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

4.4 Seasonal Flow and Development of Blocks 1, 2, and 3 for the
Alafia River and Other District Rivers
For most rivers in the SWFWMD, there is a repetitive annual flow regime that can
be described on the basis of three periods. These three periods are
characterized by low, medium, and high flows and for the purpose of developing
minimum flows and levels, are termed Block 1, Block 2, and Block 3,
respectively. To determine when these blocks may be expected to occur, flow
records for several regional rivers were evaluated.
For this analysis, flow records for long-term gage sites including the Alafia River
at Lithia, the Hillsborough River at Zephyrhills, the Myakka River near Sarasota,
the Peace River at Arcadia, and the Withlacoochee River at Croom were
reviewed. The mean annual 75 and 50 percent exceedance flows and average
median daily flows for two time periods (1940 to 1969 and 1970 to 1999) were
determined. Records from the two time periods were reviewed to evaluate
potential effects of Atlantic Multidecadal Oscillation phases on river flows. The
low flow period, Block 1, was defined as beginning when the average median
daily flow for a given time period fell below and stayed below the annual 75%
exceedance flow. Block 1 was defined as ending when the high flow period, or
Block 3, began. Block 3 was defined as beginning when the mean median daily
flow exceeded and stayed above the mean annual 50% exceedance flow and
ending when the flow fell below the 50% exceedance. The medium flow period,
Block 2, was defined as extending from the end of Block 3 to the beginning of
Block 1 (Figure 4-10).
With the exception of the gage site on the Withlacoochee River, there was little
difference in the dates that each defined period began and ended, irrespective of
the time period evaluated (Table 4-1). For the Alafia, Hillsborough, Myakka, and
Peace Rivers, Block 1 was defined as beginning on Julian day 110 (April 20 on
non-leap years) and ending on Julian day 176 (June 25). Block 3 was defined as
beginning on Julian day 177 (June 26) and ending on Julian day 299 (October
26). Block 2, the medium flow period, extends form Julian day 300 (October 27)
to Julian day 109 (April 19) of the following calendar year. Using these
definitions: Blocks 1, 2, and 3 are 66, 175 and 123 days in length, respectively
(Table 4-2). Blocks 1 and 3 began approximately 20-30 days later at the
Withlacoochee River gage site; it will be necessary to generate a separate set of
flow blocks for establishing minimum flows for the Withlacoochee River.
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