To avoid constructing expensive hydraulic structures for implementing new water resources, one may design an intentionally deficit irrigation project to use optimally the available water resources. Previous papers on this matter have not considered irrigation efficiency, quantitatively, in their analysis. In fact, efficiency is a very significant factor in optimization analysis, and the potential for increasing irrigation efficiency is one of the key reasons for deficit irrigation. In this paper the influences of irrigation efficiency (under full irrigation condition, Ep) on the performances of deficit irrigation, and the effects of deficit irrigation on improving the irrigation efficiency (under deficit irrigation, Ea) have been considered for four crops of winter wheat, spring barley, corn, and sorghum under and arid region of I.R. Iran. The results showed that considerable improvements can be achieved under this concept. Some mathematical relationships were derived to show the quantitative increase in irrigation efficiency under deficit irrigation. These results showed that performances of deficit irrigation is highly dependent on Ep, such that lower Ep values result in higher allowable water reduction level and also more field incomes. Furthermore, the most and the least allowable water reductions were obtained for barley and corn, respectively.

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