Abstract:A field experiment was conducted by using a factorial randomized complete block design (RCBD) in three replicates with four irrigation intervals (7, 8, 10 and12 days), which equivalent to 14, 12, 10, and 8 irrigations for two seasons of 2012 and 2013. Irrigation water was applied to the spring var. 5018. The results showed that different irrigation intervals applied had statistically significant effect on number of days to male and female flowering, plant height, leaf area, root dry weight, biological weight and yield. The results in both full irrigations (7and 8 days) which was equivalent to 14, 12 irrigation respectively indicated that no significant difference (P<0.05) between these two treatments, although the maximum yield was obtained from full irrigation 7 days, but these treatments have significant difference (P<0.05) with deficit irrigation treatments (10and 12 days) which equivalent to 10and 8 irrigation in above plant traits and yield. The treatment of 8 days irrigation interval gave highest productivity of irrigation water 0.631 and 0.693 kg/m3 than other irrigation intervals of 7,10 and 12 days which were 0.604, 0.622, 0.552 and 0.587, 0.415 , 0.575 kg/m3in the two seasons respectively. The irrigation interval of 8 days saved about 14% of irrigation water per hectare comparing with other intervals. It can be concluded that the deficit irrigation can improve and increase the water productivity of corn associated with increased yield within an acceptable level under Iraq's semi-arid conditions.

Soil-water-atmosphere-plant (SWAP) relationship model is used to evaluate the impact of current irrigation practices on groundwater table depth, soil salinity and crop yields and to determine optimal irrigation requirements and drain depth for the study area. The results indicate that current irrigation practices of applying 600 mm to wheat and 1000 mm to maize are wasting more than 30% of applied irrigation water as deep percolation, which causes rise in groundwater table, increase in profile salinity and reduction in crop yields. The simulation results reveal that in the absence of an effective drainage system in the study area, a groundwater table depth of approximately 200 cm together with an irrigation application of 5000 m 3 ha -1 for wheat and 6000 m 3 ha -1 for maize will be the most appropriate combination for obtaining optimum yields of wheat (3.0 t ha -1 ) and maize (1.80 t ha -1 ). However, to achieve potential yields, leaching of excessive salts from the root zone through freshwater application would be essential. Therefore a drainage system in these areas should be installed to maintain groundwater table depth around 200 cm. Installation of deeper drains would not be feasible as it will increase the costs and without much gains in crop yields.