Released March 12, 2015

Transcription

1 Low-Flow Characteristics and Flow- Duration Statistics for Selected USGS Continuous-Record Streamgaging Stations in North Carolina Through 2012 Prepared in cooperation with the North Carolina Department of Environment and Natural Resources, Division of Water Resources J. Curtis Weaver, Hydrologist USGS South Atlantic Water Science Center (Raleigh) U.S. Department of the Interior U.S. Geological Survey Presented to: 2015 WRRI Conference McKimmon Center, Raleigh, NC March 18, 2015 Released March 12, 2015 U.S. Geological Survey streamflow gage , Beetree Creek near Swannanoa in Buncombe County in western North Carolina, with records of continuous discharge dating back to February 1926 Available online via: 2 1

2 Why? Where resources permit, low-flow characteristics should be updated about once every 10 years Allows for inclusion of newer stations with sufficient data Provides regulators and planners with relevant hydrologic information reflective of the recent droughts ( , ) 3 Ecological integrity Streamflow statistics, including annual and monthly flow-duration statistics, also are a key component in evaluating ecologic integrity of the aquatic environment. Ecological Flows Science Advisory Board review confirmed that flow-duration statistics are important in developing the relations between biological response measures and hydrologic variables 4 2

3 For your consideration First statewide low-flow report since Giese and Mason in 1993 (USGS WSP 2403) however, report does not provide regional relations for estimating low-flow characteristics at ungaged sites. Regionalization study in future? Last basin-wide low-flow report in 2003 (Rocky River basin in Yadkin-Pee Dee) first report since recent droughts ( , ) 5 Previous basin-wide low-flow reports 6 3

4 Low-flow vs Flow-duration? point of confusion for some? Low-flow: defined by a set of discharges that are statistically derived values having an associated duration (expressed in days) and recurrence interval (probability of future occurrence) Flow-duration: discharges associated with a percentile, which is a value on a scale of 1 to 100 indicating the percentage of a sample distribution that is equal to, greater than, or less than the discharge during a given time period (historical assessment) 7 Example low-flow: 7Q10 7-day, 10-year low-flow discharge The annual minimum average streamflow for a 7-consecutive-day period will be equal to or lower than the 7Q10 discharge, on average, once every 10 years If the 7Q10 discharge is 5 cubic feet per second (ft 3 /s), then the annual minimum average streamflow for a 7-consecutive-day period would be 5 ft 3 /s or lower, on average, once every 10 years 8 4

5 L-F frequency curve for Elk Creek Series of annual minimum 7- day average streamflows fitted to Log Pearson Type III distribution 7Q10 = 13 cfs 9 Example flow-duration: 10 th percentile (non-exceedance) The discharge value that was equaled or not met 10 percent of the time during the period of analysis In other words, 10 percent of the historical observations in the sample were at or below 10 th percentile discharge value during the period of analysis 10 5

6 Flow-duration curve for Elk Creek 10 th (non-exceedance) percentile = 26 cfs 11 Example use of flow-duration stats Percentiles similar to that used in USGS WaterWatch depictions of streamflows 12 6

7 NC low-flow characteristics Annual and winter 7-day, 10-year low-flow discharges (7Q10, W7Q10 Nov thru Mar) 30-day, 2-year low-flow discharge (30Q2) 7-day, 2-year low-flow discharge (7Q2) Based on streamflow records available through March 31, 2012 (or the 2011 climatic year) 13 Flow-duration statistics Minimum, mean, maximum, and flow-duration statistics of daily mean discharges for categorical periods Flow-duration statistics compiled for the 5th, 10th, 25th, 50th, 75th, 90th, and 95th percentiles POR, each full water year of record, each calendar month and day, selected seasonal periods 14 7

8 Flow-duration seasonal periods Six seasonal periods: Summer_USGS April through October Winter_USGS November through March Spring_ECO(logical) April through June Summer_ECO(logical) July through September Fall_ECO(logical) October and November Winter_ECO(logical) December through March 15 Initial number of sites Total of 516 streamgages considered (224 active as of 2012 WY, 292 discontinued) 177 active and 119 discontinued streamgages had recommended minimum 10 climatic years during the periods of record 16 8

9 QA/QC analyses QA/QC analyses (including Kendall tau trend tests) and other evaluations resulted in removal of 9 active and 12 discontinued sites Five sites removed due to data not reflective of current conditions (e.g., data collected prior to impoundment) Records combined for 5 pairs of active/discontinued adjacent sites where difference in DA less than 5 percent 17 Final numbers of sites Flow statistics published for 171 active and 95 discontinued sites (266 sites total) Among the 266 sites 177 unregulated 56 regulated 33 sites either known or considered to be affected by varying degrees of minor regulations and (or) diversions upstream from the streamgages. 18 9

10 Active streamgages 19 Discontinued streamgages 20 10

11 Question we re afraid to ask did the droughts change low flows? Updated 7Q10 discharges were compared for 63 streamgages across North Carolina for which: long-term streamflow record consisted of 30 or more climatic years of data available as of the 1998 climatic year, and streamflows were not known to be regulated 21 7Q10 comparisons 63 streamgages consisting of 53 where streamflows are unregulated 10 where streamflows are affected by varying degrees of minor regulations and (or) diversions upstream from the streamgages

12 7Q10 comparisons The 7Q10 discharges: did not change at 3 sites increased at 5 sites basins affected in part by upstream diversions and (or) recent development decreased at 55 sites. The median percentage change for all 63 streamgages was 18.4 percent. 23 7Q10 comparisons 24 12

13 Access to data With publication of report, efforts now being directed to update of StreamStats application where users will be able to access the low-flow characteristics and flow-duration statistics Tables 3 and 5 in report provide low-flow characteristics and flow-duration statistics for period of records at streamgages 25 In the meantime Access to flow-duration statistics for each water year of record, calendar month and day, selected seasonal periods available via Downloads link at online site for report:

14 Flow-durations Tab-delimited ASCII file of flow-duration that can quickly be imported into other programs or software Calendar day Calendar month Period of record Six selected seasonal periods 27 Flow-durations Tab-delimited ASCII file of flow-duration that can quickly be imported into other programs or software Each full water year of record 28 14

15 Closing thoughts Plan effectively... Questions concerns complaints? Contact info: J. Curtis Weaver, Hydrologist, PE Phone: (919) USGS South Atlantic Water Science Center