01F070 - S.F. Nooksack River at Potter Road Technical Notes: 2009 Water Year Chuck Springer The telemetered stream gaging station on the South Fork Nooksack River at Potter Road operated throughout Water Year (WY) 2009. During the water year, 11 discharge measurements were made and 14 discrete manual stage readings were taken at this station. Cumulative potential error for this station for WY 2009 was ±30%. Rating Curve The station started WY 2009 on Rating Table 7. Table 7 covers a range of discharge from 92.6 to 26,700 cubic feet per second (cfs). Four of the 20 discharge measurements used to develop this rating were taken during WY 2009. The measured flows for this rating, ranging from 145 to 11,700 cfs cover only 43% of the rating curve. However, flows exceeded the highest measured flows only 1% of the time during the three months Table 7 was in effect during WY 2009. Flows greater than 11,700 cfs were modeled using a slope-conveyance discharge model, which had a confidence of ±14%. The rating curve was interpolated between discharge measurements and extrapolated to half the lowest measured flow using Johnson’s method to temporarily straighten the rating curve using a log offset (e=4.4) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±10%. A large storm event in January 2009 (the second largest during the period of record for this station) caused a moderate amount of channel scour. This shift is represented by Rating Table 8. Table 8 covers a range of discharge from 0 to 26,700 cfs. Seven of the 15 discharge measurements used to develop this rating were taken during WY 2009. The measured flows for this rating, ranging from point of zero flow (PZF) to 11,700 cfs, cover only 44% of the rating curve. However, flows exceeded the highest measured flows only 1% of the time during the nine months Table 8 was in effect during WY 2009. Flows greater than 11,700 cfs were modeled using a slope-conveyance discharge model, which had a confidence of ±14%. The rating curve was interpolated between discharge measurements and extrapolated to point of zero flow using Johnson’s method to temporarily straighten the rating curve using a log offset (e=3.6) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±15%. The weighted potential error for flows derived from the rating curves for this station for WY 2007 is ±14%. Stage Record The station logged continuously throughout WY 2009 without interruption. The staff gage at this site is generally readable to within 0.02 ft during all flow conditions. Conditions surrounding the terminal end of the bubbler orifice are similar to those around the staff gage. The stage height readings differed from manual staff gage readings by highly variable amounts, as much as 0.57 ft. Drift at this station was very erratic through much of the water year. Time- weighted corrective adjustments were made to the continuous stage record whenever the staff gage and datalogger readings differed. All adjustments are documented in the Hydstra Data Workbench. Where drift conditions are considered unstable, and adjustments result in a 20% or greater change in discharge for any given day, the data for that day are qualified as estimates. For WY 2008, due to the highly erratic drift, 130 days were qualified as estimates. Quality control measures were taken to identify potentially erroneous staff gage observations. A linear regression of staff gage versus tape down observations had an r2 of 0.999, with a standard deviation of 0.04 ft. The regression did not identify any obvious outliers; however the staff gage was damaged for half of the water year. Readings during this period (January to July 2009) had to be estimated based on this regression The calculated potential error of the continuous stage data for this station is ±16%. Future Efforts A more thorough high-flow modeling effort needs to occur for this site to solidify the upper end of the rating curve. Also, point of zero flow (PZF) measurements can only be taken at this site during the lowest of flows. PZF measurements should be taken whenever possible.