Alafia River Minimum Flows and Levels - Southwest Florida Water ...

River, Myakka River, Withlacoochee River). This, in large part, prompted an
evaluation of multidecadal differences in flow patterns.
As a result of findings in, "Florida River Flow Patterns and the Atlantic
Multidecadal Oscillation" (Kelly 2004), the District has identified two benchmark
periods. One benchmark period corresponds to the warm phase of the AMO,
and is correlated with a multidecadal period of higher rainfall and thus increased
river flows, and one corresponds to the cool phase of the AMO, and is correlated
with a multidecadal period of lower rainfall and thus climatically lower river flows.
One of several approaches could be used in developing MFLs given that two and
not one benchmark period exists. If permitting or allowing consumptive water
use (i.e., withdrawals) on a volume basis (a fixed withdrawal; e.g., 50 mgd) rather
than on a percent of flow approach (e.g., 10% of the preceding day's flow), the
more conservative approach toward ecology and aquatic resources of a system
would be to use the drier period as the benchmark period, since this would yield
the lowest withdrawal recommendation. This approach would protect from
significant harm during the low flow period, and provide even greater protection
during the higher flow period. If, however, permitting on a "percent of flow"
approach, the more conservative approach would be to base permitting on the
benchmark period that produces the lower percent recommendation. This would
allow the recommended percent to be used in either benchmark period and
would allow the actual volume withdrawn to increase when flow enters the higher
flow period. While it might seem intuitive that the lower flow period would always
produce a lower recommended percent of flow reduction, this does not have to
be the case.
A third option would be to adjust either the permitted volume or percent of flow
recommendation according to the AMO period that one happens to be in. From
a water supply perspective, this would probably be the most desirable approach,
since it would allow the maximum amount of water to be withdrawn consistent
with multidecadal climatic conditions. This option, however, would be difficult to
apply since there is currently no method for determining when a step change to a
new climatic regime has occurred except in hindsight. Given the difficulty of
determining when a step change has occurred and given that there are several
advantages to the "percent of flow" approach (e.g., maintenance of the shape
and seasonality of the natural hydrograph) over the fixed quantity approach, we
have proposed flow reductions that would yield the lowest recommended percent
withdrawal regardless of the benchmark period used. We in essence use two
benchmark periods in developing our MFLs recommendations.
Although some (e.g., Stoker et al. 1995, SDI 2003) have reported that Alafia
River flows have declined due to anthropogenic factors, we have demonstrated
that much of the reported flow decline is attributable to either a natural climatic
oscillation (step change) or to removal or reduction of mine related discharges.
For this reason, we believe that the entire flow record for the mutidecadal period
2-35