Page - 571 - in Book of Full Papers - Symposium Hydro Engineering

moisture, and water supply ability in accordance with study objective. Numerous drought indices had been developed to express droughts which result in various social effects. Typical drought indices are Standardized Precipitation Index (SPI, [5]), Palmer Drought Severity Index (PDSI, [6]), Surface Water Supply Index (SWSI, [7]), Standardized Precipitation Evapotranspiration Index (SPEI, [8]). However, most drought indices are calculated by the lack of amount over the normal phenomenon or the occurrence probability, which has a problem that does not reflect the water supply systems. Modern water use mainly depends on reservoir’s water supply system. Therefore, to respond drought in a timely manner and mitigate drought damages, the drought index that considers the dam operation is needed instead of evaluating the shortage of the average or the probability of occurrence. This study developed Dam Inflow Drought Index (DIDI) considering the criteria for reduction in water supply related to dam operation during a drought. DIDI needs threshold of the cumulative difference between the critical inflow and the observed inflow. In this study, optimal threshold calculated by Receiver Operating Characteristics (ROC) analysis. ROC analysis is commonly used in verification of weather forecast ([9]), but it is also used in verification of drought indices compared with actual drought cases ([10], [11], [12]). The DIDI could be used as the drought monitoring tool that can effectively detect drought of dam basin. Droughts, unlike floods, are characterized by slow progress. Appropriate drought index reflecting the characteristic of water supply system such as the DIDI would contribute to coping with drought and mitigating drought damages. 2. METHOD Multipurpose dams in Korea have criteria for reduction in water supply during the drought. When using this criterion, we can calculate critical inflow for keeping the criteria by “Eq. [1].” 𝐼 𝐶 ,𝑖 = 𝐴 𝐶 𝑖 − 𝐴 𝐶 𝑖 −1 + 𝑆 𝑃 ,𝑖 [1] where 𝐼 𝐶 is the critical inflow, 𝐴 𝐶 is the storage of the water supply adjustment criteria, 𝑆 𝑃 is the amount of the planned water supply, and 𝑖 is the month. The difference between the observed inflow and the critical inflow for each month can be calculated by “Eq. [2],” and cumulative the anomalies are calculated by “Eq. [3].” Then the DIDI is calculated by “Eq. [4].” 𝐷 𝐼 𝑖 = 𝐼 𝑂 ,𝑖 − 𝐼 𝐶 ,𝑖 [2] 𝐶 𝐷 𝐼 𝑖 = 𝐶 𝐷 𝐼 𝑖 −1 +𝐷 𝐼 𝑖 [3] 𝐶 𝐷 𝐼 𝑖 = 𝐶 𝐷 𝐼 /𝜎 (𝐶 𝐷 𝐼 ) [4] 571