for the first scenario the best single supplemental irrigation is at the first or second or the fifth week, but for the second scenario is at the first or sixth week ,it can be said that with good initial moisture in the root zone at the beginning the best choice is to add single irrigation at the 19thweek.
