A decision support system for determining reservoir capacity, named as KORESIDSS (KOwaco's REservoir SIzing Decision Support System), was developed. The system is composed of three subsystems; a database/information subsystem, a model subsystem, and an output subsystem. This system is operated using MS-Windows with a GUI (Graphic User Interface) system developed using Visual Basic 5.0. As a continuous runoff model, the DAWAST model (DAily WAtershed STreamflow model) developed by Noh(1991) was and its analysis module was developed. This system was applied to a newly-planned dam, the Cheongyan Dam, Which will be located in Cheongyang-Gun, Chungcheongnam-Do and it was proved to be applicable in determining reservoir storage.