Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 102

4.4.2 e StIMatIon of f Low d uratIon
The duration of lows is typically analyzed as described in the previous section, but either daily,
monthly, or annually to create a low duration curve. A low duration curve shows the percentage
of time that a given low rate is equaled or exceeded. The construction of a low duration curve
can be illustrated using the mean annual lows from the USGS Mud Creek Station (02430615 Mud
Creek near Fairview, Mississippi), which are tabulated in Table 4.5 and sorted in descending order,
ranked, and a probability assigned using Equation 4.4 (as graphically illustrated by Figure 4.21).
The low duration is usually written as Q X , where X represents the percentage that the low is
equaled or exceeded. For example, the Q50 for Mud Creek means that the annual low is about
20 cfs (based on the preceding analysis), so the 20 cfs annual low is equaled or exceeded 50% of
the time. Flow duration curves are more commonly based on daily lows. Smakhtin (2001) indicates
that the “design” low-low range of a low duration curve is in the 70%-99% range, or the Q70-Q99
range. The ratio of the discharge that is equaled or exceeded 90% of the time, to that of 50% of
the time (Q90/Q50) is commonly used to indicate the proportion of streamlow contributed from
groundwater storage (Nathan and McMahon 1990).

4.4.3 e StIMatIon of t IMInG and r ate of c HanGe
Timing describes the time of year at which particular low events occur, such as the timing of loods
or low-low extremes, while the rate of change indicates how quickly the low changes, as lows rise
and fall from day to day.
The timing of an event is usually determined by two attributes: a magnitude and a time of occur-
rence. First, some low condition is deined, such as the annual minimum or maximum low, and
then the date or Julian day for that event is recorded. The rate of change requires some “trigger”
such as a high or low low and then an algorithm to determine whether the low is the rising or the
descending limb of the hydrograph. For example, the following is the algorithm used for high lows
in the Nature Conservancy's (2007) Indicators of Hydrologic Alteration (IHA) Program:

TABLE 4.5
Year Averaged Flow and Flow Probabilities for USGS Station 02430615 Mud
Creek near Fairview, Mississippi

Rank P (%) Year Flow (cfs) Rank P (%) Year Flow (cfs)

1 5.88 1991 45.20 17 100.00 1996 19.70

2 11.76 1997 30.10 18 105.88 1976 19.10

3 17.65 1989 24.50 19 111.76 1979 19.00

4 23.53 1983 24.30 20 117.65 1999 18.80

5 29.41 2005 23.80 21 123.53 1987 17.30

6 35.29 2003 22.50 22 129.41 2001 16.10

7 41.18 1994 22.20 23 135.29 1985 15.90

8 47.06 1993 22.00 24 141.18 1977 15.60

9 52.94 1980 21.40 25 147.06 1978 15.20

10 58.82 1992 21.40 26 152.94 2006 13.20

11 64.71 1984 20.80 27 158.82 1988 11.40

12 70.59 2004 20.70 28 164.71 2000 11.40

13 76.47 1990 20.30 29 170.59 1982 10.40

14 82.35 1995 20.10 30 176.47 1981 9.85

15 88.24 1998 20.00 31 182.35 1986 9.80

16 94.12 2002 19.90 32 188.24 2007 8.13