In order to overcome the shortcomings in the difference between the observed and simulated water supply through previous studies, supply estimation method was developed using the reservoir rate data that are being monitored. To verify the supply estimation method, eight districts in Korea were selected. The data for the last 15 years (2002-2016) were set as the model period of calibration, and the previous three years (1999-2001) were set as verification period. The summary of the study is as follows:
1. We estimated the previous irrigation water supply using reservoir rate and meteorological data based on Water Balance Equation. The reservoir average irrigation supply ranged from 803 mm to 4,976 mm. This is judged as a difference in irrigation supply in addition to irrigation supply in each region and reservoir.

2. Drought years was considered based on the decrease in the irrigation water supply compared to average, and the drought coefficient was adapted into the estimation procedure using Optimization technique. As a result, the reservoir storage rate before the start of the irrigation was analyzed to be less than 57%, the annual precipitation rate was less than 57% and the drought supply coefficient was 0.7. By comparing the results before and after considering the drought years, it was confirmed that the supply accuracy has remarkably improved, which proves the water supply decreases during the drought years.

3. Using the calculated precipitation data that is closely related to the irrigation supply data, we 1.) proposed a linear regression (LR) method for estimating the amount of rainfall supply based on linear regression equations. The supply tendencies calculated for the (2002-2016) and (1999-2001) data show similarities, the evaluation resulted “Satisfactory” for Okgye, Botong, Waryong, Gopung and Hwasan reservoirs. On the other hand, due to the changes in the farming methods, the recent 15-year and prior 3-year water supply are different and resulted to “Not Satisfactory”. The “Not Satisfactory” results were calculated for Dogap and Seoknam reservoirs for both 15-year and 3-year verification analysis.

4. By including an optimization method to the existing estimation method, an improved Water Demand (WD) method was proposed. “Satisfactory” results were only observed for Gopung reservoir.

The results confirm that each reservoir is managed under different practices based on the corresponding seasons. More reliable irrigation supply was estimated using readily available measurement data from the reservoirs. It is then considered that scientific and reliable water management will be possible through the connection between the field channels in the future.