# Analysis of the Water Circulation Structure in the Paldang Reservoir, South Korea
## 3 Results
### Water Circulation of Paldang Reservoir
#### 3.2.3 Three-Dimensional Water Circulation  
The Paldang Reservoir exhibits a complex, three-dimensional water circulation structure as a consequence of various factors including its complex topography and tributary inflows, as well as the discharge patterns of weirs and dams. In the Paldang Reservoir, flow characteristics and water temperature variations were found to be drastically different for each station, and these were affected by the stratification due to the temperature differences in discharge water between the IPW in the South Han River and the CPD in the North Han River. During the vertical mixing period, flow occurred in  
Figure 10: Relationships between velocity and discharge.  
the downstream direction across all layers in the North Han River and the South Han River, and flow velocity increased rapidly with discharge. During the stratification period, temperature differences in the discharge water between the IPW and the CPD led to changes in the flow structure.  
The three-dimensional circulation structure of the Paldang Reservoir is illustrated in the cross-section diagram shown in Figure 11. The surface layer in the middle-lower reaches of the North Han River accommodates the IPW-originated high-temperature water flowing in the upstream direction. Meanwhile, the lower layer contains the CPD-originated low-temperature water, and flow occurs in the downstream direction. Moreover, a convergence zone forms near the middle reaches of the North Han River, which results in the stagnation of water. The vertical mixing of suspended materials hardly takes place on the surface layer in the middle-lower reaches of the North Han River because the surface layer remains mostly stable. The surface layer in the lower reaches of the South Han River accommodates the IPW-originated high-temperature water flowing in the downstream direction. The middle and lower layers contain the CPD-originated low-temperature water flowing in the upstream direction. The low-temperature, formerly-discharged old water from the CPD resides in the lower layer.  
The complex three-dimensional water circulation structure has a major influence on the water quality of the Paldang Reservoir. To verify this, a time series of DO variation and data on the prevalence of annual hypoxia (below 2 mg/L) were collected from key locations in the Paldang Reservoir, such as at Paldangdam Sites 1-4 (PD1-PD4) of the water quality monitoring network of the Ministry of Environment, South Korea, at approximately eight-day intervals from 2012 to 2018. The resulting data are shown in Figure 12. In the North Han River (PD4), no hypoxic conditions (below 2 mg/L)  
Figure 11: Diagram of the water circulation structure in the Paldang Reservoir (stratification period).  
were detected. In contrast, hypoxia was observed in the lower layer of the South Han River (PD1 and PD3) and in front of the Paldangdam (PD2) during summer from June through August, which is when the stratification mostly occurred. The hypoxic conditions prevailed in Paldang Dam 1 Site (PD1), which is located further upstream from Paldangdam 3 Site (PD3). This can be attributed to limited vertical mixing between the surface water with high DO concentrations and the bottom water below the thermocline with a high water mass stability and an increased water residence time caused by upstream flow occurring in the presence of stratification in the lower layer of the South Han River.