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

2.3 Hydrology
The assessment of minimum flow and levels for the freshwater segment of the
Alafia River was supported by analyses of long-term streamflow records that date
to the 1930s. Significant declining trends in flow have been documented or
reported by a number of workers (Stoker et al. 1996, Hickey 1998, SDI 2003,
Kelly 2004); however, the cause(s) of these declines has been the subject of
some debate. Kelly (2004) attributed flow declines in the Alafia River largely to
climate, and that is a primary assumption inherent in the minimum flow analyses
to follow.
Although there has been considerable phosphate mining in the Alafia watershed
(especially in the watersheds of the North and South Prongs) and substantial
groundwater withdrawals from the Floridan aquifer, comparison of river flow
declines with neighboring watersheds suggests a similar causative factor for flow
declines. Our analyses indicate that flow declines attributed by Stoker et al.
(1996) to groundwater withdrawals, and by SDI (2003) to increasing area of
mined land are due to another factor, namely the removal or reduction of
discharges from the phosphate mining industry. These flow declines actually
represent an increase in water use efficiency by the mining industry such that the
large volumes of groundwater historically used for ore extraction and processing
have been substantially reduced. In response to work done by SDI (2003), we
have compared discharge volumes from the watersheds of the South and North
Prongs of the Alafia River to demonstrate that similar amounts of water are being
discharged from both basins and thus increasing area of mined lands has not
lead to substantial nor quantifiable reductions in flow.
Kelly (2004) concluded that the Atlantic Multidecadal Oscillation (AMO; see
Enfield et al. 2001) has an important controlling effect on river flow volumes
throughout southwest Florida, and is a major factor that must be considered
when developing minimum flows and levels (Shaw et al. 2004). This
phenomenon is shown to affect the baseline or benchmark period against which
flow reductions and MFLs should be judged.
2.3.1 Florida River Flow Patterns and the Atlantic Multidecadal
Oscillation
"It would be reasonable to assume that given a fairly constant climate, the
amount of water flowing down a river's course each year would vary
evenly about an average value." (Smith and Stopp 1978)
Smith and Stopp's statement reflects the typical paradigm with respect to the
impact of climate on river flow. As a result, little attention has been paid to the
2-28