17.3 Environmental Flow Assessment and Water Resources…
401
Under this scenario, users of water from Zhuxi Reservoir would receive the
same reliability of supply as they would if the pipeline from Shisandu Reservoir
was included. The average annual volume of water supplied from Changtan Reservoir would, however, be significantly lower, which would substantially reduce the
reliability of supplies received by both urban/industrial and agricultural users.
Determining whether infrastructure operating rules under this scenario could
be modified to improve supply reliabilities would require further modeling work.
However, it is likely that any such modifications would have consequences for other
users, such as those supplied from Zhuxi Reservoir.
Consequently, transferring water via a pipeline from Shisandu Reservoir to Zhuxi
Reservoir appears necessary to ensure the predicted 2020 demands for water from
Changtan Reservoir can be met.
17.3.5.7 Effects of Sub-Optimal Environmental Flows
The last scenario (S11) that was developed was designed to demonstrate a “trade-off”
of certain environmental outcomes in favor of improved supply reliabilities.
As shown above, the provision of the recommended environmental flows
would impact supply reliability for various users, particularly urban/industrial users
supplied from Changtan Reservoir. These users would normally expect to receive a
daily reliability of at least 95%. This level is not achieved under any of the scenarios
where all the recommended environmental flows are provided (Fig. 17.10).
This final scenario was developed to attempt to raise the reliability of
urban/industrial supplies from Changtan Reservoir while still providing some of the
critical environmental flows. This required revisiting the environmental flows assessment and recommendations to ideate alternative, “sub-optimal” environmental flow
rules.
Reducing the level of environmental flows requires prioritization of the identified
assets and objectives to determine which of the recommended flows can be reduced
or ignored. It must be recognized that the sub-optimal environmental flow option
represents a departure from the recommended flow regime, so it carries a higher
risk that the identified assets will not be protected. Accepting a higher risk is a
management decision, not a scientific one.
The supply reliabilities for users from the Changtan and Zhuxi Reservoirs under
the recommended and sub-optimal environmental flow options are compared in Table
17.16. Both of the scenarios analyzed involve the changed operational arrangements
for Changtan Reservoir to maximize the supply reliabilities for urban/industrial users.
The reduced level of environmental flows has the following impacts: (1) average
annual reliability for urban/industrial users supplied from Changtan Reservoir does
not change materially; (2) there is a significant increase in the average annual supply
for irrigation users from Changtan Reservoir; (3) urban/industrial users supplied from
Zhuxi Reservoir continue to receive 100% of their demands; and (4) the reliability of
supply for irrigation users from Zhuxi Reservoir increases substantially. Under the
401
Under this scenario, users of water from Zhuxi Reservoir would receive the
same reliability of supply as they would if the pipeline from Shisandu Reservoir
was included. The average annual volume of water supplied from Changtan Reservoir would, however, be significantly lower, which would substantially reduce the
reliability of supplies received by both urban/industrial and agricultural users.
Determining whether infrastructure operating rules under this scenario could
be modified to improve supply reliabilities would require further modeling work.
However, it is likely that any such modifications would have consequences for other
users, such as those supplied from Zhuxi Reservoir.
Consequently, transferring water via a pipeline from Shisandu Reservoir to Zhuxi
Reservoir appears necessary to ensure the predicted 2020 demands for water from
Changtan Reservoir can be met.
17.3.5.7 Effects of Sub-Optimal Environmental Flows
The last scenario (S11) that was developed was designed to demonstrate a “trade-off”
of certain environmental outcomes in favor of improved supply reliabilities.
As shown above, the provision of the recommended environmental flows
would impact supply reliability for various users, particularly urban/industrial users
supplied from Changtan Reservoir. These users would normally expect to receive a
daily reliability of at least 95%. This level is not achieved under any of the scenarios
where all the recommended environmental flows are provided (Fig. 17.10).
This final scenario was developed to attempt to raise the reliability of
urban/industrial supplies from Changtan Reservoir while still providing some of the
critical environmental flows. This required revisiting the environmental flows assessment and recommendations to ideate alternative, “sub-optimal” environmental flow
rules.
Reducing the level of environmental flows requires prioritization of the identified
assets and objectives to determine which of the recommended flows can be reduced
or ignored. It must be recognized that the sub-optimal environmental flow option
represents a departure from the recommended flow regime, so it carries a higher
risk that the identified assets will not be protected. Accepting a higher risk is a
management decision, not a scientific one.
The supply reliabilities for users from the Changtan and Zhuxi Reservoirs under
the recommended and sub-optimal environmental flow options are compared in Table
17.16. Both of the scenarios analyzed involve the changed operational arrangements
for Changtan Reservoir to maximize the supply reliabilities for urban/industrial users.
The reduced level of environmental flows has the following impacts: (1) average
annual reliability for urban/industrial users supplied from Changtan Reservoir does
not change materially; (2) there is a significant increase in the average annual supply
for irrigation users from Changtan Reservoir; (3) urban/industrial users supplied from
Zhuxi Reservoir continue to receive 100% of their demands; and (4) the reliability of
supply for irrigation users from Zhuxi Reservoir increases substantially. Under the