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FACT SHEET



Rangeland Watershed Program

A Water Quality Education and Technical Assistance Program for California Rangelands

FACT SHEET

U.C. Cooperative Extension and U.S.D.A. Natural Resources Conservation Service

____________________________________________________________________________________________________
No. 39
October 1996
____________________________________________________________________________________________________

Monitoring Streamflow

Hydrologists, stream ecologists, and watershed
managers are often interested in estimating stream
discharge. Discharge (q) is the flow rate of water
passing a point on a stream at an instant in time.
Discharge is expressed as a unit volume of water per
unit of time, usually expressed as cubic feet per second
(cfs). One cfs equals 7.5 gallons per second. Current
velocity (v) is the speed at which water in the channel
is moving expressed as feet per second (ft/s). Flow
volume (Q) is estimated as discharge (q) multiplied by
the time duration of interest [Q (ft) = q (ft
3
/sec) *
time (sec)].

Whether stream discharge is measured automatically at
a flume or manually at a stream cross-section, there are
two basic concepts which must be applied to monitor
stream discharge. This paper will introduce the area-
velocity and the stage-discharge concepts and their
application. This is not a detailed description of how to
monitor streamflow. Monitoring streamflow requires
training.

A

rea-Velocity


The area-velocity concept allows one to estimate
discharge as the area (A) of water (ft
2
) within a stream
channel cross-section multiplied by the current veloc-
ity (v) at which the water in the cross-section is
traveling (ft/s).

q (ft
3
/s) = A (ft
2
) * v (ft/s)

Current velocity at the surface of the stream will be
greater than at the bottom. Current velocity at the edge
of the stream will be lower than at the middle of the
stream. This is simply due to the resistance to flow
presented by the stream channel.

Because current velocity will vary within the stream
channel cross-section, a stream channel cross-section
is often divided into portions and the area and current
velocity within each portion of the stream cross-
section determined individually.

Figure 1 illustrates the division of a stream channel
cross-section into 5 portions and the estimation of area
within each portion. The number of portions a cross-
section should be divided into depends upon the
characteristics of the cross-section, and the intended
use of the information. The more complex or irregular
the cross-section and the greater the need for an
accurate estimate, the more portions needed.


A current velocity can be used to estimate velocity
within each portion of the cross-section. The reading
of current velocity should be taken at 0.6 times the
water depth from the top of the water column.


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University of California and U.S. Department of Agriculture cooperating.

Stream discharge is simply the sum of the discharge in
each portion of the cross-section.
q = A1*v1 + A2*v2 + A3*v3 + A4*v4 + A5*v5

or

q = w1*d1*v1 + w2*d2*v2 + w3*d3*v3
+ w4*d4*v4 + w5*d5*v5

Often, a velocity meter is not available. In this case a
float and stopwatch can be used to estimate average
stream profile velocity. The average stream profile
velocity is substituted for v1 though v5 in the calcula-
tion of discharge (i.e., v1 through v5 have the same
value).

An orange makes a good float because it is submerged
enough not to be affected by wind and it is easy to see
in the water. Figure 2 illustrates the use of a float to
estimate average stream surface velocity. The time of
travel over a known distance is measured, and velocity
is computed as distance divided by elapsed time. The
stream reach the float is used in should be straight with
uniform flow, and the distance should be such that the
travel time is at least 20 seconds. The reach should be
near the cross-section. The procedure should be
repeated several times, and an average stream surface
velocity calculated. The average stream surface
velocity should then be multiplied by 0.85 to
approximate the average stream profile velocity.

S

tage-Discharge Relationship


The stage-discharge relationship is based upon the
relationship between stream water depth (stage) and
discharge at a flume or permanent cross-section. A
stage discharge relationship is an equation which
computes discharge from stage. Stage-discharge
relationships have been calculated for most standard
flumes and weirs, making discharge estimation at a
flume simply a matter of monitoring stage and then
calculating discharge.

Where discharge is to be monitored in a channel
without a flume, the stage-discharge relationship must
be established. The relationship will be different for
every cross-section, and will change at a cross-section
as the cross-section changes through time (aggrades or
degrades).

The first step is to identify a permanent cross-section.
The cross-section should be in a stable portion of the
stream (i.e., little erosion or deposition). The cross-
section should be easily assessable and should not be
located in a flood plain or where flow might be
affected by a downstream lake, pond, or estuary.

The second step is to install a stage staff or depth
meter in the channel at the cross-section. One often
sees stage staffs attached to the side of bridges or
pilings. The stage staff must be fixed and permanent.
Reference points should be identified in the event the
stage staff moves during a flood event. The perma-
nence of the stage staff and the stability of the cross-
section are crucial for developing a meaningful stage-
discharge relationship.

The next step is to repeatedly use the area-velocity
method at the cross-section to estimate discharge over
a wide a range of discharge levels as possible (low
flow to flood flow). Stage must be recorded each time
discharge is estimated. It is important that the obser-
vations represent a wide range of discharges, numer-
ous readings of similar flow events are of limited
value. When enough observations are made, a stage-
discharge relationship (equation) can be developed to
relate discharge at the cross-section to water depth at
the cross-section. The number of observations needed
to establish the stage-discharge relationship varies

Page 2
FS#39 with each cross-section. Developing a stage-discharge
relationship requires experience.

Once the stage-discharge relationship is developed,
discharge can be estimated as often as desired simply
by monitoring the water level at the cross-section over
time. One must be careful when extrapolating beyond
the range of discharge used to develop the stage-
discharge relationship (equation).

Although the stage-discharge method may seem
complex, it can be accomplished with sufficient train-
ing. The initial effort exerted in developing the stage-
discharge relationship (equation) is worthwhile if one
is interested in monitoring discharge at fairly frequent
intervals. Once the discharge relationship is estab-
lished, it must be checked periodically to insure it has
not changed due to changes in the stream channel
cross-section (erosion or deposition).

Monitoring stream discharge does require training and
an initial investment of time. Anyone monitoring
discharge should have a clear objective in mind before
expending time and energy monitoring discharge.































__________________
Kenneth W. Tate, Department of Agronomy & Range Science, University of California, Davis, CA 95616-8515, and Glenn Nader,
Livestock Advisor, Yuba-Sutter-Butte counties, UCCE.