Environmental Engineering Reference
In-Depth Information
States are available through the US Geological
Survey ( Table 2.1 ). However, direct measure-
ment of surface flow may be necessary in some
studies. Judicious selection of boundaries of
a control volume or study area can facilitate
measurement of surface flow. Small water-
sheds with no surface inflow make good con-
trol volumes; in this sense, only streamflow out
of the watershed would need to be measured.
In semiarid and arid regions with permeable
soils, runoff is often assumed to be negligible.
Surface flow can occur in stream channels or
as overland flow (sheet flow). Measurement of
surface flow onto and off a site is straightfor-
ward if flow occurs only in stream channels.
Overland flow is much more difficult to dir-
ectly measure (the estimation methods dis-
cussed in Section 2.4.4 are more commonly
applied). So it is difficult to construct an accur-
ate water budget for a control volume (such as
a small field plot) where overland flow across
boundaries is substantial.
Streamflow can be measured directly with
an acoustic Doppler current profiler (Olson
and Norris, 2007 ); this device can provide dis-
charge data in real time if it is maintained at
a single location. Because of the high cost of
the Doppler profiler, few studies can afford to
install one permanently. Streamflow at gaug-
ing stations is more commonly determined by
measuring stream stage and converting that
stage to a discharge by means of a rating table.
Rating tables can be developed theoretically
or empirically. Hydraulic structures, such as
flumes, culverts, and weirs, have theoretical
rating curves (Kennedy, 1984 ; Chow et al ., 1988 ;
Maidment, 1993 ). Installation of a hydraulic
structure in a small stream channel removes
the need for direct discharge measurements.
In the absence of a Doppler profiler, streamflow
can be determined by careful measurement of
the cross-sectional area of the stream (normal
to the direction of flow) and measurement of
flow velocity at a number of locations in the
cross section (Olson and Norris, 2007 ). Velocity
is measured with a mechanical current meter.
Stream stage can be monitored with a float or
pressure transducer; real time data on stream
stage are available for many stream gauge sites
in the United States ( Table 2.1 ). Stream dis-
charge determined from a Doppler profiler or
from a rating table is generally thought to be,
at best, within 5% of actual discharge (Rantz,
1982 ; Oberg et al ., 2005 ). Additional discussion
on streamflow measurement is contained in
Section 4.2 , in which the stream water-budget
method is described. Steep channels and ice-
affected streams are not amenable to standard
methods of discharge measurement; tracer
injection methods (discussed in Section 4.6.2 )
can be used in these streams (Kilpatrick and
Cobb, 1985 ; Clow and Fleming, 2008 ).
Overland flow can be measured on small
plots, but it is a tedious procedure, and the
measurement technique may alter natural flow
rates. Impermeable edging can be pressed into
the ground surface so as to funnel overland flow
to a collector. A tipping-bucket mechanism,
similar to that used in precipitation gauges, can
be used at the collection point to measure flow
(Wilkison et al ., 1994 ).
2.3.5 Subsurface flow
Quantifying groundwater inflow to and out-
flow from a small study area is problematic
because of the three-dimensional nature of
groundwater flow and the inherent variability
in properties of subsurface materials (Alley et al .,
2002 ). Knowledge of hydraulic conductivity and
hydraulic head at multiple locations and depths
is required for determining groundwater flow
rates. As with surface flow, selection of boundar-
ies for control volumes will influence the accuracy
with which boundary fluxes can be determined.
Groundwater divides provide natural boundaries
because there is no flow across them. For some
small basins, all groundwater discharges to a
stream near the mouth of the watershed, thus
simplifying the water-budget study.
One-dimensional groundwater fluxes can be
estimated with the Darcy equation:
q
=− ∂ ∂
KHx
s
/
(2.25)
where q is specific flux (flux per unit cross-sec-
tional area), K s is saturated hydraulic conduc-
tivity in direction x , and H is total head. Data
on hydraulic conductivity and water levels
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