Environmental Engineering Reference
In-Depth Information
1
Figure 4.7 Relation between
mean annual base flow, Q 50 , and
Q 90 for five physiographic regions
within the state of Virginia (after
Nelms et al ., 1997 ).
Base flow
Q 50
Q 90
0.8
0.6
0.4
0.2
0
Valley and
Ridge
Blue Ridge
Piedmont/
Blue Ridge
Transition
Piedmont
Northern
Piedmont
Southern
v a r i a b l e s a t u n g a u g e d s i t e s h a s b e e n a t of p i c of f c of n -
siderable research (Smakhtin, 2001 ). Regression
methods, based on basin and climate character-
istics, are the most commonly used approach.
Singh ( 1971 ) developed equations for predicting
Q Pr on the basis of drainage area for 13 different
regions in Illinois. Dingman ( 1978 ) developed
similar equations using drainage area and eleva-
tion for streams in New Hampshire. Santhi et al .
( 2008 ) developed regression equations to predict
base-flow index across the entire conterminous
United States.
Various low-flow indices have been proposed
to represent the variability of base flow, for
example [ Q 25 / Q 75 ] 0.5 (Stricker, 1983 ) and [ Q 20 / Q 90 ]
(Arihood and Glatfelter, 1991 ). Nelms et al .
( 1997 ) used log( Q 50 / Q 90 ) as an index and found
a median value of about 0.6 for the sites stud-
ied in Virginia. Higher index values indicate
higher base flow variability in time at a site.
These indices cannot be used to estimate base
flow by themselves, but they can be useful for
comparing characteristics among sites.
Streamflow duration curves also are use-
ful for the analysis of ephemeral streams that
receive little or no groundwater discharge.
in other areas suggest that mean annual base
flows can be estimated from flow exceedance
percentages of 40 to 90 ( Table 4.1 ).
For estimating base flow, the flow duration
curve is usually determined for the entire
period of record. However, duration curves can
be constructed for individual months (such as
for all Januarys) or seasons (such as winter) over
the period of record. For gauges with a long his-
tory of flow data, duration curves constructed
over fixed time intervals (such as decades) can
be used to examine trends in base flow that may
be attributable to changes in climate, land and
water use, or watershed characteristics such
as vegetation (Buytaert et al ., 2007 ; Callow and
Smettem, 2007 ; Tuteja et al ., 2007 ). The stream-
l of w d u r a t i of n c u r v e r e l e c t s t h e e f f e c t s of f c l i m a t e
and watershed characteristics over the entire
drainage area. Streamflow duration curves are
available for many USGS stream gauging sites
within the United States (http://streamstats.usgs.
gov; accessed February 11, 2009).
The streamflow duration curves described
above were constructed with data from gauged
locations where stage is continually recorded.
Prediction
of
various
low-flow
frequency
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