## 2 Materials and Methods  
### Drought Determination Criteria Considering Measurement Data and Hydrologic Models  
Low flow (LF) refers to the minimum flow of natural rivers, which humans and nature share during the dry season. LF functions as the reference flow for setting intake volumes based on the evaluation of the capacity of river water to meet demand. In Japan and South Korea, average and standard LFs are used, whereas the 10-year frequency and 7-day flow are used as LF indicators in the U.S. and U.K. [34]. Although historical observation flow data are required to calculate the amount of water, water level observation facilities are limited in Korea. Therefore, it is difficult to calculate the amount of water that might be available during the dry season.  
In this study, the average water intake amount and actual or simulated flow were calculated by applying a drainage-area ratio (DAR). The DAR is commonly used to transfer a measured streamflow time series to an ungauged basin based on the ratio of the respective drainage areas (Equation (1)). The daily average volume provided by the water source was subtracted from the result. Subsequently, the value was divided by the absolute water supply volume and the amount of water insufficiency at the supply facility was noted as a drought-index as a percentage value:  
\[DAR=A_{WIC}\ /\ A_{WLOS} \tag{1}\]  
where \(DAR\) is the drainage-area ratio, and \(A_{WIC}\) and \(A_{WLOS}\) are areas of the water intake station and water level observation stations, respectively. The water intake capacity (WIC) was simply estimated using the following equation (Equation (2)).  
\[WIC=DAR\times(DF_{WLOS}-LF_{WLOS}) \tag{2}\]  
where \(WIC\) is the water intake capacity (m\({}^{3}\)/day), and \(DF_{WLOS}\) and \(LF_{WLOS}\) are the daily flow and low flow at the water level observatory, respectively.  
Detailed methods for calculating the measurement data, hydrologic model-based water intake, application of DAR, and water insufficiency calculations at the water intake facility are presented in Figure 1.  
Flow data are required for calculating water intake. In this study, three methods were used to calculate the water intake:  
(1) When data from a nearby water level observatory were available, the measurement data-based water intake capacity (WIC) was calculated using the flow data derived from the flow rating curve.  
(2) At the intake point, a localized rainfall-runoff model was applied to calculate the WIC of the water source when data from a nearby water level observatory were not available. In this study, the Tank model, which is a storage-type hydrologic model widely used to determine runoff processes, was applied, but the hydrologic model can be selected depending on the status of the basin.  
(3) If data were missing because of winter freezing, or for other reasons, the WIC of the water source was calculated based on the two above-mentioned methods, even if daily flow data could be obtained from the flow duration curve (FDC) obtained at a water level observation station nearby.  
Based on the WIC, the drought index of water shortage in water intake facilities was calculated as the ratio of WIC minus daily average water intake to WIC value (Equation (3)):  
\[WSI\ (\%)=100\ *\frac{WIC-WIC_{AVG}}{|WIC|} \tag{3}\]  
where \(WIC_{AVG}\) is the daily average water intake capacity of the water intake sources.