## 1 Introduction  
Recently, as the importance of water resource management increases due to the occurrence of drought in an environment such as global warming, interest in forests that play a large role in the global water cycle is increasing. Forests with high water storage capabilities contribute to drought as well as flood control [1]. An accurate understanding of forest water storage capacity in water circulation functions is crucial for preventing water-related disasters and for sustainable water management at the national level [2, 3]. However, information on the forest vertical structure for estimating the exact water storage capacity of forests is not quantified in most forest regions because of the complexity and continuous change of the forest. Therefore, forest vertical structure information is essential for understanding the water storage function of forests and for quantifying water storage capacity of forest. As most water resources originate in forests, forests should be well preserved and managed to protect and maintain the quality of downstream rivers, lakes, and seawater [4]. Forests have thick layers of organic matter and high water penetrating soils that hold a lot of rainwater. By regulating the timing of water supply, forests act as a huge reservoir that mitigates the extreme damage caused by water. The forest vertical structure, which is one of the important factors controlling the amount of water the forest can store, is generally divided into four layers of canopy, understory, shrub, and grass [5]. Forests differ in the degree of water storage by major species, and hardwoods have a higher water storage capacity than conifers [6]. Natural forests with a variety of vertical structures have high vitality and high water storage capacity, whereas artificial coniferous forests have low water storage capacity; artificial forests generally have lower levels than natural forests so that forest management can reduce the amount of evapotranspiration and increase the available water resources [7]. Therefore, it is necessary to manage forests with high water storage capacity, considering the vertical structure along with forest species. In recent years, the Korean Forest Service is investigating forests based on aerial photographs to produce forest maps. It is difficult to figure out the internal structure of the forest with this method, so the vertical structure information is inevitably constructed through field survey by manpower. Due to the characteristic of forest areas that are inaccessible to people, a large amount of budget is required for field surveys in which 70% of the country is mountainous in South Korea. In addition, it takes a certain time to survey a specific region, making it difficult to build data at the same time for multiple regions with delayed updates. Satellite images enable periodic forest mapping as an effective approach to continuous exploration over large areas where human access is not easy or impossible [8; 9; 10]. It is not possible to directly estimate the water storage capacity of the forest, but it is possible to estimate the forest vertical structure [11]. For the vegetation researches, an index map using multiple wavelength bands of an image is generally used to detect vegetation characteristics [12]. Indicators widely used in the vegetation field include normalized difference vegetation index (NDVI) [13], normalized difference water index (NDWI) [14], texture, etc. For Sentinel-2 images, the red edge channels can be provided to produce a normalized difference red edge index (NDre) [15]. As the reflectivity of the red edge wavelength band increases in vegetation, it is effectively known for vegetation discrimination [16]. NDVI and Red Edge Band of Sentinel-2 images were used to distinguish field crops and showed its usefulness for classifying vegetation [17].