The problem of optimal allocation of a limited water supply for irrigation of several crops, grown in the same area, is addressed. Both intraseasonal and interseasonal competition for water between crops are considered.

The specific objectives of this study are:

(1) to develop an optimization model to choose the optimal cropping pattern, among different alternatives, which satisfies the existing climatic, agronomic, soil, economic, and land and water availability constraints for a selected area,

(2) to design an efficient water distribution system that fulfils the crop water requirements, in time and quantity,

(3) to choose the most economic alternative based on cropping pattern profitability and distribution system cost. Ghazzah, a village in South Bekaa - Lebanon, has been chosen as a pilot study area.

A linear programming (LP) mathematical model is developed to solve the problem of water allocation. The objective function of the formulated model is to maximize net returns from a cropping pattern selected from crops grown in South Bekaa. The constraining variables include water and land availability as well as agronomic and production conditions. Two different scenarios, based on different constraints, are tested within the model. Results obtained from the LP model indicate that the current pattern is not optimal and should be replaced by another one. Accordingly, new crops are introduced to the area, the acreage of some current crops are rearranged, and some of the current crops in the cropping pattern are eliminated. Water distribution systems for the two scenarios are designed to meet the irrigation requirements of the cropping patterns. Finally, the recommended scenario is that having the highest net benefit to distribution system cost ratio.