Environmental Engineering Reference
In-Depth Information
to streams is assumed to be a continuous pro-
cess. Recession-curve displacement meth-
ods are based on the assumption of episodic
recharge; recharge and rises in stream stage
occur only in response to storms. Recharge
is estimated by determining the amount of
streamflow that can be attributed to ground-
water discharge for each rise in stage and
summing over all storm events.
Hydrograph analysis techniques require
careful selection of the streamflow gauge and
watershed to be analyzed. If too large a water-
shed is selected, rainfall may not be uniformly
distributed over the watershed for all storm
events and the watershed may overlie several
aquifers. Rutledge ( 1998 ) suggested that the
techniques be applied at stream sites with drain-
age areas less than about 1300 km 2 . Hydrograph
analysis should not be applied to watersheds
with flood-control features or reservoirs.
Rutledge ( 2007 ) and Halford and Mayer ( 2000 )
provided some guidance and illustrated basins
where hydrograph analysis is inappropriate.
Table 4.1. Published values of Pr such that Q Pr , daily
streamflow that is exceeded Pr percent of days, is
equivalent to base flow.
Pr , in percent
Region
42
Valley Ridge Province, eastern
US (Rutledge and Mesko, 1996)
50
Virginia (Nelms et al ., 1997)
(Cushing et al ., 1973)
60-65
Southeast US Coastal Plain
(Stricker, 1983; Kuniansky, 1989)
55
Long Island, nonurbanized
(Reynolds, 1982)
60-90
Ohio (Pettyjohn and Henning,
1979)
Ephemeral streams are an important source
of focused recharge in many arid and semi-
arid regions. Duration curves indicate the
percentage of days in which flow is expected.
The number of days in which flow occurs is
an important factor in calculating the total
amount of recharge occurring from an ephem-
eral stream (Dahan et al ., 2007 ).
4.5.1 Empirical hydrograph separation
methods
Prior to the advent of computers, a number of
arbitrary graphical techniques of varying com-
plexity were used for hydrograph separation
(Barnes, 1939 ; Chow et al ., 1988 ). One such tech-
nique consisted of plotting stream discharge
vs. time on a log-linear scale, determining the
slope of the groundwater recession at the point
in time after the peak where the recession
became approximately linear (at which time
streamflow is assumed to be entirely derived
from groundwater discharge, point A in Figure
4.8 ), and extending the recession curve back-
ward to the point in time of the first inflection
point on the hydrograph after the peak (point
B in Figure 4.8 ). A straight line was drawn con-
necting this point to the point of initial rise in
the hydrograph (point C in Figure 4.8 ), and the
area under line segments AB and BC in Figure
4.8 is the calculated base flow (Chow, 1964 ).
The method is subjective, results may not be
reproducible, and the method can become com-
plicated when streamflow rises occur in quick
succession.
4.5 Physical streamflow
hydrograph analysis
Hydrologists have studied streamflow hydro-
graphs since at least the mid 1800s in efforts
to identify portions of streamflow that can be
attributed to surface runoff and groundwater
discharge (Hall, 1968 ). Over the course of time,
various theoretical and empirical approaches
have been invented and reinvented for quanti-
fying base flow or recharge. Several empirical
hydrograph separation methods are described
here, along with the recession-curve displace-
ment analysis. Both approaches are used to
develop estimates of diffuse recharge inte-
grated over the area of the aquifer that con-
tributes to groundwater discharge. With
hydrograph separation methods, base flow
is calculated by summing groundwater dis-
charge to a stream on a daily basis over the
period being analyzed; groundwater discharge
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