Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 100

needed are the mean, standard deviation, and coeficient of skewness of the common logarithms of
the annual peak lows.
However, complications can result where data are missing in any given year or between years,
due to outliers, annual peaks falling below the lower limit of measurements, and other factors that
may impact the probability distribution. To account for these complications, a methodology was
established by an Interagency Advisory Committee on Water Data (1982) and published in Bulletin
17B (Figure 4.18). This methodology has been implemented in a computer program available from
the USGS, called PeakFQ (Flynn et al. 2006). So, peak low data can be downloaded from the
USGS NWIS database, loaded into the PeakFQ program and the output of that program includes
tabular or graphical low magnitudes, exceedance probability estimates, and 95% conidence for
those estimates limits, as illustrated in Figures 4.19 and 4.20 for the Mud Creek gage.
In addition to peak lows, statistical data are available for selected gage locations from the USGS
NWIS, including mean daily, monthly, and yearly lows, which are analyzed using the same meth-
ods to determine the return probabilities. It should be noted that the “year” for the USGS record is a
water year and not a calendar year. The USGS deines a “water year” as the 12-month period from
October 1 through September 30 of the following year. Other agencies may use different deinitions;
for example, for low-low calculations, the U.S. Environmental Protection Agency (U.S. EPA) typi-
cally uses April 1 through March 31.
The low conditions of interest for water quality, such as in setting discharge permits, are typi-
cally low lows rather than peak lows. Since the instantaneous minimum low is not a realistic mea-
sure, typically a minimum low over some speciied duration is used, such as a X Q T R low, where X
is some averaging period and T R is the return period. For example, a 7Q10 is the 7-day average low
low with a recurrence interval of 10 years (expected to occur once every 10 years). To determine
the 7-day average low for a particular year, a 7-day running average is used and then the low low
for the year determined, and this becomes a single data point. When compiled for consecutive years,
the 7-day minimum low can be used to estimate the magnitude of the low with a 10-year recur-
rence interval. For example, a 7Q10 low of 12 cfs (cubic feet per second) indicates that there is a
10% chance (return period = 1/0.1 = 10 years) that the low for 7 consecutive days will be less than

2000

1800

Peak FQ estimate

1600

1400

1200

1000

Graphical estimate

800

600

400

200

0

0.001 0.01 0.033

Probability

0.1 1

FIGURE 4.18 Peak low probability curve for Mud Creek using graphical and Bulletin 17B estimate from

PEAKFQ.