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

2.3.2 Benchmark period
According to Beecher (1990), an instream flow standard should include certain
elements, one of which is the establishment of a benchmark period. The
benchmark period is selected as the measuring stick against which acceptable
flow reductions will be compared. A major impediment to the establishment of
MFLs in the SWFWMD has been the difficulty of identifying and establishing a
benchmark period for available flow records. This has apparently not posed a
significant problem for most working on minimum flow issues; in many instances
a 20 to 30-year period of record has generally been deemed acceptable (Richter
et al. 1996). Identification of a benchmark period has not been recognized as a
problem in much of the United States for two possible reasons:
1. it is generally assumed that except for anthropogenic factors, river flows
are the consequence of a sequence of random independently and
identically distributed random variables (Olsen et al. 1999); and
2. while river flows in peninsular Florida have declined when two
multidecadal periods are examined (Kelly 2004), they have increased in
most of the United States.
The traditional method of selecting a benchmark period would be to simply
identify a sufficient period of the historic record believed to have been minimally
impacted by humans. Since many of the rivers in the SWFWMD have periods of
record extending back 50 years or more, this approach would typically lead to
selection of a flow period in the 1940s, 50s or 60s. In general, except where
major structural alterations have occurred (e.g., construction of a dam), one
would expect man's impact in the 1940s, 50s and 60s to be less than from 1970
onward. This period would predate Florida's major population growth, high
agricultural use of groundwater, and most large-scale water supply development
projects. However, as has recently been demonstrated, the period from 1940 thru
1960 represents a period when peninsular Florida was experiencing a
multidecadal period of higher rainfall and consequently river flows (Enfield 2001,
Basso and Schultz 2003, Kelly 2004). It is believed that even without the
intervention of man, that flows in many stream and river systems would show a
decline of 20 to 40% when two multidecadal periods are compared (i.e., 1940 to
1969 and 1970 to 1999).
Unless cognizant of this multidecadal climate oscillation, the danger in using a
flow record pre-1970 as a benchmark for setting MFLs on rivers in the SWFWMD
is that it would likely be assumed that these declines are somehow related to
human factors. In this case, there is the potential for setting unrealistically high
MFLs. Not only would this affect society's ability to use a portion of the flow, but
could lead to the development of unneeded recovery strategies.
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