The objectives of the research were to modify and validate a daily water balance model, DIROM, for irrigation reservoirs using field data, and to estimate daily water supply from the reservoirs in the Han river basin. Modified DIROM was applied to three test sites, and validated with field data. The relative errors between the simulated and observed water supply were less than 10 percent. Historical records on daily or ten-day’s storage for 110 reservoirs from the twenty Farmland Improvement Associations, FIA were collected and used to estimate the daily water supply during 1993 to 1997. The results were applied to the other 723 reservoirs that are not owned by FIA. The five-year averaged annual water supply from the reservoirs was estimated to be 180 million ㎥/yr. Maximum yearly water supply was recorded to be 190 million ㎥/yr in 1996.