## 3 Results
### Drought Impact Assessment and Drought Monitoring  
The RSIDI was evaluated for the 2001 drought (Figures 5 and 6). The severe spring drought in 2001 began in the fall of 2000 and lasted until the spring of 2001. In spring, when the agricultural water demand was the highest due to the rice planting, the supply of agricultural water was insufficient, causing serious agricultural damages. In most parts of the Korean Peninsula, less than 50% of the average annual rainfall was recorded, and in some areas, only 10 to 30% of the average annual rainfall resulted in the most extensive agricultural drought damage in June [43]. The drought that occurred in 2001 was mostly resolved after more than 150 mm of rainfall since mid-June.  
The SPI and WBDI illustrate the drought from April to May was a serious event, and the drought centered in the central region since September also appears to be approaching the serious stage (Figure 5). The ROC analysis between the RSIDI and three drought indices also showed that the WBDI had the highest with 0.90, followed by SPI at 0.78, and ADCI at 0.65. For ADCI, the observed indicators showed significant spatial variation compared to the other drought indices, and in the 2000 drought, the effects of changes in the SMSI resulted in a weaker or earlier drought peak than those observed in cases of other drought indices (Figure 6). These features appear to be more sensitive to short-term droughts as the ADCI applied in this study was used only for the surface soil moisture.  
Figure 4: Time series of integrated drought index (IDI) for 2001–2019 in the Gojicheon stream in the Han River basin. (**a**) IDI index and (**b**) relative contribution evaluation.  
Figures 7 and 8 show the results of the evaluation of the 2014 drought. In 2014, a drought occurred around the Han River basin; in the same year, the Gangwon Province had 70% of the average annual rainfall and the Gyeonggi Province area around Seoul had 59%,  
Figure 5: Spatial change of each drought index: SPI, ADCI, WBDI, and IDI (integrated drought index) for the 2001 drought.  
Figure 6: Spatial change of each drought index: SMSI (soil moisture saturation index), VCI (vegetation condition index), TCI (temperature condition index), and ADCI for the 2001 drought.  
which was less compared to the previous years' average annual rainfall. In particular, in 2014, a dry monsoon phenomenon occurred in which a drought that began in spring (due to the El Nino phenomenon) continued to cause no rainfall or a significantly lower amount of rainfall [44]. The rainfall between June 2014 and July 2014 was 48% (compared to the average in the past years), and the national water storage level also dropped significantly to 64% over the previous year. In August, the average water level of multi-purpose dams in Korea was only 36.1%. The droughts lasted until 2015, resulting in less than 70% of the average rainfall, and the hydrological droughts, with reservoirs in many multipurpose dams, reached dangerous levels [45]. Similar to the results of 2001, the RSIDI obtained could effectively describe the time and spatial occurrence patterns of the SPI and the WBDI, while the ADCI was analyzed to have delayed drought due to changes in SMSI. The ROC statistical analysis also confirmed that WBDI was highest and ADCI was relatively low in 2014 (SPI: 0.81, ADCI: 0.61, WBDI: 0.88). Through evaluation of past droughts, the RSIDI explained the three drought characteristics (meteorological, agricultural, and hydrological) well and confirmed the applicability of the integrated drought index through ROC analysis.  
Figure 7: Spatial change of each drought index: SPI (standardized precipitation index), ADCI (agricultural dry condition index), WBDI (water budget-based drought index), and IDI (integrated drought index) for the 2014 drought.