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As for the dry condition (Case II), it is found that the ‘natural’ river regimes cannot provide the necessary water storage to sustain basic ecosystem functioning (Table 7); neglecting the effects of reservoirs (Scenario 0), the EWD satisfaction rates are around 80–85% during the storage period. If the existing operation policy is adopted (Scenario 1), all lake regions' EWD satisfaction rates are to be reduced by about 15%. The optimal reservoir operation schemes in dry condition are similarly estimated as in Case I. Similar to the normal condition, the relative optimization strategies of the TGR (Figure 10) and tributary reservoirs (Figure 11) in Case II are to enlarge the storage periods by planning the operation in advance. Table 7 summarizes the predicted EWD satisfaction rates under different scenarios of Case II, and the optimal policy in Scenario 4 is also the best one to be considered. Although the optimal operation is believed to have a positive effect on the lake ecosystem, no significant improvement in EWD satisfaction rate could be detected when compared with Case I. Due to the relatively small background discharges available, the general EWD satisfaction rate under the optimal joint operation is predicted to be 67.74%. It is interesting to note that, in the dry condition, the lake is more sensitive to the operations of tributary reservoirs, especially in the southern and western lake regions.
Table 7

EWD satisfaction rates for different scenarios (late September to late October, Case II)

Lake regionScenario 0Scenario 1Scenario 2Scenario 3Scenario 4
The eastern region 80.67% 63.90% 66.70% 64.48% 67.32% 
The southern region 80.24% 64.78% 63.48% 68.30% 66.69% 
The western region 85.48% 70.96% 70.14% 71.49% 70.35% 
Dongting Lake 81.50% 65.58% 66.42% 67.03% 67.74% 
Lake regionScenario 0Scenario 1Scenario 2Scenario 3Scenario 4
The eastern region 80.67% 63.90% 66.70% 64.48% 67.32% 
The southern region 80.24% 64.78% 63.48% 68.30% 66.69% 
The western region 85.48% 70.96% 70.14% 71.49% 70.35% 
Dongting Lake 81.50% 65.58% 66.42% 67.03% 67.74% 
Figure 10

Discharges and water levels of TGR under different scenarios in Case II.

Figure 10

Discharges and water levels of TGR under different scenarios in Case II.

Figure 11

Operation schemes of the reservoirs on lake tributaries under different scenarios in Case II.

Figure 11

Operation schemes of the reservoirs on lake tributaries under different scenarios in Case II.

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