*2.1. Site Description*

Seomjin River is one of the four major watersheds in South Korea and is located in the southwestern part of the Korean peninsula (34◦55–35◦45 N, 126◦57–127◦55 E) (Figure 1). The catchment area is approximately 4900 km2, and the length of the river is 223 km. The primary land use is a forest area (48%), and the second dominant land use is an agricultural area (33.6%) (Figure 1). The urban lands comprise approximately 4% of the total catchment area. The agricultural lands are mainly adjacent to stream channels. The annual precipitation has been 1384 ± 317 mm (mean ± SD) in the Seomjin River catchment for the last decade. The unique climate conditions, such as monsoon and typhoons, bring about >50% of the precipitation concentrated in summer between June and September (Figure 2). For this reason, several multi-purpose dams are operational to effectively manage water resources for irrigation, potable use, and flow regulation. The Korean Ministry of the Environment manages the Seomjin River watershed, dividing it into nine subwatersheds in compliance with their geophysical characteristics, specifically the main channel, tributaries, upstream, downstream, and dam location. Particularly for the two groups of subwatersheds; (i) Seomjin Dam in the upper site (SDU) and the Seomjin Dam in the lower site (SDL), and (ii) Juam Dam (JD) and Bo-Seong (BS), the main characteristics of these subwatersheds are separated by dam location. The former subwatersheds form part of the headwater of the main channel, while the latter are of the main tributary (Figure 1).

**Figure 1.** Map of the study area and land-use information in the Seomjin River catchment. The red circles indicate a total of 124 study sites. The entire watershed was divided into nine subwatersheds.

**Figure 2.** Total precipitation of the Seomjin River basin in 2017, based on nine major subwatersheds.
