2.3 Field Methods
67
measuring flow are detailed by Rantz et al. (1982). This section provides a
synopsis of this material. In addition, the USGS has produced a training video
for measuring discharge (http://training.usgs.gov/TEL/Nolan/SWProcedures/
Index.html). Field personnel may want to review this video as part of their field
training program.
To measure stream flow, some type of channel control is necessary. This
control may be constructed as a temporary feature, such as a weir or dam, or
a natural control, such as a bedrock outcrop or channel-width restriction. An
effective control provides a predictable relationship between water level (stage)
and flow (discharge) that does not change over time. A pressure transducer
installed in the deepest part of the stream channel, just upstream of the channel control, can be used to record the water level, commonly at 15-min intervals.
A line must be secured in the stream to transmit the response of the pressure
transducer to a data logger. Thus, the pressure transducer measures changes in
stage; these stage measurements then must be converted to estimates of flow.
Alternatively, stage can be measured using a measuring rod or yardstick held
vertically in place at a specific location. That location must be clearly defined
using a permanent structure of some sort, such as a rock or large tree.
To establish the relationship between stage and flow (referred to as a rating curve), simultaneous stream stage and flow measurements are needed over
as wide a range of stream flows as possible. To conduct the stream flow measurements, a cross section is chosen in the general vicinity of where the waterlevel measurements are taken. The ideal cross section chosen for measurement
should provide a regular cross-sectional channel shape that provides laminar
flow throughout the channel. The more closely these conditions are met, the
more accurate will be the resulting estimates of discharge.
The cross section is divided into intervals such that at least one pair of depth
and stream velocity measurements can be made in each interval. The number
and width of the intervals are dependent on the shape of the cross section.
Stream flow measurement determined with a single stream velocity measurement is not sufficient for obtaining an accurate representation of discharge.
An additional common method of estimating stream velocity relies on measuring the velocity of a neutrally buoyant object, such as a small orange, traveling downstream. This approach can provide grossly inaccurate flow estimates.
The object can follow preferentially rapid flow paths or, conversely, be temporarily impeded by stones or wood in the channel. We do not recommend use
of this method for estimating stream discharge, although it can be useful for
instructional purposes.
The stream velocity can vary considerably along the cross section and with
depth, requiring a number of velocity measurements to obtain an accurate
flow measurement. A large variety of stream velocity meters with varying precision and accuracy are available commercially. The velocity of each interval is
67
measuring flow are detailed by Rantz et al. (1982). This section provides a
synopsis of this material. In addition, the USGS has produced a training video
for measuring discharge (http://training.usgs.gov/TEL/Nolan/SWProcedures/
Index.html). Field personnel may want to review this video as part of their field
training program.
To measure stream flow, some type of channel control is necessary. This
control may be constructed as a temporary feature, such as a weir or dam, or
a natural control, such as a bedrock outcrop or channel-width restriction. An
effective control provides a predictable relationship between water level (stage)
and flow (discharge) that does not change over time. A pressure transducer
installed in the deepest part of the stream channel, just upstream of the channel control, can be used to record the water level, commonly at 15-min intervals.
A line must be secured in the stream to transmit the response of the pressure
transducer to a data logger. Thus, the pressure transducer measures changes in
stage; these stage measurements then must be converted to estimates of flow.
Alternatively, stage can be measured using a measuring rod or yardstick held
vertically in place at a specific location. That location must be clearly defined
using a permanent structure of some sort, such as a rock or large tree.
To establish the relationship between stage and flow (referred to as a rating curve), simultaneous stream stage and flow measurements are needed over
as wide a range of stream flows as possible. To conduct the stream flow measurements, a cross section is chosen in the general vicinity of where the waterlevel measurements are taken. The ideal cross section chosen for measurement
should provide a regular cross-sectional channel shape that provides laminar
flow throughout the channel. The more closely these conditions are met, the
more accurate will be the resulting estimates of discharge.
The cross section is divided into intervals such that at least one pair of depth
and stream velocity measurements can be made in each interval. The number
and width of the intervals are dependent on the shape of the cross section.
Stream flow measurement determined with a single stream velocity measurement is not sufficient for obtaining an accurate representation of discharge.
An additional common method of estimating stream velocity relies on measuring the velocity of a neutrally buoyant object, such as a small orange, traveling downstream. This approach can provide grossly inaccurate flow estimates.
The object can follow preferentially rapid flow paths or, conversely, be temporarily impeded by stones or wood in the channel. We do not recommend use
of this method for estimating stream discharge, although it can be useful for
instructional purposes.
The stream velocity can vary considerably along the cross section and with
depth, requiring a number of velocity measurements to obtain an accurate
flow measurement. A large variety of stream velocity meters with varying precision and accuracy are available commercially. The velocity of each interval is