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This paper aims to solve an optimal irrigation water resources allocation problem applied to Alqueva River Basin irrigation system in Portugal. A multiobjective model was developed for this purpose in which the economic, social and environmental benefits are considered. This work makes use of a what-if analysis tool intended to help decision makers and stakeholders evaluate proposed sets of weights and intends to identify real and applicable water resources allocation plans for an irrigation system. The possible interaction with these actors in order to get more adequate results is an added value compared with the studied models. The results obtained based on an augmented weighted Chebyshev's approach represent a commitment between the results achieved in each single-objective model, having two main solutions vectors: Solution 1 for a large variability of sets of weights, with higher representation; and Solution 2 for a specific set of weights. For a few sets of weights, there are several solutions with low representation. The 5 crops under analysis were distributed to the 18 counties and to three water sources according to the weight coefficients assigned to each objective function. The outcomes of the study are valuable for supporting the adjustment of the existing irrigation patterns and to identify a desired irrigation plan. If the decision maker considers that the crop area should be different, a constraint is introduced, gathering a different crop allocation.