## 2 Materials and Methods  
### Study Site and Data Used  
Boryeong Dam watershed located in South Chungcheong Province of South Korea was selected as the study site (Figure 1). It is near the coast of the West Sea and belongs to a monsoonal climatic region with seasonal cycle of precipitation. The dam's height and crest length are 50 m and 291 m, respectively [22]. The dam has an effective storage of \(108.7\times 106\) m\({}^{3}\) and supplies municipal and industrial waters to three cities (Boryeong, Seosan and Dangjin), five counties (Seocheon, Cheongyang, Hongseong, Yesan and Taean) and two electrical power plants (Dangjin and Seobu). The watershed drainage area upstream of the dam site is 163 km\({}^{2}\)[1]. The average slope is 36.9%, and the average elevation is 233.4 m. The basin's topography is mainly covered with forest (72%) and agricultural land (17%) and had undergone very little changes since early 1900s [23]. This watershed is a fluvial system, which is governed by rainfall-driven peak in wet season. The annual average precipitation of the watershed is about 1305 mm/year, and approximately two-thirds of the annual precipitation occurs during warm seasons from June to September. The annual mean discharge and runoff ratio are 4.19 m\({}^{3}\)/s and 0.58. The average annual temperature, monthly maximum temperature and monthly minimum temperature are 12.8, 36, and \(-\)13.6 \({}^{\circ}\)C, respectively.  
In this study, monthly reservoir inflow data of Boryeong Dam are obtained from Water Resources Management Information System [24] of Korea (1998-2016) to be used as predictands. Our target seasons for forecasts are dry periods spanning from January to May and from November to December, and the wet seasons (June to October) are excluded because the prediction of reservoir inflow becomes more important for water supply during this season. In the literature, there are similar studies in spirit, which focused on forecasting specifically for dry season streamflow, which is the critical season that allows for the decision to be made on optimal allocation of precious water resources in the context of the drought condition [25; 26; 27]. Monthly long-term observed precipitation data, which are the areal precipitation from stations within Boryeong Dam watershed, were taken from WAMIS and K-water and were used for the analysis. The gauging stations are located in Figure 1. The monthly average precipitation, dam inflow and runoff coefficient are shown in Table 1 and Figure 2.  
We also used time series of many climatic indices available from the Climate Prediction Center [28] for the same period, which are updated monthly (Table 2).  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Data** & **Jan** & **Feb** & **Mar** & **Apr** & **May** & **Jun** & **Jul** & **Aug** & **Sep** & **Oct** & **Nov** & **Dec** \\ \hline Precipitation (mm) & 28.7 & 38.6 & 50.3 & 96.3 & 103.4 & 163.4 & 326.6 & 274.8 & 165.2 & 67.7 & 45.6 & 39.9 \\ Inflow depth (mm) & 10.9 & 17.0 & 18.4 & 44.6 & 50.3 & 63.8 & 237.9 & 198.5 & 119.1 & 33.6 & 13.5 & 15.0 \\ Runoff coefficient & 0.38 & 0.44 & 0.37 & 0.46 & 0.48 & 0.39 & 0.72 & 0.72 & 0.72 & 0.50 & 0.30 & 0.38 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Monthly average precipitation, reservoir inflow depth and runoff coefficient (1998–2016).  
Figure 1: Location of the Boryeong Dam watershed and its elevation.  
Figure 2: Monthly average precipitation, inflow depth and runoff coefficient of study area.