The conjunctive use of a surface reservoir and a ground-water aquifer was analyzed from the point of view of optimal operation. A mathematical model was established considering the system to be operated for N consecutive years. The method of dynamic programming was used in developing an optimal operating rule. A computer program was developed for the solution of the algorithm by means of the IBM 7090 digital computer. An optimal policy was obtained as a steady state solution, when the operating rule remained constant, irrespective of how many stages remained in the operation of the system. The computer program was tested for several combinations of input data, and an optimal policy was established for each combination. An illustrative example is included.