A method was developed to determine the best possible set of design parameters for regional, complex, multi-purpose water resources system. The parameters were the design capacities of a complex reservoir system. The optimization was subject to the requirements of the many constraints that must be imposed, including hydrological, economic, social, political and legal constraints, as well as the usual physical limitations. The original problem of high dimensionality was decomposed by successive approximations into a series of subproblems of low dimensionality in such a way that the sequence of optimizations over the subproblems converged back to the original one. The optimal design was obtained by maximizing the net benefits subject to the required constraints. (Author)