Instream Flow Needs Analysis - Okanagan Basin Water Board

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FINALOkanagan Basin Instream Flow Needssubstrate of the creek, which itself can vary substantially among sites and years. The meta-analysisprediction nevertheless approximates the average empirical condition.This analysis is somewhat qualitative in nature and limited in scope, since it addresses only five streams,but we believe it provided reasonable support for our use of the meta-analysis to developrecommendations for optimal fish flows for salmonids at the Okanagan stream nodes. Additionalvalidation will be possible in the future as more empirical work is completed.2.4.4 MAD correction factorMean annual discharge (MAD) for each node is required for use with the Hatfield and Bruce (2000) metaanalysisapproach using the 1996-2006 period of record. However, the 1996-2006 record included severalyears with exceptionally high annual streamflow yields resulting in elevated MAD values relative to thosethat would result from a longer dataset. This discrepancy made it necessary to develop corrections for the1996–2006 MAD values.To develop estimated MAD corrections for catchment nodes with short datasets (i.e.,
FINALOkanagan Basin Instream Flow Needs2006 derived MAD by the estimated MAD correction. These corrected MADs were then used in theHatfield and Bruce (2000) meta-analysis-based optimal flow regressions for each node.2.5 Method 2: British Columbia Instream Flow Guidelines for Fish (BCIFN)The Ministry of Water, Land and Air Protection (MWLAP), Ministry of Sustainable ResourceManagement (MSRM), Land and Water BC Inc. (LWBC), and Fisheries and Oceans Canada (DFO) havedeveloped the British Columbia Instream Flow Guidelines for Fish to aid in the process of settinginstream flows in British Columbia streams. The guidelines deal specifically with instream flowrequirements to support aquatic ecosystem values, and are made up of two main components, FlowThresholds (Hatfield et al. 2003) and Assessment Methods (Lewis et al. 2003). The BCIFN methoddeveloped by Hatfield et al. (2003) for fish-bearing streams calculates recommended minimum instreamflow thresholds based on daily stream discharge data over a 20+ year period of record. Thresholds aredetermined on a monthly basis and water diversions/allocations are restricted to periods when streamflows are greater than the corresponding thresholds. A maximum diversion rate (i.e., infrastructure limit)is defined as equivalent to the 80th percentile flow over the period of record is in effect at all times.During times when flows exceed the sum of the minimum flow threshold and the maximum diversionrate, diversions are restricted to the maximum diversion rate.The BCIFN method for fish-bearing streams calculates the median of all daily flows for each calendarmonth (grouping all years together) and the monthly medians are then ordered from lowest to highest.The minimum instream flow threshold for the lowest median flow month is set to the 90th percentile ofthe mean daily flows for that month. Conversely, the minimum flow threshold for the highest medianflow month is set to the 20th percentile of the mean daily flows for that month. The flow threshold foreach of the other 10 months is also calculated as a percentile of mean daily flows in the respective month,but the percentile varies between 20 th and 90 th according to the following formula:⎡⎛median − ⎞90 − ⎢⎜imedianmin⎟ × 20⎢⎣⎝ medianmax− medianmin⎠⎤( 90 − ) ⎥⎥⎦ Where: median i is the median of mean daily flows for month i,median min is the month of lowest median flows, andmedian max is the month of highest median flows.Figure 2.7 provides an example from Hatfield et al. 2003 that illustrates the mechanics of the BCIFNflow threshold approach. In essence the BCIFN guideline permits diversion/allocation of flows within theband demarcated by the two dark blue lines in the figure that represent the minimum and maximumdiversion thresholds. Flows below the band are not available for diversion, and would combine with“residual flows” above the band when present.25 ESSA Technologies Ltd. &Solander Ecological Research
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Instream Flow Needs Analysis - Okanagan Basin Water Board

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