17.3 Environmental Flow Assessment and Water Resources…
393
The benefits to the trunk stream of providing for environmental flows (from Zhuxi
Reservoir) can be seen by analyzing the mean annual flows at the reporting nodes
under each of the scenarios (Table 17.14).
Providing environmental flows would, however, impact the reliability of water
supply (Table 17.15).
• It would reduce the daily reliability of supply for urban/industrial users from
Zhuxi Reservoir from 100 to 97%; this is still within the target range of urban
water reliability under the Taizhou water resources comprehensive plan.
• It would reduce the average annual supply (as a percentage of demand) for irrigation from Zhuxi Reservoir from 100 to 95%; this translates into a reduction in the
annual reliability of these supplies (i.e., the proportion of years in which irrigators
would receive their full demands) from 100 to 67%.
• It would reduce the reliability of urban/industrial supplies from Changtan
Reservoir as follows:
– The average annual supply reduces by between 1 and 3% of the sector’s
demands.
– The daily reliability reduces considerably from 82 to 74% unless the Changtan
Reservoir operation rules are also changed, in which case the reduction is
relatively minor, dropping from 96 to 93%.
• The annual average supply for irrigation from Changtan Reservoir would be
between 5 and 7% lower to fall between 60 and 67%.
The impact of the provision of environmental flows on supply from Changtan
Reservoir can also be understood by considering the volume of water piped from
Zhuxi Reservoir to Changtan Reservoir.
Under the planned pipeline operational rules, water is only transferred to Changtan
Reservoir depending on the water level in Zhuxi Reservoir (relative to Changtan
Reservoir). The provision of environmental flows reduces the times when the prerequisites for transfer are met, which in turn reduces the annual volume transferred by
about 40% (Table 17.15).
The provision of these flows, should they be adopted and included in a water
resources allocation plan, would require amending the operational rules for Zhuxi
Reservoir.
17.3.5.3 Effects of Changing the Changtan Reservoir Operation Rules
The modified Changtan Reservoir operation rules (applied in S4, S5, and S10) involve
lowering the threshold at which water for urban supplies must be restricted. Under
the currently planned rules, urban water supply is restricted, and irrigation water
supply is reduced to zero when the water level falls to 21.05 m (72 million m
3 ). The
modified arrangements involve lowering this restriction threshold to 15.05 m (12
million m
3 ) for urban supplies, but they do not change the threshold for irrigation
supplies; this allows urban/industrial users to be supplied from what would otherwise
be “dead” storage.
393
The benefits to the trunk stream of providing for environmental flows (from Zhuxi
Reservoir) can be seen by analyzing the mean annual flows at the reporting nodes
under each of the scenarios (Table 17.14).
Providing environmental flows would, however, impact the reliability of water
supply (Table 17.15).
• It would reduce the daily reliability of supply for urban/industrial users from
Zhuxi Reservoir from 100 to 97%; this is still within the target range of urban
water reliability under the Taizhou water resources comprehensive plan.
• It would reduce the average annual supply (as a percentage of demand) for irrigation from Zhuxi Reservoir from 100 to 95%; this translates into a reduction in the
annual reliability of these supplies (i.e., the proportion of years in which irrigators
would receive their full demands) from 100 to 67%.
• It would reduce the reliability of urban/industrial supplies from Changtan
Reservoir as follows:
– The average annual supply reduces by between 1 and 3% of the sector’s
demands.
– The daily reliability reduces considerably from 82 to 74% unless the Changtan
Reservoir operation rules are also changed, in which case the reduction is
relatively minor, dropping from 96 to 93%.
• The annual average supply for irrigation from Changtan Reservoir would be
between 5 and 7% lower to fall between 60 and 67%.
The impact of the provision of environmental flows on supply from Changtan
Reservoir can also be understood by considering the volume of water piped from
Zhuxi Reservoir to Changtan Reservoir.
Under the planned pipeline operational rules, water is only transferred to Changtan
Reservoir depending on the water level in Zhuxi Reservoir (relative to Changtan
Reservoir). The provision of environmental flows reduces the times when the prerequisites for transfer are met, which in turn reduces the annual volume transferred by
about 40% (Table 17.15).
The provision of these flows, should they be adopted and included in a water
resources allocation plan, would require amending the operational rules for Zhuxi
Reservoir.
17.3.5.3 Effects of Changing the Changtan Reservoir Operation Rules
The modified Changtan Reservoir operation rules (applied in S4, S5, and S10) involve
lowering the threshold at which water for urban supplies must be restricted. Under
the currently planned rules, urban water supply is restricted, and irrigation water
supply is reduced to zero when the water level falls to 21.05 m (72 million m
3 ). The
modified arrangements involve lowering this restriction threshold to 15.05 m (12
million m
3 ) for urban supplies, but they do not change the threshold for irrigation
supplies; this allows urban/industrial users to be supplied from what would otherwise
be “dead” storage.