SECTION 785:46-1-6. Determination of regulatory low flow

(a)   General.

(1)   7Q2. The 7Q2 is calculated as a moving average of seven consecutive days for each year in a given record. These seven-day low flow values are ranked in ascending order. An order number (m) is calculated based upon the number of years of record (n), with a recurrence interval (R) of two years, as m = (n+1)/R, where R = two years. A value of flow corresponding to the m^th order is taken as the seven-day, two-year low flow for those historical data.

(2)   Seasonal 7Q2. The seasonal 7Q2 is calculated as a moving average of seven consecutive days for the applicable dates specified in Table 1 of Appendix G of OAC 785:45 in a given period of record. These seven-day low flow values are ranked in ascending order. An order number (m) is calculated based upon the number of seasons (n) specified in Table 1 of OAC 785:45 Appendix G during the period of record, with a recurrence interval (R) of two years, as m = (n+1)/R, where R = two years. A value of flow corresponding to the m^th order is taken as the seasonal seven-day, two-year low flow for those historical data.

(b)   Primary method for determination. If the 7Q2 or seasonal 7Q2 for a given stream or stream segment is determinable from the United States Geological Survey publication entitled "Statistical Summaries of Streamflow in and near Oklahoma Through 2007" or the latest version of the Water Quality Management Plan published by the Department of Environmental Quality, then that 7Q2 and seasonal 7Q2 shall be conclusive except as provided otherwise in this section.

(c)   Alternative methods for determination of 7Q2 or seasonal 7Q2.

(1)   In lieu of determining the 7Q2 or seasonal 7Q2 as provided in (b) of this Section, the 7Q2 for a given stream or stream segment may be determined by an affected person or the permitting authority if all of the following conditions are satisfied:

(A)   A hydrological modification affecting the flow in the stream is documented to the satisfaction of the Oklahoma Water Resources Board and permitting authority;

(B)   At least 10 years of daily flow data comporting with the requirements of this section are available; and

(C)   Data from the entire period of record for the stream, unless a different time frame of record is approved by the Board and the permitting authority, are used in the calculation.

(2)   If the 7Q2 or seasonal 7Q2 for a given stream or stream segment is not determinable as provided in (b) or (c)(1) of this Section or if additional daily flow data have been collected, then the 7Q2 or seasonal 7Q2 for that stream or stream segment may be determined by an affected person or the permitting authority using the calculations provided in (a) of this Section, provided at least 10 years of daily flow data are available for that stream.

(3)   If the flow is affected by contributions from gauged tributaries or other permitted discharges, then the 7Q2 or seasonal 7Q2 for a given stream or stream segment may be determined taking those contributions at 7Q2 or seasonal 7Q2, or both, into account on a case-by-case basis if approved by either the Board or the permitting authority.

(4)   If the 7Q2 or seasonal 7Q2 for a given stream or stream segment is not determinable as provided in (b), (c)(1), (c)(2) or (c)(3) of this Section, then the 7Q2 or seasonal 7Q2 for that stream or stream segment may be determined by an affected person or the permitting authority using an estimate based upon limited data only if both the method for estimating, and the estimate itself, are approved by both the Board and permitting authority.

(d)   Additional rules for 7Q2 and seasonal 7Q2 determinations.

(1)   Any 7Q2 or seasonal 7Q2 determined with a period of record less than 20 years shall be invalid for any purpose except the issuance of the permit or establishment of the site specific criteria based upon and developed contemporaneously with such 7Q2 or seasonal 7Q2. Any subsequent renewal of such permit must be based upon a fresh determination of the 7Q2 or seasonal 7Q2 until the pertinent period of record equals or exceeds 20 years.

(2)   Any subsequent renewal of a permit based upon a 7Q2 or seasonal 7Q2 determined pursuant to (c)(3) or (c)(4) of this Section must be based upon a fresh determination of the 7Q2 or seasonal 7Q2 that takes into account all discharge and flow data from the time the 7Q2 or seasonal 7Q2 was previously determined.

(3)   Any subsequent renewal of a permit based upon a 7Q2 or seasonal 7Q2 determined pursuant to (c)(1) of this Section must be based upon a fresh determination of the 7Q2 or seasonal 7Q2 that takes into account whether the hydrological modification continues to exist.

(e)   Alternative method for determination of regulatory low flow. In stream segments where dams or other structures have substantially affected the historic flow regime of the stream segment, including but not limited to the portions of the Verdigris and Arkansas Rivers constituting the McClellan-Kerr Arkansas River Navigation System, on a site-specific basis a properly designed and implemented hydrologic study approved by the permitting authority and the Board may be used to determine the appropriate regulatory low flow.

[Source: Added at 19 Ok Reg 2512, eff 6-27-02; Amended at 20 Ok Reg 1429, eff 7-1-03; Amended at 23 Ok Reg 1568, eff 7-1-06; Amended at 30 Ok Reg 974, eff 7-1-13]