# Analysis of the Water Circulation Structure in the Paldang Reservoir, South Korea
## 3 Results
### Water Circulation of Paldang Reservoir  
#### 3.2.1 Water Stability  
In this study, hydraulic, water temperature, and meteorological data were compared in an attempt to investigate the characteristics of water stability and circulation in the Paldang Reservoir (Figures 6-8). At PC1, a small range in the water temperature gradient was observed as a result of vertical mixing until 22 May, but the range increased afterwards, which led to lower water temperatures and a thicker low-temperature layer than those in the South Han River basin. During the vertical mixing period, flow velocities increased across all layers due to the discharge from the CPD. During the stratification period, flow velocities escalated in the middle and lower layers as the cross-sectional area of downstream flow decreased with water temperature, and, as a result, two-layer flow was clearly observed. The Richardson number was deemed stable in the surface and middle layers and unstable in the lower layers during most of the observation period, and all layers became unstable and vertical mixing intensified with strong winds and increasing discharge rates. The stratification coefficient mostly remained below 1/m\({}^{3}\) and was momentarily close to 1 J/m\({}^{3}\) until 29 May, which was the period when the vertical water temperature gradient was not large and when the vertical mixing and vertical water temperature gradient repeatedly increased. After 29 May, the stratification coefficient remained constant above 1 J/m\({}^{3}\), which corresponded to the intensifying stratification.  
Overall, at PC2, the water temperature was relatively high and the low-temperature layer was relatively thin compared to the conditions in the North Han River basin. Flow occurred in the downstream direction at all depths during the vertical mixing period and was accelerated in the upstream direction at the middle and lower layers during the stratification period, thus showing a distinct two-layer flow. The Richardson number remained stable in all layers during most of the observation periods. The stratification coefficient varied while exhibiting similar characteristics to PC1, but, overall, the values were higher than those at PC1. This was because the vertical water temperature gradient at PC2 remained higher than that at PC1. The stratification threshold of 1 J/m\({}^{3}\) remained constant after 19 May, which roughly corresponded to the time at which average daily temperatures became constant above 20 \({}^{\circ}\)C.  
At PC3, the vertical water temperature gradient was small until around 19 May and increased afterwards. Flow velocity intensified in all layers due to dam discharges during the vertical mixing period, and the velocity decreased at times to less than 0.02 m/s in the lower layer, depending on water discharge from the PDD, during the stratification period. The Richardson number was stable in the surface and middle layers during most of the observation periods, but it was mostly unstable in the lower layer. The stratification coefficient log (PEA) remained above 1 J/m\({}^{3}\) during the entire observation period. The coefficient gradually increased from the initial stage of observations until 2 June and remained constant at 3 J/m\({}^{3}\) afterwards.