In this study, we propose a method for systematically estimating agricultural water demand using the EPA-SWMM model. We utilize watershed and water supply system data to ensure accurate estimations. A comparative analysis was conducted on agricultural water demand over the past ten years, from 2014 to 2023, for the Yongdang, Jangchi, and Geumgye reservoirs. The demand was compared and analyzed among three sources: the design criteria demand calculated using the Hydrological Operation Model for Water Resources System (HOMWRS), the demand calculated with the EPA-SWMM model using irrigation canal network, and the demand outlined in the National Water Management Basic Plan. The average design criteria water demand for the three study areas was found to be approximately 672 mm, while the water demand, taking into account the water supply system, was approximately 1,182 mm. On average, the water demand considering the water supply system was calculated to be 1.76 times the design criteria demand. The demand considering the water supply system was found to be less than the demand presented in the National Water Management Basic Plan but greater than the design criteria demand. The findings of this study can serve as foundational data for addressing the discrepancies between agricultural water demand and supply.