## 1 Introduction  
Like many regions in developed countries, demands on water resources of the Southern Chungnam province, fourth largest in South Korea, are continuously growing, but there is only one water source--Boryeong multi-purpose Dam. The available reservoir storage of Boryeong Dam is 116.9 million tons and designed to supply 23,800 tons per day for agricultural, residential and industrial use serving 420,000 of people in Western Chungnam Province [1]. Boryeong Dam is also supplying water to four major steam power plants, which generate about 25% of South Korea's electricity. Currently, the water use of Southern Chungnam Province is about 21,000 tons per day, which is approaching 90% of design demand. Water demand from steam power plants is expected to increase because of population growth. Furthermore, because of the region's adjacency to Seoul, the capital of South Korea, construction of an industrial complex has been planned in the near future, which will also increase water demand. These increasing water demands have raised several concernsand triggered discussions on additional dam building and water allocation issues. However, those structural measures need at least 5-10 years to solve the water shortage problem in Chungnam Province, which means, if the operational failure of the dam occurs in the meantime during drought, water availability may be threatened because of the storage shortage. In 2015, Western Chungnam Province had to reduce water supply by 20% for 5 months because of serious drought. Thus, reliable long lead-time reservoir inflow forecasting is essential because water resources manager must decide when to reduce environmental flows to secure more water during water shortage or to increase water release to prevent dam overflows. To reduce the negative impacts of climatic variability on dam operation, dam inflow forecasting model with at least 3 months lead time is needed to prepare for potential threat such as floods and droughs. For the water supply purpose, it is important to know in advance to save some water if the shortage in water is on the horizon of the near future. Boryeong Dam is very vulnerable to drought as its effective storage to annual water supply is 1:1 and is related with many different types of stakeholders including government, municipalities, agriculture, environmental organizations, enterprises, etc. [1]. To address the complex need for water resources before any decision on the adjustment of dam releases to be reached, a month or two is not a sufficient amount of time to do so. Thus, at least 3-months lead time, thus a season-ahead forecast, is needed, and it is quite common in forecasting practices [2; 3; 4]. In summary, the time required for local stakeholders to take operational actions and also the medium time horizon for forecasting needs were factored into the selection of lead time to be 3 months. There are two basic approaches for long-range hydroclimatic predictions: dynamic models and empirical statistical models. The dynamic approach is based on process models for climate and hydrology. The statistical approach is based on data driven models where the predictor-predictand relationship is the essence of the technology. Although it is anticipated that dynamic models may become superior to empirical models in the future, empirical forecasting models are still able to compete because they are less expensive to develop and use [5]. The hydroclimatic models used to forecast seasonal precipitation or streamflow are still far from perfect, and there are a lot of uncertainties to be considered. Especially, the models need to be in better compliance with climatic observations such as ocean-atmospheric indices, which are connected to climatic variability globally. These oceanic-atmospheric forcing predictors, called hydroclimatic teleconnections are widely used to forecast long range seasonal streamflow or precipitation.