Water supply plan by optimal conjunctive use of surface and ground water is studied to prepare expected water deficit in near future. The optimization model for conjunctive use of surface and ground water is developed using discrete differential dynamic programming technique to maximize net benefit by water supply. As a result of applying the model to Namdaechun river located in Yangyang, it is found that water supply reliability using optimal conjunctive use of surface and ground water is much higher than reliability using surface water alone.???