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Pressure or Head (H): People often use the phrase “head

of water.” A foot of head usually implies that the water level

is one foot above some measuring point. However, head can

also mean pressure. For example, as the level of water rises

in a barrel, the pressure at the bottom of the barrel increases.

One foot of water exerts 0.43 pounds per square inch (psi)

at the bottom of the barrel. Approximately 2.31 feet of water

equals 1 psi. Thus, if a tank of water were to be raised 23.1 feet

(2.31 x 10) in the air with a hose connected to it, the pressure

in the hose at the ground would be about 10 psi.

Area: The cross sectional area of a ditch is often required

to calculate flow. Some ditches are trapezoids and others or

more like ellipses. To find the area of a trapezoid (Fig. 1a),

measure the width of the bottom (b) and the width of the

ditch at the water surface (s) and add them together. Divide

that number by 2 and then multiply by the height (h) of the

water. If the ditch is more elliptical in shape (Fig. 1b), take the

depth of the water (h), multiply it by the width of the ditch at

the surface (s), divide by 4 and then multiply by PI (3.14). To

calculate the cross-sectional area of a pipe, the formula is PI x

r2, where PI is 3.14 and “r” is the radius of the pipe. NOTE:

All measurements should be in feet.

ARIZONA COOPERAT IVE

E TE NSION

M easuring W ater F loW in s ur Face

irrigation D itches an D g ate D P iP e

Measuring water in surface irrigation systems is critical for

peak efficiency management. Without knowing the amount

of water being applied, it is difficult to make decisions on

when to stop irrigating or when to irrigate next. A good

irrigation manager should know the flow rate of the irrigation

water, the total time of the irrigation event and the acreage

irrigated. From this, the total amount of water applied can

be determined, which will help determine whether the

irrigation was adequate and when the next irrigation should

be. Irrigation management decisions should be made based

on the amount of water applied and how this relates to the

consumptive use demands of the plants and the soil water

holding capacity.

Units of Measuring Water

There are many ways to express water volume and

flow. The volume of water applied is usually expressed in

acre-inches or acre-feet for row crops or gallons per tree in

orchards. Flow rate terminology is even more varied. Flow

rate is expressed as cfs (cubic feet per second), gpm (gallons

per minute) and in some areas, miner ’s-inches. Below is a

description of each.

Acre-inch (ac-in.): An acre-inch is the volume of water

required to cover an acre of land with one inch of water. One

acre-inch equals about 3,630 cubic feet or 27,154 gallons.

Acre-foot (ac-ft): An acre-foot is the volume of water

required to cover an acre of land with 1 foot of water. One

acre-foot equals about 43,560 cubic feet, 325,848 gallons or 12

acre-inches.

Cubic feet per second (cfs): One cubic foot per second is

equivalent to a stream of water in a ditch 1- foot wide and

1-foot deep flowing at a velocity of 1 foot per second. It is also

equal to 450 gallons per minute, or 40 miner ’s-inches.

Gallons per minute (gpm): Gallons per minute is a

measurement of the amount of water being pumped, or

flowing within a ditch or coming out of a pipeline in one

minute.

Miner’s inches: Miner ’s-inches was a term founded in the

old mining days. It is just another way of expressing flow.

Some areas in the West still use this measurement unit.

Caution needs to be taken because there are Arizona miner ’s-

inches, California miner ’s-inches and probably some that

are locally used. Approximately 40 Arizona miner ’s- inches

equals 1 cfs or 450 gpm.

Revised 12/11

College of Agriculture and Life Sciences

AZ1329

Edward C. Martin

h s

b (a)

h

s

(b)

Figure 1. Cross-sectional dimensions for trapezoidal (a) and elliptica\

l

(b) ditches. (Diagram by J.S. Jones, 2003)

Arizona Water

Series No.312

Measuring Water Flow in Ditches

The Float Method: This method is useful to get a rough

estimate of flow. First, choose a 100-foot section of ditch that

is fairly uniform in depth and width. Mark the zero point and

the 100 ft point with a flag or stick. The 100 ft mark should

be downstream from the zero point. For most people, one

good, long stride equals three feet. If there is no tape measure

available, step off about 33 paces. Next, calculate the ditch

cross sectional area (see “ Area ” above for details). Use an

average of several measurements along the ditch.

Now, take a float (tennis balls, apples, oranges, etc.) and

place it a few feet up stream from the zero point, in the center

of ditch. Once the float hits the zero point, mark the time

(probably to the nearest second). Then, mark the time the

float passes the 100 ft mark. Record the time. Do this several

times. Try to place the float in the center of the ditch flow

so that it won’t bounce off the sides or get caught up in any

weeds. After 5-10 tries, average the recorded times.

The flow rate is determined by calculating the velocity of

the water and multiplying it by the cross sectional area of the

ditch. First, take the length of the ditch (100 ft) and divide it by

the time (in seconds). This will give the surface velocity (speed)

in feet per second. However, water at the surface flows faster

than water in the center of the flow and it is the average flow

or center flow that is needed. Therefore, a conversion factor

must be used to determine the mean channel velocity. The

factor by which the surface velocity should be multiplied by

is a function of the depth of the water in the ditch. Table 1

gives the coefficients to be used. Find the depth measured on

the left and the corresponding coefficient on the right. Then

multiply the surface float velocity by the coefficient to obtain

the mean channel velocity.

Finally, take the cross sectional area of the ditch (ft 2) and

multiply it by the corrected velocity (ft/sec) and this will

compute the flow rate in cubic feet per second (cfs). To

convert to gallons per minute, multiply the cfs by 450.

Tracer Method: This method is very similar to the float

method but with one exception, a colored dye or salt is used

instead of a float. Estimates of the ditch area are still required.

Pour the dye upstream of the zero point, and record how

long it takes the dye to travel from the zero point to the 100

ft mark. Then the calculations are exactly the same as the

float method. This method often works well if the float keeps

getting caught on the sides of the ditch. However, in many

cases the dye is difficult to see because of the color of the

water itself. Test the dye first to make sure it can be seen. The

correction factors used with the float method (Table 1) are not

required for the tracer method.

Velocity Head Rod: The velocity head rod is used to

measure the velocity of water in a ditch and is relatively

inexpensive and fairly accurate. The rod is in actuality a ruler

used to measure the depth of the water. The water height is

first measured with the sharp edge of the ruler parallel with

the flow and the again with the ruler turned 90 degrees (Fig.

2). The difference in the height of water is the head differential

and using Table 2, an estimate of the velocity (feet per second)

can be made. From there, follow the same formula as with the

float or tracer method, i.e., multiply the velocity by the cross

sectional area of the ditch to get cubic feet per second. The

velocity head rod method works only for velocities greater

than 1.5 ft/sec and less than about 10 ft/sec.

The procedure is:

▪ Place the rod with the sharp edge upstream. Record the

depth of the water (normal depth).

▪ Place the rod sideways. This will cause some turbulence

and the water level will “jump” causing the water level to

rise. Record the level again (turbulent depth).

▪ Subtract the normal depth from the turbulent depth and

this will be the jump height.

▪ Find the corresponding velocity from Table 2.

▪ Multiply the velocity by the cross sectional area of the ditch

to get the flow rate (cfs).

Weirs: There are several different types of weirs that can be

constructed and used to determine the flow rate in a ditch

or stream. The three most common weirs are: (1) V-Notch or

Triangular (2) Rectangular and (3) Cipolletti.

The simplest design is to make the weir out of a sheet of

plywood or sheet metal. Cut the wood or metal to fit ditch

with the particular shape notch cut out of the top. Make sure

Average Depth (ft) Coefficient

1 0.66

2 0.68

3 0.70

4 0.72

5 0.74

6 0.76

9 0.77

12 0.78

15 0.79

20 0.80

Table 1. Coefficients to correct surface float velocities to mean channel

velocities. (from “Water Management Manual, USDI/BOR, 1997).

Jump (inches) ½ 1 2 3 4 5 6 7 8 9 10 11 12 15 18

Velocity (ft/sec) 1.6 2.3 3.3 4.0 4.6 5.2 5.7 6.1 6.5 6.9 7.3 7.7 8.0 9.0 9.8

Table 2. Conversion chart for velocity head rod measurements from inches to ft/sec.

The University of Arizona Cooperative Extension3

the weir is sturdy enough to hold up against the flow of the

water. Figure 3 shows an example of the three different types.

The top two are rectangular weirs. The first is a rectangular

contracted weir and is one of the most commonly used. The

second is another rectangular weir but since the sides of the

weir are actually the sides of the ditch, it is called a suppressed

rectangular weir. The third type shown in Figure 3 is the

Cipolletti weir. This type of weir has a trapezoidal shaped

notch. The last type shown is a triangular or V-notched type.

With proper installation, all of these weirs can be accurate.

Figure 2. Using a velocity Rod. (Waterwatch, 2002).

1

0 2

34 5

67 8

910

1

0 2

3 4 5

6

78

91

0

11

Depth

D1 Depth

D2H = Head

H= D2 - D1

The dimensions for a contracted rectangular weir are given

in Figure 4. An estimate of the actual flow rate must be made

before construction of the weir in order to make sure the notch

size is correct. For the V-notch, the dimension requirements

are the same and for the Cipolletti, the requirements are also

the same but with a 25% slope rising outward at the sides

of the notch. To measure the head or height of the water for

these weirs, pound in a stake about 6 feet upstream so that

the top of the stake is even with the bottom of the notch in

the weir. Once in place, the water will rise behind the weir.

Measure the depth of water above the stake. Then, use charts

like the ones in Tables 3-5 to estimate the flow rate. The length

(L) refers to the width of the opening at the base of the weir

notch.

CAUTION: Installing a weir in a ditch will cause the water

behind the weir to rise. Make sure there is enough freeboard

or the water in the ditch will overflow.

Other Methods: There are several other methods available

and many devices that can be purchased “off the shelf.” One is

a current meter, which is a propeller meter that is lowered into

the stream of water and records velocity. The flow rate (cfs)

is calculated by multiplying the velocity (ft/sec) by the area

(ft2). There are flumes, submerged orifices and even acoustic

ultrasonic meters that use ultrasonic pulses to measure the

velocity of the flow stream. All of these methods have limits

to their use. For more information, refer to the Arizona

Cooperative Extension publication “Measuring Water Flow

and Rate on the Farm”, publication AZ1130, Arizona Water

Series No. 24 (Martin, 2009).

Counting Tubes: If siphon tubes are used to irrigate out

of an open ditch, an estimate of the flow rate can be obtained

by counting the number of tubes. The size of the siphon tube

and the distance from the water level in the ditch to the water

level in the field (the drop) is needed to estimate the flow rate.

Figure 5 shows two possible conditions. In Condition I (free

flowing) the drop is the distance from the water level in the

ditch to the end of the tube on the field side (usually level

with the field). In Condition II (submerged), the drop is the

distance from the water level in the ditch to the water level

in the field. The larger the tube size or the greater the drop,

the higher the flow rate. Table 6 shows some typical sizes and

drops used for irrigation.

Contracted Rectangular

Contracted Triangular or V-NotchSuppressed Rectangular

Cipolletti Contracted

Figure 3. Diagrams of various types of weirs used to measure flow rate

in an open ditch. (USDI-BOR, 1997).

H

L a

Figure 4. Diagram of a rectangular weir where L = width of weir opening

(4 to 8 times H), H = head of weir (measured 6 ft upstream of weir) and

a = at least 3*H.

The University of Arizona Cooperative Extension4

Head

(inches)

(H)

Crest length (L)

(L): 1 foot (L): 2 feet (L): 3 feet (L): 2 feet

gpm ac-in/hr gpm ac-in/hr gpm ac-in/hr gpm ac-in/hr

2 98 0.22 198 0.44 298 0.66 398 0.88

3 181 0.40 366 0.81 552 1.22 738 1.63

4 278 0.62 560 1.24 852 1.88 1140 2.52

5 772 1.70 1164 2.58 1560 3.54

6 1010 2.22 1535 3.40 2055 4.54

7 1270 2.80 1980 4.27 2590 5.75

8 1540 3.40 2330 5.18 3120 6.90

Table 3. Approximate flow over rectangular weirs. (Peterson and Cromwell, 1993).

Head

in inches

(H)

Gallons

per minute

(gpm)

Acre-inches

per hour

(ac-in/hr)

3 36 0.08

4 74 0.16

5 126 0.28

6 200 0.44

7 294 0.65

8 405 0.89

9 548 1.21

10 714 1.58

11 895 1.98

12 111 8 2.48

13 1365 3.05

13.5 1495 3.34

14 1630 3.63

Table 4. Approximate flow over 90-degree triangular weirs. (Peterson and Cromwell, 1993).

Head

(inches)

(H)

Crest length (L)

(L): 1 foot (L): 2 feet (L): 3 feet (L): 2 feet

gpm ac-in/hr gpm ac-in/hr gpm ac-in/hr gpm ac-in/hr

2 101 0.22 202 0.45 302 0.67 404 0.89

3 190 0.42 376 0.83 560 1.24 750 1.66

4 296 0.65 580 1.28 864 1.91 1160 2.56

5 802 1.77 1196 2.66 1500 3.52

6 1062 2.34 1530 3.50 2100 4.64

7 1350 2.98 2000 4.42 2660 5.88

8 1638 3.62 2430 5.38 3220 7.14

Table 5. Approximate flow over trapezoidal weirs. The length “L” refers to the length of the bottom of the trapezoid. (Peterson and Cromwell, 1993).

The University of Arizona Cooperative Extension5

Drop

Condition I

Drop

Condition II

Figure 5. Diagrams where to measure the drop distance for siphon tubes. (Diagram by J.S. Jones, 2003).

Pipe Size (in.) Flow Rate (gallons per minute)

Drop (in.) 4" 6" 8" 10"

¾" 3.6 4.4 5.0 5.6

1" 6.4 7.9 9.0 10.0

1 ¼" 10.4 12.7 14.6 16.2

1 ½" 14.3 17.5 20.2 22.5

2" 25.6 31.8 35.9 40.0

3" 57.2 70.0 80.8 90.0

Table 6. Approximate flow rate in gallons per minutes for siphon tubes.

Figure 6. Three photos demonstrating how to measure the “drop” in a surface system. The drop is the distance from the level of the water in the ditch

to the water level in the field. (a) Use the hose to siphon water out of the ditch; (b) Raise the hose up until water stops flowing out of the hose end; (c)

Measure the distance between the end of the hose and the water level in the field.

It is often difficult to measure the difference in water levels

between the ditch and the field. One easy way is to do this is

to get a piece of hose and a tape measure. Put the hose in the

ditch and use it to siphon water into the field (Fig. 6a). Next,

slowly raise the hose in the field until the water stops coming

out (Fig. 6b). Now, use your measuring tape to measure the

distance between the end of the hose and the water level in

the field or the outlet of an irrigation siphon tube (Fig 6c).

Make sure to keep the end up just at the level where the water

stops coming out. This distance is your drop!

Measuring Flow in Gated Pipe

Measuring water flow in gated pipe can be accomplished

many different ways. Probably the most commonly used

method is the propeller meter. These meters are normally

installed inside a section of pipe at the distributor ’s shop.

The buyer then simply buys a meter section for whatever

diameter pipe used. There are some other methods that

can be used but for convenience and ease of measurement,

the propeller is a simple and accurate method.

The University of Arizona Cooperative Extension6

Meter size

(inches)

Minimum flow

(gpm)

Maximun flow

(gpm)

4 50 400

6 90 900

8 100 1200

10 125 1500

12 150 2000

Figure 7. A Mc® Propeller from McCrometer, Inc. This propeller meter is

installed inside a pipe section.

Figure 8. Two photos showing how to measure the head (ft) in a gated pipe system. The head is the distance between the water level in the tube and

the center of the pipe. These are Rite-Flow™ gates and there is about 3 ft of head. According to Table 8, the flow is approximately 39 gpm per gate.

Table 7. Typical range of flows for different size propeller meters.

Head (ft)

Flow Capacities (gpm)

Rite-Flow™ Epp™ Snap-Top

Boot Gate

Epp™

Fly Gate

Tex-Flow™

Yellow Top

0.25 (4") 11 12 15 22

0.50 (6") 16 17 21 32

1.00 22 24 30 46

2.00 32 35 42 67

3.00 39 42 52 82

Table 8. Approximate flow capacities in gallons per minute (gpm) for some commercially available gates. Gates are wide open. (Burt, 1995).

The University of Arizona Cooperative Extension7

The U niversiTy of A rizonA

College of A griCUlTUre And life s CienCes

TUCson , A rizonA 85721

dr. e dw Ard C. M ArTin Professor and Irrigation Specialist, Ag & Biosystems Engineering

ConTACT : edwArd C.M ArTin edmartin@cals.arizona.edu

This information has been reviewed by university faculty. cals.arizona.edu/pubs/water/az1329.pdf

Originally published: 2006

Other titles from Arizona Cooperative Exten sion can be found at:

cals.arizona.edu/pubs

ARIZONA COO PERATIVE

E TE NSION THE UNIVERSITY OF ARIZONA COLLEGE OF AGRICULTURE AND LIFE SCIENCES

Any products, services or organizations that are mentioned, shown or indirectly implied in this publication

do not imply endorsement by The University of Arizona.

Issued in furtherance of Cooperative Extension work, acts of May 8 and J\

une 30, 1914, in cooperation with the U.S. Department of Agriculture, Kirk A. Astroth, Interim Director, Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.

Propeller meters are permanent pipeline devices that

measure and record the volume and flow of water moving

through a pipe. The pipe must be running at full flow

for the meters to operate properly. Also, there must be a

straight length of pipe upstream from the meter at least

10 times the diameter of the pipe. This is to reduce the

turbulence in the water as it enters the meter section. Thus,

a 6-inch pipe would require 60 inches of straight pipe

upstream from the meter. Table 7 gives the range of flows

for various size meters and Fig. 7 shows a cross-sectional

view of a typical meter.

The meters are usually placed inside a length of

aluminum pipe that is inserted into the gated pipe system.

If poly-type plastic pipe is being used, there are connectors

that will allow a meter section to be put in place. If you

don’t want to pay the expense for the meter, you can use

a piece of tubing, similar to the tube method for ditches.

Find a piece of tubing (preferably clear) that either fits

tight inside a gate or even better, can be attached tightly to

the outside of the gate. Raise the tubing into the air until

the water stops flowing out. Measure the distance from the

water level in the tubing to the center of the gated pipe.

If clear tubing is used, then you can raise the tube well

above the point when the water stops coming out and it

makes for an easier measurement (Fig. 8). Table 8 gives

some estimate of flow rates for various manufacturers

gates. Most manufacturers should be able to supply this

information.

Summary

There are many methods that can be used to measure

flow rate and only the most common have been covered in

this paper. In addition, there are meters that use ultrasound

waves to measure flow in pipes, flumes, gates and even a

Doppler-type acoustic meter. Although these are relatively

expensive, the price has come down in recent years and the

technology is being applied throughout the agricultural

sector. Measuring flow is the first step in determining

how much water is being applied to a field. With the flow

rate, the area irrigated and the time of irrigation, you can

calculate the amount of water applied. For information

on calculating how much water was applied, read the

University Arizona Cooperative Extension publication

Determining the Amount of Water Applied to a Field, Pub.

No. AZ1157, Arizona Water Series No. 29 (Martin, 2011).

References

Burt, Charles M. 1995. The Surface Irrigation Manual.

Waterman Industries, Inc. Exeter, CA. First edition.

Martin, E.C. 2011. Determining the Amount of Water

Applied to a Field. Cooperative Extension Pub. No.

AZ1157, Arizona Water Series No. 29. University of

Arizona, Tucson, AZ.

Martin, E.C. 2009. Measuring Water Flow and Rate on the

Farm. Cooperative Extension Pub. No. AZ1130, Arizona

Water Series No. 24. University of Arizona, Tucson, AZ.

Peterson, M and C.F. Cromwell, Jr. 1993. Measuring

Irrigation Water in a Ditch, Stream or Reservoir. Ag.

Pub. G01681. University of Missouri, Columbia.

Water Measurement Manual. 1997. A Water Resources

Technical Publication. U.S. Dep. of the Interior, Bureau

of Reclamation. Third edition.

Waterwatch Australia National Technical Manual,

2002. Module 4 - Physical and Chemical Parameters,

Waterwatch Australia Steering Committee Environment

Australia.

The University of Arizona Cooperative Extension

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