40D-8.041. Minimum Flows

(1) Minimum Flows for the Lower Hillsborough River. (a) For the purposes of Minimum Flows, the Lower Hillsborough River is defined as the River downstream of Fletcher Avenue. A tributary of the Lower Hillsborough River is Sulphur Springs, an artesian spring which enters the River via a short spring run at a point 2.2 miles downstream of the City’s dam. (b) The Minimum Flows for the Lower Hillsborough River are based on extending a salinity range less than 5 ppt from the Hillsborough River Dam toward Sulphur Springs. The Minimum Flows for the Lower Hillsborough River are 20 cubic feet per second (“cfs”) freshwater equivalent from July 1 through March 31 and 24 cfs fresh water equivalent from April 1 through June 30 at the base of the dam as adjusted based on a proportionate amount that flow at the United States Geological Survey Gauge No. 0203000 near Zephyrhills, Florida (“Gauge”) is below 58 cfs. The adjustment is that for each one cfs that Hillsborough River flow at the Gauge is below 58 cfs, when 20 cfs freshwater equivalent is otherwise required, the Minimum Flow is adjusted by reducing it by 0.35 cfs; when 24 cfs freshwater equivalent is otherwise required, the Minimum Flow is adjusted by reducing it by 0.40 cfs. For purposes of this paragraph 40D-8.041(1)(b), F.A.C., freshwater equivalent means water that has a salinity concentration of 0.0 ppt for modeling purposes. (2) Minimum Flows for the upper Hillsborough River. (a) The Minimum Flows are to ensure that the minimum hydrologic requirements of the water resources or ecology of the natural systems associated with the river are met. (b) Minimum Flows for the upper Hillsborough River at the USGS Hillsborough River near Morris Bridge Gage USGS #02303330 (“Morris Bridge Gage”) are set forth in Table 8-12 below. The long-term compliance standards set forth in Table 8-13 are established based on the application of the Minimum Flows to the lowest anticipated natural flow conditions. Minimum Flows for the upper Hillsborough River are both seasonal and flow dependent. Two standards are flow based and applied continuously regardless of season. The first is a Minimum Low Flow threshold of 52 cfs at the Morris Bridge Gage. The second is a Minimum High Flow threshold of 470 cfs at the Morris Bridge Gage. The Minimum High Flow is based on changes in the number of days of inundation of floodplain features. There are also three seasonally dependent or Block specific Minimum Flows. The Block 1 and Block 2 Minimum Flows are based on potential changes in habitat availability for fish species and macroinvertebrate diversity. The Block 3 Minimum Flow is based on changes in the number of days of connection with floodplain features. Table 8-12 Minimum Flow for Upper Hillsborough River at USGS Hillsborough River near Morris Bridge Gage Period Effective Dates Where Flow on Previous Day Equals: Minimum Flow Is Annually January 1 to December 31 (52 cfs (52cfs and (470 cfs ≥470 cfs 52 cfs Seasonally dependent – see Blocks below Previous day flow minus 8% Block 1 April 20 to June 24 (52 (52cfs and (470 cfs ≥470 cfs 52 cfs previous day flow minus 10% Previous day flow minus 8% Block 2 October 28 to April 19 (52 (52cfs and (470 cfs ≥470 cfs 52 cfs previous day flow minus 11% previous day flow minus 8% Block 3 June 25 to October 27 (52 cfs (52 cfs and (470cfs ≥470 cfs 52 cfs previous day flow minus 13% previous day flow minus 8% (c) Compliance – The Minimum Flows are met when the flows in Table 8-13 are achieved. Table 8-13 Compliance Standards for the Hillsborough River near Morris Bridge Gage Minimum Flow Hydrologic Statistic Flow (cfs) Annual Flow 10-Year Mean