# An Estimation Method of River Dry Runoff Alteration after Upper New Reservoirs Storage
## 3 Results and Discussion
### Runoff Prediction  
In practice, the construction of the Baihetan and Wudongde dams was completed in 2021. However, it should be noted that the full impact of these dams on river runoff will take several years to manifest. Additionally, the recorded hydrological data for the most recent year has not yet been published. Therefore, the runoff increments in the driest month, February, after the implementation of these two dam projects, were predicted using the estimation method proposed in this paper. According to the designed parameters of the dams, the additional adjustable capacity was 13.5 billion m\({}^{3}\). Similarly, the theoretical increment of the driest month could be calculated as 893 m\({}^{3}\)/s. Given the practical bias of the theoretical result at 26%, a more precise increment of 654 m\({}^{3}\)/s could be calculated using Equation (5), as seen in Table 4.  
In other words, the predicted result suggests that the monthly average runoff in February is expected to increase from 2212 m\({}^{3}\)/s to 2866 m\({}^{3}\)/s following the construction of the Baihetan and Wudongde dams. The differences in the regulating storage capacity and operational scheduling methods of reservoir groups in various basins, along with factors such as in-stream water withdrawals and evaporation, result in variations in the increase of dry season flow caused by the addition of the same adjustable storage capacity in the Jinsha River and Min River, as depicted in Figure 7.  
For channel regulation engineering, the assurance runoff is a critical parameter in the design process. Currently, the design standard for the channel from YinBin to Chongqing aims to ensure that ships with a loading capacity of 2000 t can navigate 95% of the time throughout the year. This corresponds to a 95% assurance runoff. Based on the recorded data of the Jinsha River, the assurance runoff since 2014, when the Xiluodu and other dams were constructed, has been measured at 1749 m\({}^{3}\)/s. This represents an increase of 527 m\({}^{3}\)/s compared to the period prior to the construction of any dams before the year 2000. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Reservoir** & **Monthly Average Runoff Increase in February** \\ \hline
**Name** & **Adjustable Volume (Billion m\({}^{3}\))** & **Theoretical (m\({}^{3}\)/s)** & **Deviation** \\ \hline Ertan + Xiluodu + Jinping 1 st & 15.7 & 1028 & 26\% \\ + Xiangjiaba & & & \\ \hline Wudongde + Baihetan & 13.4 & 654 (Deduction deviation) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Prediction of downstream river flow increase due to hydropower development in the Jinsha River Basin. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Monthly Average Runoff Increase in February** \\ \hline
**Name** & **Adjustable Volume (Billion m\({}^{3}\))** & **Theoretical (m\({}^{3}\)/s)** & **Deviation** \\ \hline Zipingpu + Pubugou + Houziyan + Changheba & 5.5 & 492 & 13\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Increase in downstream river flow due to the construction of key projects in the Min River Basin. According to the analysis of the daily averaged runoff distribution throughout the year, it is observed that the period with daily averaged runoff below the 95% assurance runoff threshold is primarily concentrated in the driest month, which is February. It is likely that the runoff increments will also occur for most of the month of February. Therefore, we can estimate the predicted increment of the 95% assurance runoff by considering the ratio of the assurance runoff increment to the monthly average runoff increment in February, which has been found to be approximately 0.65.