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The Vu Gia Thu Bon catchment has a complicated river system. Although the length of the two main rivers reaches 200 km, there is only one flow measuring station in the middle of each main river: Nong Son on the Thu Bon branch and Thanh My on the Vu Gia branch. This situation creates several difficulties for comparison of the results between simulations and observations. Especially, this is not only an inconvenience for predicting flood risk in the downstream region, but it is also a factor that produces uncertainty when assessing the impact of climate change on runoff. The lack of observation data for comparing with simulation results degrades the performance of the distributed model. Many hydrologists have suggested realizing calibration by multi-site, with not only the discharges but also the water levels (Wang et al. 2012). Accordingly, the water level at Ai Nghia station, Cam Le Station on the Vu Gia branch and Hiep Duc, Giao Thuy, Cau Lau stations on the Thu Bon branch are also compared with the MIKE SHE model outputs in order to increase the confidence and simultaneously to reduce the uncertainty in the projected climate scenario. The model assessment is performed with statistical measures of the root mean squared error (RMSE) (Equation (2)), the correlation coefficient (R) (Equation (3)), and Nash–Sutcliffe coefficient (E) (Equation (4)). The smaller the RMSE value is, the higher the model performance will reach. The perfect value of RMSE is 0 (Moriasi et al. 2007). The performance levels of R and E are classified in Table 1. The optimal values of these two factors are all 1 (Safari et al. 2012; Wang et al. 2012).

Table 1

Performance criteria for model evaluation (Wang et al. 2012)

Performance indicatorExcellentGoodFairPoor
>0.85 0.65–0.85 0.5–0.65 <0.5 
>0.95 0.85–0.95 0.75–0.85 <0.75 
Performance indicatorExcellentGoodFairPoor
>0.85 0.65–0.85 0.5–0.65 <0.5 
>0.95 0.85–0.95 0.75–0.85 <0.75 

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