Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 103

50.00

45.00

40.00

35.00

30.00

25.00

20.00

15.00

10.00

5.00

0.00

0.00 10.00 20.00 0.00 40.00 0.00 60.00 0.00 80.00 0.00 100.00

% equalled or exceeded

FIGURE 4.21 Flow duration curve for mean annual lows. (From USGS Station 02430615 Mud Creek near

Fa i r view, MS.)

• Following a low-low day, the next day is assigned to the ascending limb of a high-low

event if the daily low is higher than the 75th percentile of all lows, or if the low is higher
than the 50th percentile of all lows and the daily increase is more than 25%. Otherwise,
it continues as a low low.

• The ascending limb of a high-low event continues until the daily low decreases by more

than 10%, at which time the descending limb of the event is started.

• During the descending limb of a high-low event, the ascending limb is restarted if the

daily low increases by more than 25%.

• During the descending limb of a high-low event, the event is ended if the rate of decrease

of the low drops below 10% per day (meaning that the change in low is between -10%
and 25%), unless the low is still above the 75th percentile of all lows, in which case the
descending limb continues.

4.4.4 d ata r equIreMentS and c onSIderatIonS
An analysis of the aforementioned components (magnitude, return interval, frequency, duration,
timing, and rate of change) shows that they all have at least one thing in common. They all require
a lot of low data. What is usually required is a long-term historical record of lows from a particu-
lar gauging location. The number of gauging stations with an adequate historical record is limited
and is becoming increasingly limited. Hirsch and Costa (2004) indicated “the most crucial policy
question remains how the stream gauging network should be supported to foster continued mod-
ernization and improved eficiency, and to assure the continuation of valuable long-term stream
low records.” This is an important issue since, due largely to the lack of support, about 640 USGS
stream gages with more than 30 years of records (Hirsch and Costa 2004) have been discontinued
during the last decade. Making management decisions today based on low data, the most recent
records of which are a decade or more old (therefore not relecting current conditions), may have
costly consequences.