## 1 Introduction  
Drought has complex environmental impacts that affect the quality, structure, and diversity of many systems (e.g., soil, air, plants, forests, aquatic systems, and wildlife) [1; 2; 3]. Drought increases the possibility of forest fires and degrades the health of vegetation [4]. Global warming induces the increased evaporation of lake water, leading to desiccation in semi-arid regions, the replacement of forests by grasslands, and ecological degradation [5]. Woody plant mortality due to widespread drought is observed worldwide and may be exacerbated by future climate change [6]. Over the last 10-20 years, the number of studies on the effects of drought on water quality has increased, mainly in North America, Europe, and Australia [7]. Drought fundamentally alters nutrient cycling and biota in watersheds and reservoirs, and has long-term negative effects on drinking water quality [8]. Drought also affects the biodiversity by changing habitat conditions. Changes in water environments are associated with increased mortality and decreased birth rates of aquatic organisms [9]. [PERSON] et al. [10] predicted that global warming will increase drought risk and severity throughout the subtropics and mid-latitudes in both hemispheres over the next century. Drought events have wide-ranging transboundary, environmental, and socioeconomic effects on various sectors, including agriculture, energy production, public water supply, and water quality [11; 12; 13]. Under the combined influence of climate change and human activities, a series of water problems, such as insufficient supply, are being observed worldwide [14]. [PERSON] et al. [15] reported that more than 50% of Earth's accessible freshwater runoff is utilized for human use, and more than one billion people have no access to clean drinking water. In addition, almost three billion people lack basic sanitation services. Semi-arid and arid regions primarily rely on precipitation or glacial meltwater; therefore, climate change greatly affects the water supply in such areas [16]. Most urbanized coastal cities lack freshwater resources and rely on supplies from adjacent inland watersheds [17]. Owing to economic development and population concentrations in coastal areas, the water supplies therein face significant challenges under climate change. Water shortages have led to the increased utilization of alternative sources such as imported water, desalinated seawater, and reclaimed water [18; 19]. In addition, methods for the enhancement of the water supply, such as water recovery, reuse, and recycling (including rainwater harvesting) have been proposed and implemented [19]. Climate change will lead to new challenges for water supply planning and management in many regions in the 21 st century [20]. Since 2012, South Korea has experienced major challenges due to drought. According to the national drought information portal [21], the number of people affected by a drought-limited local water supply was 508 in 2012. The water supply for domestic use was adjusted (20% reduction in seven cities in the Chungnam Province) in 2015. In 2016, 11,123 people in 35 cities and municipalities were affected by water shortages. This number increased to 26,853 (51 cities) in 2017 and to 111,473 (22 municipalities) in 2018. In 2019, 9789 people in 15 municipalities were affected by water shortages. However, with respect to the local water supply, preemptive responses to drought (e.g., controlling the water supply as river flow decreases) are challenging, owing to the absence of river flow monitoring facilities. Hydrological drought is defined as a significant decrease in water availability in all its forms appearing in the land phase of the hydrological cycle [22]. Runoff data are often used to assess hydrological droughs [23]. Hydrological drought indices, such as the Palmer Hydrological Drought Index (PHDI) [24] or the Surface Water Supply Index (SWSI) [25], have problems with complex data collection and large computational efforts.