Chapter 1 Minimum Flows and Levels - Southwest Florida Water ...

2.4.4 Benchmark Periods
Climate-based differences in flows associated with ocean warming and cooling
phases of the AMO and identification of step-trends for Florida river flows
suggest that separate benchmark periods should be utilized for evaluating
minimum flow criteria. For peninsular Florida, a benchmark period from 1940
through 1969 corresponds to a warm phase of the AMO, and is correlated with a
multidecadal period of higher rainfall and increased river flows; the period from
1970 through 1994 corresponds to a cool phase of the AMO, and is correlated
with a multidecadal period of lower rainfall and lower river flows. An apparent
shift to a warmer AMO phase in the mid-1990s has recently been identified (e.g.,
see Goldenberg et al. 2001, Sutton and Hodson 2005), suggesting that
consideration of at least three benchmark periods may be appropriate for
development of minimum flows and levels.
Several approaches could be used to develop minimum flows and levels given
that high and low flow benchmark periods have been identified. If permitting or
allowing consumptive water use is conducted on a fixed-quantity basis (e.g., 50
million gallons per day) a conservative approach for protecting the ecology and
aquatic resources of river systems would be to use the drier period as the
benchmark period, since this would yield the lowest withdrawal recommendation.
This approach would prevent significant harm from withdrawals during the low
flow benchmark period, and provide greater protection during the period of higher
flows. If, however, permits are issued on a percent-of-flow basis (e.g., 10% of
the preceding day's flow is available for use), the most conservative approach
would be to base permitting on the benchmark period that produces the lower
percent-of-flow reduction associated with the criterion or key resources identified
for protection from significant harm. This would allow the recommended percentof-flow
reduction to be used in either benchmark period while affording protection
to the key resource(s) during both flow periods. A third option would be to adjust
either the fixed quantity or percent-of-flow withdrawal restrictions according to the
current AMO period or phase. 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amount of water to be withdrawn irrespective of the multidecadal phasing of the
AMO. 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.
Based on the difficulty of determining when a step change in flows has occurred
and given that there are several advantages to the "percent-of-flow" approach
(e.g., maintenance of the seasonality and distribution of flows in the natural flow
regime) over the fixed-quantity approach, we have developed minimum flow
criteria that are based on percent-of-flow reductions. Under most circumstances
we anticipate that for most rivers, these criteria will be based on the most
restrictive flow reductions associated with analyses involving two benchmark
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