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3.2. COMPARISON WITH TYPICAL DROUGHT INDICES This chapter compared DIDI with typical drought indices such as SWSI and SPI. SWSI and SPI are widely used as drought indices. SWSI is calculated by monthly precipitation, snowpack, streamflow, and reservoir storage. Fig. 8 shows DIDI compared with SWSI. SWSI defines -1 or less as drought. The SWSI indicates drought at the time of reduction in water supply adequately, but it can be seen that the drought by SWSI is very common even in the normal water supply condition. SPI most widely used drought index and it was agreed that using SPI as universal drought index to cope with climate risks at ‘The Inter-Regional Workshop on Indices and Early Warning Systems for Drought’ held at the University of Nebraska-Lincoln. SPI can calculate simply and easily collect data due to using rainfall only. SPI is calculated by standardizing value of cumulative distribution function (CDF) of Gamma distribution that is estimated using sums of rainfall for a preset period (3 months, 6 months, 12 months, etc.). “Fig. 9” shows DIDI and SPI of 3, 6, 9, and 12 months. When visually confirmed, DIDI seems to reflect the point of the water supply reductions better. As shown in “Fig. 9,” one can see that 6-months and 12-months SPIs that indicate long-term drought are better able to monitor drought than 3-month and 6-month SPIs that show short- and mid-term. In order to evaluate the drought monitoring ability of SWSI and SPI, ROC analysis was also conducted for these. “Table 2” indicates ROC scores of DIDI, SWSI, SPI(3), SPI(6), SPI(9), and SPI(12). ROC score of DIDI was 0.93 which was 0.22 greater than it of SWSI and 0.33, 0.22, 0.16, and 0.17 greater than it of SPI(3), SPI(6), SPI(9), and SPI(12), respectively. By applying the dam operation and threshold, DIDI was able to express the drought of dam inflow adequately, which indicates that the DIDI is useful for monitoring and responding the drought, and its application would help to mitigate drought damages. Fig. 8 Comparison with DIDI and SWSI 576