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Group Title: Bulletin Florida Cooperative Extension Service
Title: Agricultural water measurement
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00008522/00001
 Material Information
Title: Agricultural water measurement
Series Title: Bulletin Florida Cooperative Extension Service
Physical Description: 21 p. : ill. ; 23 cm.
Language: English
Creator: Smajstrla, A. G ( Allen George )
Harrison, D. S ( Dalton Sidney ), 1920-
Zazueta, F. S ( Fedro S )
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville
Publication Date: 1985?
Subject: Water consumption   ( lcsh )
Water-supply, Agricultural   ( lcsh )
Water conservation   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: A.G. Smajstrla, D.S. Harrison and F.S. Zazueta.
General Note: Cover title.
 Record Information
Bibliographic ID: UF00008522
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6784
ltuf - AEG9838
oclc - 14635685
alephbibnum - 000872577

Table of Contents
    Front Cover
        Front Cover
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
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        Page 21
    Back Cover
        Page 22
Full Text

Bulletin 207.

SAgricultural Water

Measure .pe .-';.

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:; F.A.S..Univ. of FIor:,


A.G. Smajstrla, D.S. Harrison, and ES. Zazueta

Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida
Gainesville, Florida
John T Woeste, Dean for Extension

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A.G. Smajstrla, D.S. Harrison, and FS. Zazueta*

The growing need to conserve water and to use our natural resources
wisely has increased the importance of being able to measure water
supplies accurately. Water measurements are essential in irrigation,
where the amount of water applied to a crop must be known. Most irri-
gation and water management districts grant permits for the amount
of water to be used for crop production. Also, in irrigation it is important
that flow rates be measured to ensure the most efficient use of water, as
well as of the power required to obtain it.
Many methods of water measurement are used at the present time.
These vary from measuring a volume of water collected during some
time interval to the use of devices which give the flow rate directly.
Measuring flow rates in open channels usually requires different
methods from measuring pipe flow. All methods are based upon applica-
tion of the equation of continuity,
Q = av (1)
where Q = flow rate, volume per unit time,
a = cross-sectional area of flow, and
v = average velocity of flow.
Of course, units of measurement must be used consistently.

Units of Measurement

Volume (V)
Volume is the amount of water measured. For agricultural applica-
tions, the volume units commonly used are the gallon, cubic foot, acre-
inch, and acre-foot. The relationships among these units are as follows:
1 cubic foot (ft3) = 7.48 gallons (gal)
1 acre-foot (ac-ft) = 43,560 cubic feet (ft3)
1 acre-foot (ac-ft) = 12 acre-inches (ac-in)
1 acre-inch (ac-in) = 27,154 gallons (gal)

*Associate Professor, Professor, and Visiting Assistant Professor, respectively,
Agricultural Engineering Department, IFAS, University of Florida, Gainesville,
Florida 32611.

Velocity (v)
Velocity is the average speed at which water moves in the direction of
flow. The unit of velocity commonly used is feet per second. bT aid in
understanding the size of this unit, it is compared with a more familiar
1 foot per second (ft/sec or fps) = 0 68 miles per hour

Cross-sectional Area (a)
The cross-sectional area is the size of the surface that is perpendicular
to the direction of flow. For example, in a pipe flowing full this is the
area of a circle with a radius equal to the inside diameter of the pipe.
The cross-sectional area is normally given in square feet (ft2) or
square inches (in2) where
1 square foot (ft2) = 144 square inches (in2).

Flow Rate (Q)
Flow rate is the volume of water flowing through a given cross-
sectional area per unit time. As indicated in equation (1), flow rate is the
multiple of velocity and cross-sectional area. Thus, a small-diameter
pipe may have the same flow rate as a larger-diameter pipe, if the
velocity in the smaller pipe is greater than that in the larger pipe.
The units of flow rate commonly used for agricultural applications
are gallons per minute, cubic feet per second, and acre-inches per hour.
The relationships between these units are as follows:
1 gallon per minute = 0.00223 cubic ft per second
(gal/min or gpm) (ft3/sec or cfs)
1 gallon per minute = 0.00221 acre-inches per hour
(gal/min or gpm) (ac-in/hr)
1 cfs = 449 gpm = 0.992 ac-in/hr
1 ac-in/hr = 453 gpm = 1.01 cfs
Also, water management districts sometimes use units of millions of
gallons per day in issuing permits.
1 million gal/day (mgd) = 1.55 cfs = 1.53 ac-in/hr

Volumetric Flow-Rate Measurements
Volumetric flow-rate measurements are made by collecting a volume
of water for some interval of time. A graduated container or container of
known size can be used with a stopwatch for these measurements. The

measured volume of a container divided by the time required to fill the
container is the volume rate of flow. For example, if the time required to
collect a 1-cubic-foot container of water was 20 seconds, the flow rate, Q,
was 1/20 cfs or 22.4 gpm.
Another method of volume measurement is based on the weight ofthe
water collected in a container. Water is allowed to fill the container for a
given period of time, and the water and container are weighed. The con-
tainer is then weighed empty. The difference in the two weights is the
weight of the water collected. The volume may be found by using the
8.34 lbs = 1 gallon or 62.4 lbs = 1 cubic foot
For example, a container weighing 6.32 lbs was used to collect water
for a period of 1 minute. The container and water were then found to
weigh 89.72 lbs. The weight ofthe water was 89.72 lbs 6.32 lbs = 83.40
lbs. The volume rate of flow was (83.40 lbs/1 min) (1 gallon/8.34 lbs) =
10 gpm.
Another method of volume measurement uses a tipping (tilt) bucket
with two compartments, as shown in Figure 1. The tilting container
must be under the outlet of the water supply for this method to be used.
This flow measuring device depends on the force of gravity to tip the
bucket about some fixed pivot point when a known quantity of water

Figure 1. Two-way tipping bucket for low flow-rate measure-

has flowed into one of the compartments. When the container tips, the
other compartment shifts under the outlet, and the previously filled
compartment drains. If only a volume measurement is desired, all that
is required is a counting mechanism that is triggered for each tip of the
container. For flow-rate measurements, a timing mechanism must also
be used to determine the length of time required for a given number of
tips. This method is often used to measure low flow rates, such as that of
rainfall. The resolution of this instrument is the volume of water re-
quired to tip the bucket.
Finally, volumes and flow rates can be measured directly by gauging
the change of level in a reservoir or some other gathering place for the
period of time that water is added to or extracted from it. Seepage losses
and other water losses must be estimated if this method is to be

Velocity-Area/Flow-Rate Measurements
When the mean velocity in an open channel or pipe can be determined
and the cross-sectional area of the flow can be measured, the flow rate pass-
ing a point can be calculated from equation (1). The following velocity-
area/flow-rate methods are often used for agricultural water measure-

Float Method
The float method provides a way of estimating surface velocity in an
open channel. The velocity of a floating object can be obtained by measur-
ing the time required for the object to move a known distance. The mean
channel velocity is normally 0.8 to 0.9 of the velocity of the floating object.

Manning's Equation
The mean velocity in a channel can be calculated using Manning's equa-
1.49 /a\ (2)
v = --() (s)"
n p
where n = coefficient of roughness,
a = cross-sectional area of flow (ft),
p = wetted perimeter of the channel (ft),
s = channel slope (ft/ft),
v = mean velocity (fps).
Values of n are obtainable from published tables for various channel
characteristics. Some values for common types are given in Table 1.

Table 1. Roughness coefficients, n, for the Manning equation.
Type and Description of Channel n values
Channels, Lined
Asphalt 0.015
Concrete, smooth 0.012-0.018
Concrete, rough 0.017-0.030
Metal, smooth 0.011-0.015
Metal, corrugated 0.021-0.026
Plastic 0.012-0.014
Wood 0.011-0.015
Channels, Vegetated
(Dense, uniform stands of green
vegetation more than 10 in. tall)
Bermuda grass 0.04-0.20
Kudzu 0.07-0.23
Lespedeza 0.047-0.095
Earth Channels and Natural Streams
Clean, straight bank, full stage 0.025-0.040
Winding, some weeds and stones 0.033-0.045
Sluggish river sections, weedy
or with deep pools 0.050-0.150
Asbestos cement 0.009
Cast iron 0.011-0.015
Clay or concrete (4 to 12 in.) 0.010-0.020
Metal, corrugated 0.021-0.0255
Plastic, corrugated (2 to4 in.) 0.016
Steel, riveted and spiral 0.013-0.017
Vitrified sewer pipe 0.010-0.017
Wrought iron 0.012-0.017

Another method of estimating the mean velocity of flow is by using
tracers. A dye or colored fluid may be injected into the flow below the sur-
face at about 0.2 of the flow depth. The time required for the tracer to move
a measured distance from the point of injection is recorded. This should be
done several times and the distance divided by the average time to obtain
the velocity.

Current Meters
Current meters can be used, where accurate point velocities within the
flow are required. The flow rate can be obtained by measuring a series of

point velocities, multiplying the mean velocity of each segment of channel
by the cross-sectional area of the segment, and adding these incremental
flow rates to obtain the total flow rate in the stream.
A Price current meter (Figure 2) may be used to make point-velocity
measurements. This meter is composed of several small cone-shaped cups
mounted symmetrically about a pivot point. The velocity of the water
causes the cups to rotate. The number of rotations or revolutions are in-
dicated by an electrical sounding device connected to earphones worn by
the operator.
Current meters are either mounted on a rod or suspended from a cable
above a heavy weight. Rod-mounted current meters are used in shallow
streams, while most larger streams and rivers require the use of a sus-
pended cable meter.




Figure 2. Price current meter for stream velocity measurements.

Current meters must be calibrated before being used to measure
point velocities. The calibration is made by counting the number of
revolutions made for a given interval of time for several known
velocities. The revolutions per unit of time are then graphed versus the
known velocities. From the resulting curve, velocities are obtained
when revolutions per minute are measured in the field.
For wide streams the channel cross section is divided into segments
(Figure 3) and point velocities are taken in the vertical direction at the
horizontal midpoints of the segments. The mean velocity of a segment
multiplied by the cross-sectional area of the segment is the flow rate for
that segment. As in the example in Table 2, adding the flow rates for all
segments gives the total flow rate of the stream shown in Figure 3.
In Figure 3, two methods are shown for using a current meter to deter-
mine the mean velocity of a channel segment. These are the single-
point and two-point methods. In the two-point method, point velocities
are obtained at 0.2 and 0.8 of the depth. The average of the two values is
used as a mean velocity for that segment of the channel. The single-
point method is generally used for shallow flow depths (less than one
foot), and the reading is taken at 0.6 of the depth.
The channel section where current meter measurements are taken
should be straight with an approximately uniform cross section.
Obstructions in the channel, such as bridge piers, should be avoided
because of additional turbulence in the stream flow.

8' 4- 20'---- 20' -- 8'-_

LL 2 -----
--I --
I I I I l I I

0 10 20 30 40 50 60


Figure 3. Subdivision of stream cross section for current meter
flow-rate measurements.

Table 2. Computations of stream flow using a current meter.
Dist. Depth Obser- Time Revo- Point Mean Area Flow
from of vation (sec) lutions Velo- Velo- (ft2) Rate
initial water depth city city (ft2) (cfs)
point (ft) (ft) (fps) (fps)
0 0 0 0 0 0 0 -
6 0.9 0.54 56 12 0.58 0.58 3.6 0.21
10 2.4 0.48 56 18 0.84 0.81 10.8 8.10
1.92 56 16 0.77
14 3.4 0.68 57 22 1.01 0.95 13.0 12.35
2.72 58 20 0.89
18 2.8 0.56 64 18 0.74 0.73 10.8 7.88
2.24 59 16 0.72
28 1.1 0.66 61 14 0.63 0.63 4.8 3.02
Total Stream Flow 31.56

Pitot Tubes

The pitot tube is often used to obtain the velocity of flow in an open
channel or pipe. The pitot tube consists of a small-diameter, L-shaped
tube pointed upstream, i.e., into the current. Water in the L-shaped
tube will rise to a small height above the water surface in the stream.
This small height is the velocity head, v2/2g. For low velocities, the
rise of the water in the pitot tube may be too small for accurate
The pitot tube is primarily used for flow measurements in pipes,
where the velocities are usually much higher than stream velocities. In
pipe flow the water is also subjected to an internal pressure, which in-
fluences the height the water rises in the pitot tube. The height of rise,
h, in feet is given by
Sv2 (3)
h= +-
w 2g
where p = the pressure Obs/in2),
v = the velocity of flow (fps), and
g = the acceleration of gravity (32.2 ft/sec2).
If the height (h) and the pressure (p) in the pipe are read, the velocity
(v) may be obtained from

p )2g) (4)
v=((h 2g
where all factors are as previously defined.

For high-pressure systems, the practical application of equation (4) is
limited by the accuracy with which pressure can be measured. For
these conditions, pitot-static tubes can be used with both piezometers
and pitot tubes, so velocity heads can be measured directly using
U-tube manometers.
Pitot-tube principles are used in some commercially available flow
meters. These meters use special water-air manometers to measure
mean velocity directly. The conventional pitot tube only measures
point velocities, thus requiring several readings to obtain the mean.
In each of the pitot-tube applications, the cross-sectional area of the
flow must be obtained to calculate the flow rate in the pipe or channel.
For a pipe, the area may be easily obtained by measuring the inside
diameter. Some pitot-tube gauges are constructed for a specific pipe
diameter and are thus calibrated directly in terms of flow rates.

Flow-Rate Methods Employing
Constriction in the Flow Channel
Flow rates may be measured by constricting the flow in a channel and
forcing it over or through a cross section of known geometry. Several
devices of this type exist. Weirs, orifices, and flumes are commonly used
for agricultural flow measurements and will be discussed here.

A weir may be defined as a restriction in a plane through which water
passes (Figures 4 through 8, pp. 10-11). The restriction is normally
created by constructing a plate with a notch in it. Weir notches have
been constructed in various shapes. Weirs are classified by the shapes of
the notches, the types of crests, and whether they are contracted or sup-
pressed. Some examples are the 90 degree V-notch weir (Figure 4), the
Cippolletti weir (Figure 5), the rectangular contracted weir (Figure 6),
and the rectangular suppressed weir (Figure 7). Suppressed weirs have
crest lengths approximately equal to the width of a channel, and con-
tracted weirs have crest lengths much shorter than the width of the
upstream channel so that the stream must converge as water
approaches and flows through the weir notch.
Weirs can be either broad-crested (Figure 7) or sharp-crested (Figure
8). The sharp-crested weir has a sharp upstream edge that causes the
water to cross only a line as it passes over the edge of the weir. The
broad-crested weir has either a curved or rounded upstream edge or a
crest, so that the water passes over a surface rather than a line. Because
water passes over only a thin edge, much greater accuracy is obtained
with sharp-crested weirs, which are used almost exclusively for
measuring flow rates in irrigation canals or ditches.

Figure 4. 90-degree V-notch weir for
open-channel measurements.

Figure 5. Cipolletti trapezoidall) weir
for open-channel use.

Figure 6. Rectangular contracted weir
for open-channel use.

Figure 7. The rectangular suppressed
weir for open-channel use.

Figure 8. Procedure for measurement of head on a weir. (Top of
stake is set at elevation of the crest of the weir.)

Rectangular Weirs. The flow rate through a rectangular weir depends
on the approach conditions, the downstream water level, and the shape
of the weir. A general equation for a rectangular, contracted weir is
Q = C(L 0.2h)h"' (5)
where Q is the flow rate (cfs),
C = a constant that depends on approach conditions and the con-
struction of the weir notch,
L = the length of the weir crest (ft), and
h = the height of the water above the weir crest (ft) measured
upstream of the weir about 4 feet (Figure 8).
For suppressed rectangular weirs, the general equation is
Q = CLh"' (6)
with each term as previously defined in equation (5).C is usually 3.33
for both contracted and suppressed sharp-crested weirs. C must be
determined by calibration of broad-crested weirs.
V-notch Weirs. The V-notch (triangular) weir (Figure 4) has a greater
practical range of flow-rate measurements than most other types. The
shape of the notch permits accurate measurements for low flow rates
through the notch. Because of the large head loss, however, flows over
this weir should be limited to a maximum of 4 cfs. Most V-notch weirs
are constructed with a 90-degree angle. For a 90-degree, sharp-crested,
V-notch weir, the general equation is
Q = 2.49h2.48 (7)
with each term as previously defined. For accurate flow-rate
measurements, the head should be measured to the nearest 0.01 foot.

Cippoletti Weirs. Some weirs are constructed so that the sides of the
notch slope out from the weir crest at the rate of 1 to 4. These weirs,
called Cippoletti weirs (Figure 5), discharge through the triangular
portions at the ends of the weir the same amount the discharge per unit
of head is decreased because of end contractions.
This permits the use of the equation
Q = 3.67Lh' (8)
with all terms as previously defined.
In the field, weirs can easily measure flows accurately (within 15%);
however, there are some precautions and limitations to their use. Silt
and sediment deposition behind a weir increases the flow rate for a
given head. A weir also requires a difference in elevation between the
upstream water level and downstream water level, which may be dif-
ficult to attain for channels having very little slope. When the water
level on the downstream side of a weir becomes high enough so that
water does not spill freely over the weir, then the weir is submerged and
equations (5) to (8) are no longer valid.

Open-channel Orifices. An orifice for measuring flow rates in an open
channel usually consists of either a round or a rectangular opening in
an obstruction placed across the channel. The edges of the opening are
usually sharp and constructed of metal. The size of the opening is small
compared to the area of the stream, therefore water backs up on the
upstream side of the obstruction. The orifice equation is
Q = 0.61 a (2gh)' (9)
where a is the cross-sectional area of the opening (ft2),
g is the acceleration of gravity (32.2 ft/sec2), and
all other terms are as previously defined.
This equation may be written
Q = 4.89 ah' (10)
since 2g = 8.02. The head is measured as shown in Figures 9 and 10. The
first illustration (Figure 9) is of free-flow conditions, when the
downstream water level is below the orifice. The second illustration
(Figure 10) is of submerged conditions, when the downstream water
level is above the orifice and both the upstream and downstream water
elevations must be accurately measured. The difference in the water
elevations is the head, h, for the submerged condition. Conditions
where the downstream water level submerges only part of the orifice
should be avoided. The orifice then acts like a submerged weir.


Figure 9. Head measurement for free flow from an orifice.
(Head is measured to center of orifice.)





Figure 10. Head measurement for a submerged orifice.

Pipe Orifices. Pressure differences on either side of an orifice con-
stricting flow in a pipe can be related to velocity. Pipe-orifice flow
meters are based on that principle and, for a given pipe size, are com-
mercially available to read directly in gpm or cfs.
An end-cap orifice is frequently used to measure flows from pipes to
the atmosphere, as shown in Figure 11, p. 14. In this case, h is the
manometer reading above the centerline of the pipe as taken 2 to 3 ft
upstream of the orifice. Equation (10) is still applicable.




Figure 11. Pipe end-cap orifice for flow measurement from a

Flumes are specially shaped sections of channels which are used for
flow-rate measurement. Depth measurements in these channel sec-
tions can be used to calculate flow rates, if the flumes are properly
Parshall Flume. The Parshall flume (Figure 12) is a flow measuring
device which is used extensively for irrigation water measurement. It is
shaped so that water passes through a sudden drop in the floor of the
flume, causing a change in the flow depth. As water enters the flume, it
is caused to converge in a restricted area called a throat. The drop in the
floor occurs at the throat. The flow rate passing through the flume can
be computed by measuring the throat width and the depth of flow above
the throat. The equation is rather complicated and will not be given.
Manufacturer's calibration curves and tables are commonly used to
obtain the flow rate for the heads measured.
The successful operation of the Parshall flume depends on setting the
crest of the flume at the proper elevation above the bed of the channel.
This is difficult for steep slopes and high banks. Also, when the banks
are low and the slope of the channel is very flat, care must be taken in
choosing the right size so that water does not back up behind the flume.
In addition, the flume must be set level in the channel for accurate
When the downstream elevation of the water near the exit of the
flume reaches a height that retards the flow through the flume, a
submerged condition exists and a correction factor is required for

L \a




Figure 12. The Parshall flume for open-channel flow measure-

accurate flow measurement. The flume is considered submerged when
the depth of flow downstream of the sudden drop in the floor of the
flume, Hb, exceeds 0.7 of the depth of flow upstream ofthe drop, Ha. The
correction depends on the depth of flow at Ha and Hb. The correction is
obtained from the manufacturer's calibrations and is subtracted from
flow calculated for the unsubmerged condition.
The advantages of the Parshall flume are: (1) it passes sediment and
small trash easily, (2) it requires only a small head loss, and (3) it allows
accurate flow measurements even when partially submerged.
A disadvantage of the Parshall flume is that it is not accurate at low
flow rates. It is therefore not entirely satisfactory for measuring widely
fluctuating flow rates.

Type H Flume. The Type H flume is particularly adapted to runoff
measurement. Shown in Figure 13, this flume has a V-shape which
gives accuracy at low flows, in addition to providing for high capacity.
Dimensions of the flume are given in Figure 13. As with the Parshall
flume, manufacturer's calibrations of flow rates as functions of heads
would again be used in tabular or graphical form because the equation
is complex and varies with the flume dimensions.

Other Flow-Rate Measurement Methods
There are a wide variety of other methods of measuring flow rates in
either open channels or closed conduits. Those commonly used in
agricultural flow measurement are presented below.

Impeller Meters
The velocity of flowing water in a pipe can be measured by installa-
tion of an impeller which rotates at a speed proportional to the fluid
velocity. Impeller meters measure flow rates directly, since in most
cases the cross-sectional area is a constant.
Impeller meters have many advantages. They normally totalize
volumes of flow but can measure rates directly as well. They are
relatively low-cost and accurate (within 2%). They can be mounted on
pipes horizontally (Figure 14) or vertically (Figure 15, p. 18), provided
the flow fills the pipe entirely. They do, however, require routine main-
tenance and are subject to losses of accuracy with age due to failure of
mechanical components.
The shunt-flow impeller meter is a relatively new, low-cost version. A
portion of the flow is shunted through a small impeller meter by using a
constriction in the main pipe. Only the shunted flow is measured using
a small low-cost, totalizing impeller flow meter. The flow in the main
pipe is obtained from the manufacturer's calibration. Mechanical com-
ponents are restricted to the low-cost impeller meter, thus reducing
repair and maintenance costs.

Elbow Meters
Another type of flow measurement device is the elbow flow meter
(Figure 16, p. 18). Differences in pressure in the elbow of a pipe are
measured and correlated with velocity. Such a meter must be
calibrated for each size of elbow and degree of curvature.



1-1.1 D- : 1-0.6D <^Tan \m

--1.9D--- I- 1.35D- WELL

Figure 13. The Type H flume for open-channel flow measure-

Figure 14. Horizontally mounted impeller flow meter for pipe-
flow measurement.

Trajectory Method
Flow rates can be estimated by applying fundamental physical laws.
Where a horizontal pipe discharges to the atmosphere, the trajectory of
the jet is a function of the velocity of discharge. The laws controlling the
velocity and displacement of falling bodies are applied to calculate the
velocity of the jet. Measurement of the coordinates of one point on the
surface of the jet with the reference (origin) on the jet at the point of
discharge permits estimation of the flow rate.

Figure 15. Vertically mounted Figure 16. Elbow flow meter
impeller flow meter, for pipe-flow use.

Figure 17 is a nomograph for estimating flows from pipes flowing full
and only partially full using this method. The use of the nomograph is
illustrated in the following examples:
1. Pipe flowing full. If pipe diameter = 10 inches, X = 11 inches, and
Y = 6 inches, then connect X = 11 inches on Scale A (eft side for Y = 6
inches) and diameter = 10 inches on Scale B with a straight line. Pro-
ject the straight line to Scale C and read flow rate = 1300 gpm.
2. Pipe flowing partially full. If pipe diameter = 10 inches, X = 11
inches, Y = 6 inches, and Z = 2 inches (where Z is the depth of pipe
flow below full), then Z/diameter = 2 inches/10 inches 0.2. First
solve the problem as in Example 1, assuming the pipe is flowing full.
Then with a straight line, connect the flow rate (1300 gpm) on Scale C
with 0.2 on Scale D. Project this line to Scale E and read flow rate =
1100 gpm.

2- 3000
30 2500
-4 -26
3 24 2000
1 22 .90 -100
-5 N 20
4- I
IN -6 D 1500
IN E 16 .85
N P 14 50
C -8 N I F .80 A
H 6- C P 2 D
E 9 H E 1000 75 J200
S 7 10E 900 Z/D U20
D 190 1 .70 S
H 12 A 800E -250
N 9 H 8 L700 60 D
N E 00 M8 L 700
T 65050 F -300
S -16 E 7 600 L
E 12 6 550 40 0 -350
Q 2 E P 500 30 4
U 14--20 I E .20 400
A U N R 450 o10 G
6 A C 5 A 450
L A H M 400L
S 18 25 L M 400
S 25 E L -500
6" 20- 30 1 -4 N 30 -550
3012' U 60
T S -600
.35 E 300
40 250 E -
3o- \ R :800
M 900
5- M -900
N2 0 '" N -1000

S. . E -1200


Figure 17. Nomograph for estimating pipe flow rates using the
trajectory method.

(Note: Pipe must be horizontal for best results. All quantities ob-
tained are approximate.)

Water-Level Recording Equipment

Stream gauging stations, weirs, and flumes are often equipped with
continuous water-level recording devices. Such a device and a stream
gauging station are shown in Figures 18 and 19, p.20. The float rests on

Figure 18. Float-actuated water-level recorder.


Figure 19. Water-level recorder and stream gauging station.


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the water in a float well which is connected to the main channel by a
pipe or trench. As the flow rises or falls, the float actuates a pen which
records the level on a clock-driven chart. From this information, flow
rates and volumes can be calculated if the stream flow rates versus
stages were previously measured.

Units of flow-rate measurement for agricultural purposes were de-
fined. Measurement methods were presented for open-channel and
pipe-flow conditions. Methods were classified as volumetric flow-rate
methods, velocity-area/flow-rate methods, methods involving a con-
striction in the flow channel, and other (miscellaneous) methods. Appli-
cations, accuracies, advantages, and disadvantages of the various
methods were given. The use of water-level recording equipment for
flow-rate measurement was also discussed.

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This public document was promulgated at a cost of $1,578.00, or 31.6
cents per copy, to inform agriculturists of techniques for the measure-
ment of water flow. 12-5M-85.


K. R. Tefertiller, director, In cooperation with the United States A
Department of Agriculture, publishes this information to further the
purpose of the May 8 and June 30, 1914 Acts of Congress; and is
authorized to provide research, educational information and other
services only to individuals and institutions that function without regard to race, color,
sex or national origin. Single copies of Extension publications (excluding 4-H and Youth
publications) are available free to Florida residents from County Extension Offices.
Information on bulk rates or copies for out-of-state purchasers Is available from C. M.
Hlnton, Publications Distribution Center, IFAS Building 664, University of Florida,
Gainesville, Florida 32611. Before publicizing this publication, editors should contact
this address to determine availability.

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