Presented is the study on design and implementation of a fuzzy system to approximately reason using measured actual storage rate in irrigation reservoirs. To design Fuzzy reasoning systems. Minimum Operation Rule by Mamdani was applied. Fuzzy variable and membership functions are determined after identifying storage-rate affecting factor and followed simulation. Hydrological model to express actual situation within drought areal boundary is generally too complex. Hereby, considering irregularity of time-rate storage change during irrigation, this system uses irrigation water and meteorological data as a IN-data. It was abvious the results were closely corresponding to the actual data observed.