## 3 Results and Discussion
### Analysis of Results of Water Management Optimization Calculations  
In order to determine the impact (positive or negative) of climate change on the water management situation in the basin of the VKC reservoirs, an water management optimization calculation was performed for the inflow series 1916-1978 and 1979-2020 according to the mathematical model, (6)-(20), presented in Section 2.3.2. Optimization was performed in the Microsoft Excel software using the generalized reduced gradient (GRG) method built into the Solver Add-in. The reliability for eight water user requirements (Table 10) for the inflow series 1916-1978 and 1979-2020, obtained as a result of performing water resource optimization calculation, is shown in Table 10 and Figure 11. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \begin{tabular}{c} **The** \\ **Long-Term** \\ **Series** \\ \end{tabular} & **Indicators** & \begin{tabular}{c} **1. No** \\ **Forcing,** \\ **Cubic km** \\ \end{tabular} & \begin{tabular}{c} **2. Filling** \\ **to NOL,** \\ **Cubic km** \\ \end{tabular} & \begin{tabular}{c} **3. Winter** \\ **Power,** \\ **MW** \\ \end{tabular} & \begin{tabular}{c} **4. Normal** \\ **Transport** \\ **cub.m/s** \\ \end{tabular} & \begin{tabular}{c} **5. Reduced** \\ **Transport,** \\ **cub.m/s** \\ \end{tabular} & \begin{tabular}{c} **6. Normal** \\ **Fishery,** \\ **cub.m/s** \\ \end{tabular} & \
\begin{tabular}{c} **7. Reduced** \\ **Fisbery,** \\ **cub.m/s** \\ \end{tabular} &
\begin{tabular}{c} **8.** \\ **Feicological** \\ **Release,** \\ **cub.m/s** \\ \end{tabular} \\ \hline
1916–1978 & Reliability & 98\% & 98\% & 100\% & 85\% & 95\% & 30\% & 70\% & 93\% \\ \hline
1979–2020 & Reliability & 100\% & 100\% & 100\% & 98\% & 100\% & 63\% & 93\% & 100\% \\ \hline  & Threshold & 100\% & 100\% & 90\% & 90\% & 100\% & 50\% & 75\% & 100\% \\ \hline
1916–1978 &
\begin{tabular}{c} Depth of \\ violation \\ \end{tabular} & 1.54 & 1.78 & 0 & 730 & 114 & 27 & 28 & 20 \\ \hline
1979–2020 &
\begin{tabular}{c} Depth of \\ violation \\ \end{tabular} & 0.00 & 0.11 & 0 & 9 & 0 & 23 & 5 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Reliability for water users’ requirements and the depth of violations for the inflow series 1916–1978 and 1979–2020. Analysis of the results of water resource optimization calculation showed that the requirements of water users obtained from the hydrological series 1916-1978 are violated for almost all criteria, except for criterion 3 (winter HPP capacity). The requirements for normal fishery (agricultural) releases are especially strongly violated (by 20%), which significantly affects the ecological situation in the Lower Volga basin due to the disturbance of the fauna and flora balance. The water resource optimization calculation according to the hydrological series 1916-1978 provides the standard reliability for all water users' requirements. At the same time, the violation depth is much lower than that for the 1916-1978 series. This shows that the climate change that has occurred has significantly improved the water management and environmental situation in the VKC reservoir basin. Unfortunately, this fact is not natural in the case of possible climate change. Research carried out for the Lower Don basin showed a significant deterioration in the fulfillment of fishery requirements. Over the last 29 years, the normal fishery release from the Tsimlyanskoye reservoir has never been carried out. To clarify the conflict of interest (_icing on the cake_) between the normal fishery release and the rest of the water users' requirements [29, 30] for the series 1916-1978 (element of multi-criteria analysis), a water resource optimization calculation was performed with penalty coefficients in favor of fishery release (agriculture) and ecology (criteria 6, 7, and 8).