Alafia River Minimum Flows and Levels - Southwest Florida Water ...
Alafia River Minimum Flows and Levels - Southwest Florida Water ... 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 This would allow the recommended percent to be used in either benchmark period and would allow the actual volume withdrawn to increase when flow enters the higher flow period.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perspective, this would probably be the most desirable approach, since it would allow the maximum amount of water to be withdrawn consistent with multidecadal climatic conditions.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
extending from 1940 to 1969 can be used as a benchmark period for evaluating flow reductions during the wetter (i.e., AMO warm period) climatic oscillation. Because the flows of the Alafia River were actually augmented during the 70's (and for at least part of the 60's), the appropriate benchmark for the cooler (drier) AMO oscillation is a subset of the multidecadal period that extends from 1970 to 1999. It is most appropriate (especially during the lower flow part of the year, late April through mid-June) to use the period 1980 to 1999, as the benchmark against which flow reductions should be compared for the low flow (AMO cool) period. Despite assertions by others (Stoker et al. 1995, SDI 2003), there have not been measurable anthropogenic declines in Alafia River flows. In fact, seasonal low flows (April-June) during the multidecadal "low flow" period were often equal to or greater than the corresponding flows during the multidecadal "high flow" period. 2.3.3 Seasonal Flow Patterns and the Building Block Approach 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 seasonally, we evaluated flow records for several regional rivers. 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), corresponding to climatic phases associated with the Atlantic Multidecadal Oscillation were examined. On a seasonal basis, a 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 average median daily flow exceeded and stayed above the mean annual 50% exceedance flow. The medium flow period, Block 2, was defined as extending from the end of Block 3 to the beginning of Block 1. With the exception of the gage site on the Withlacoochee River, there was very little difference in the dates that each defined period began and ended, irrespective of the time period evaluated (Table 2-5). For the Alafia, Hillsborough, Myakka, and Peace Rivers, Block 1 was defined as beginning on Julian day 110 (April 20 in non-leap years) and ending on Julian day 175 (June 24). Block 3 was defined as beginning on Julian day 176 (June 25) and ending on Julian day 300 (October 27). Block 2, the medium flow period, extends from 2-36
- Page 11 and 12: Figure 5-12. Habitat Gain/Loss for
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extending from 1940 to 1969 can be used as a benchmark period for evaluating<br />
flow reductions during the wetter (i.e., AMO warm period) climatic oscillation.<br />
Because the flows of the <strong>Alafia</strong> <strong>River</strong> were actually augmented during the 70's<br />
(<strong>and</strong> for at least part of the 60's), the appropriate benchmark for the cooler (drier)<br />
AMO oscillation is a subset of the multidecadal period that extends from 1970 to<br />
1999. It is most appropriate (especially during the lower flow part of the year,<br />
late April through mid-June) to use the period 1980 to 1999, as the benchmark<br />
against which flow reductions should be compared for the low flow (AMO cool)<br />
period. Despite assertions by others (Stoker et al. 1995, SDI 2003), there have<br />
not been measurable anthropogenic declines in <strong>Alafia</strong> <strong>River</strong> flows. In fact,<br />
seasonal low flows (April-June) during the multidecadal "low flow" period were<br />
often equal to or greater than the corresponding flows during the multidecadal<br />
"high flow" period.<br />
2.3.3 Seasonal Flow Patterns <strong>and</strong> the Building Block Approach<br />
For most rivers in the SWFWMD, there is a repetitive annual flow regime that can<br />
be described on the basis of three periods. These three periods are<br />
characterized by low, medium, <strong>and</strong> high flows <strong>and</strong> for the purpose of developing<br />
minimum flows <strong>and</strong> levels, are termed Block 1, Block 2, <strong>and</strong> Block 3,<br />
respectively. To determine when these blocks may be expected to occur<br />
seasonally, we evaluated flow records for several regional rivers.<br />
For this analysis, flow records for long-term gage sites including the <strong>Alafia</strong> <strong>River</strong><br />
at Lithia, the Hillsborough <strong>River</strong> at Zephyrhills, the Myakka <strong>River</strong> near Sarasota,<br />
the Peace <strong>River</strong> at Arcadia, <strong>and</strong> the Withlacoochee <strong>River</strong> at Croom were<br />
reviewed. The mean annual 75 <strong>and</strong> 50 percent exceedance flows <strong>and</strong> average<br />
median daily flows for two time periods (1940 to 1969 <strong>and</strong> 1970 to 1999),<br />
corresponding to climatic phases associated with the Atlantic Multidecadal<br />
Oscillation were examined. On a seasonal basis, a low flow period, Block 1, was<br />
defined as beginning when the average median daily flow for a given time period<br />
fell below <strong>and</strong> stayed below the annual 75% exceedance flow. Block 1 was<br />
defined as ending when the high flow period, or Block, 3 began. Block 3 was<br />
defined as beginning when the average median daily flow exceeded <strong>and</strong> stayed<br />
above the mean annual 50% exceedance flow. The medium flow period, Block<br />
2, was defined as extending from the end of Block 3 to the beginning of Block 1.<br />
With the exception of the gage site on the Withlacoochee <strong>River</strong>, there was very<br />
little difference in the dates that each defined period began <strong>and</strong> ended,<br />
irrespective of the time period evaluated (Table 2-5). For the <strong>Alafia</strong>,<br />
Hillsborough, Myakka, <strong>and</strong> Peace <strong>River</strong>s, Block 1 was defined as beginning on<br />
Julian day 110 (April 20 in non-leap years) <strong>and</strong> ending on Julian day 175 (June<br />
24). Block 3 was defined as beginning on Julian day 176 (June 25) <strong>and</strong> ending<br />
on Julian day 300 (October 27). Block 2, the medium flow period, extends from<br />
2-36
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