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Water Measurement. Brady S. McElroy, P.E. USDA-NRCS Lamar, Colorado. Objectives. Why is water measurement important to IWM? Explain some of the mathematics of water measurement Discuss some of the common measuring devices encountered in NRCS work Discuss other opportunities for measurement

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water measurement

Water Measurement

Brady S. McElroy, P.E.


Lamar, Colorado

  • Why is water measurement important to IWM?
  • Explain some of the mathematics of water measurement
  • Discuss some of the common measuring devices encountered in NRCS work
  • Discuss other opportunities for measurement
  • Work some example problems
why is water measurement important
Why is water measurement important?
  • Difficult to effectively manage irrigation without measurement
  • Positive aspects
    • Maximize use of available water supply
    • Reduced cost due to leached nutrients
    • Reduced environmental impact from over-irrigation
why is water measurement important4
Why is water measurement important?
  • Some measurement may have a negative connotation
    • Regulatory (mandated by state, etc.)
    • Billing
why is water measurement important6
Why is water measurement important?
  • Water is one of the most precious resources in the West
    • Increased competition among water users


  • States that NRCS’ reference shall be the Bureau of Reclamation’s Water Measurement Manual, 3rd edition, published in 1997
  • Available online at http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/

Primary reference for NRCS is Chapter 9 of Part 623 (Irrigation) of the National Engineering Handbook


Other useful references

  • Other NRCS documents
  • Irrigator’s Guides
  • Extension publications
  • Hydraulic texts
    • King’s Handbook of Hydraulics

Volume: length3

Flow Rate (Q): volume/time

Velocity: length/time

Area: length2


Head- measurement of the energy in a fluid. Units are typically length.

  • Total head at a given point is the sum of three components
    • Elevation head, which is equal to the elevation of the point above a datum
    • Pressure head, which is the height of a column of static water that can be supported by the static pressure at the point
    • Velocity head, which is the height to which the kinetic energy of the liquid is capable of lifting the liquid

Pressure- measurement of the force acting on a surface. Units are force/length2

Often convenient to express in terms of feet of fluid (pressure head)


(multiply psi x 2.31 for feet of H20)

  • Typically in U.S. Customary units for irrigation work.
  • Units vary depending on type of measurement
    • Q vs. volume
    • Open channel vs. pipe flow

Flow rate units expressed in volume/time

  • Open channel flow
    • Cubic feet per second (cfs)
      • second-feet
  • Pipe flow
    • Gallons per minute (gpm)

Handy Conversion Factor

1 cfs = 448.8 gpm


1 cfs ≈ 450 gpm


May also vary regionally

  • Shares
  • Some canals refer to a head of water as a delivery unit
    • Not the same as energy measurement
  • Miner’s inches
    • 38.4 miner’s inches = 1 cfs (Colorado)
    • 40 miner's inches = 1 cfs (California, et al.)
    • 50 miner’s inches = 1 cfs (New Mexico, et al.)

A share is not a share is not a share

CanalAllocation/share (cfs)

Bessemer 0.0150

Colorado 0.0125

Rocky Ford Highline 0.180

Oxford 0.0960

Otero 0.050

Holbrook 0.0250

Catlin 0.0180

Rocky Ford 0.140

Fort Lyon 0.0150

Amity 5 cfs at 0.6 hr/share

Lamar 0.0100


Volume units are often expressed in units of area x depth or depth

Acre-foot = volume of water that would cover 1 acre to a depth of 1 foot

  • 12 acre-inches
  • 43,560 cubic feet
  • 325,851 gallons

Handy Conversion Factor

1 cfs for 24 hours ≈

2 acre-feet


1 cfs ≈ 1 ac-in/hr

water measurement mathematics22
Water Measurement Mathematics

Continuity Equation


Irrigator’s Equation


continuity equation







Continuity Equation


Q = flow rate

v = velocity

A = area

continuity equation24
Continuity Equation




continuity equation25

12 in.


v=2.5 ft/s

Continuity Equation

Given: d=12 inches

v=2.5 ft/s

Find: Q in cfs

continuity equation26

12 in.


v=2.5 ft/s

Continuity Equation

Solution: Q = vA

A = 0.785 ft2

Q = 2.5 ft/s x 0.785 ft2 = 1.96 ft3/s

irrigator s equation
Irrigator’s Equation

Qt = Ad

Q = flow rate

t = time

A = area

D = depth

irrigator s equation28
Irrigator’s Equation

d = Qt/A

Q = Ad/t

t = Ad/Q

A = Qt/d

irrigator s equation29
Irrigator’s Equation

Given: d = 3 inches

A = 50 acres

Q = 2 cfs

Find: Time required to apply d

irrigator s equation30
Irrigator’s Equation

Solution: t = dA/Q

1 cfs ≈ 1 ac-in/hr

t = 75 hours

irrigator s equation31
Irrigator’s Equation

Given: t = 36 hours

A = 20 acres

Q = 2 cfs

Find: Depth of applied water, d

irrigator s equation32
Irrigator’s Equation

Solution: d = Qt/A

1 cfs ≈ 1 ac-in/hr

d = 3.6 inches

water measurement devices
Water Measurement Devices

Most water measurement devices either sense or measure velocity, or measure either pressure or head.

Tables, charts, or equations are then used to calculate the corresponding discharge

water measurement devices34
Water Measurement Devices

Devices that sample or sense velocity

  • Current meters
  • Propeller meters
  • Vane deflection meters
  • Float and stopwatch
water measurement devices35
Water Measurement Devices

Devices that measure head or pressure

    • Open channel devices commonly use h
    • Pipeline devices may use p
  • Flumes
  • Orifices
  • Venturi meters
  • Weirs
    • Velocity is computed from h, so weirs are classifed as head measuring devices
open channel devices
Open Channel Devices
  • Weirs
  • Flumes
  • Submerged Orifices
  • Other devices

A weir is an overflow structure installed perpendicular to open channel flow

  • Has a unique depth of water at an upstream measuring point for each discharge
  • If the water springs clear of downstream face, acts as sharp-crested weir
  • A long, raised channel control crest is a broad-crested weir
  • Usually named for the shape of the overflow opening
    • Rectangular
    • Triangular
    • Cipolletti
  • Lowest elevation on overflow is zero reference elevation for measuring h

Rectangular weirs can be either contracted or suppressed

  • Suppressed weirs use side of flow channel for weir ends
    • No side contraction occurs
    • Often used in divide boxes
  • Canal overshot gates can act as weirs

Cipolletti Weir


Weir Box Turnout with Cipolletti Weir


Compound Weir

90 degree triangular and suppressed rectangular



  • Simple to construct
  • Fairly good at passing trash
  • 1 head measurement


  • High head loss
  • Susceptible to sedimentation problems
  • Sensitive to approach and exit conditions

Conditions needed for sharp-crested weirs

  • Upstream face should be plumb, smooth, normal to axis of channel
  • Entire crest should be level for rectangular and Cipolletti. Bisector of V-notch angles should be plumb for triangular.
  • Plate should be thin enough to act as a sharp-crested weir
    • Chamfer downstream edge if necessary
    • Upstream edge must be straight and sharp
    • Thickness should be uniform for entire length
  • Maximum downstream elevation should be at least 0.2 ft below crest
  • Head measurement should be greater than 0.2 ft for optimal elevation
  • Head is measured upstream 4 X maximum head on crest
  • Approach must be kept free of sediment deposits

Given: Standard Contracted Rectangular Weir

L = 2 feet

h = 0.40 feet

Find: Q, in cfs

Solution: Refer to Table A7-2 in BoR Water Measurement Manual, 3rd edition


Inspection of Existing Structures

  • Approach flow
  • Turbulence
  • Rough water surface at staff gage
  • Velocity head
  • Exit flow conditions
  • Worn equipment
  • Poor installation
    • Crest must be correctly installed

Poor approach condition


Sediment in approach pool


Flumes are shaped open channel flow sections.

  • Force flow to accelerate
    • Converging sidewalls
    • Raised bottom
    • Combination
  • Force flow to pass through critical depth
    • Unique relationship between water surface profile and discharge

Two basic classes of flumes

  • Long throated flumes
    • Parallel flow lines in control section
    • Accurately rate with fluid flow analysis
  • Short throated flumes
    • Curvilinear flow in control section
    • Calibrated with more precise measurement devices
short throated flumes
Short Throated Flumes

Parshall Flume is most well-known example

of short throated flumes

  • Developed by Ralph Parshall at Colorado Agricultural College (now Colorado State University)
  • ASAE Historic Landmark
parshall flumes
Parshall Flumes

Since the beginning of irrigated agriculture, it has been important to measure flows of irrigation water. Accuracy of early water measurement methods often suffered because of trash or sediment in the water, or unusual flow conditions. Ralph L. Parshall saw this problem when he began working for the USDA in 1915, as an irrigation research engineer. In 1922 he invented the flume now known by his name. When this flume is placed in a channel, flow is uniquely related to the water depth. By 1953 Parshall had developed the depth-flow relationships for flumes with throat widths from 3 inches to 50 feet. The Parshall flume has had a major influence on the equitable distribution and proper management of irrigation water. Thousands of flumes have been used to measure irrigation water, as well as industrial and municipal liquid flows throughout the world. This plaque marks the site of the original Colorado Agricultural College Hydraulics Laboratory, where Parshall carried out his historic experiments.


parshall flumes57
Parshall Flumes
  • Designated by throat width
    • Measure 0.01 cfs with 1 inch flume
    • Measure 3000 cfs with 50 foot flume
  • Dimensions are standardized for each flume
    • Not geometrically proportionate
      • A 12 ft flume is not simply 3x a 4 ft flume
  • Relate Ha (or Ha and Hb ) to discharge with rating equation, or consult appropriate chart
parshall flumes58
Parshall Flumes
  • Flow occurs under two conditions
    • Free flow
      • Downstream water surface does not reduce discharge
      • Requires only 1 head reading (Ha)
parshall flumes59
Parshall Flumes
  • Submerged flow
    • Downstream flow is high enough to reduce discharge
    • 2 head readings required
    • 50% submergence (Hb/Ha) on 1-3 inch flumes
    • 80% submergence (Hb/Ha) ≥8 feet flumes
    • After 90% submergence, flume is no longer effective



parshall flumes60
Parshall Flumes


  • Relatively low head loss (1/4 of sharp crested weir)
  • Handle some trash and sediment
  • Well accepted
    • May be mandated
  • Many sizes are commercially available
parshall flumes61
Parshall Flumes


  • Complicated geometry for construction
  • Tight construction tolerances
  • Aren’t amenable to fluid flow analysis
  • BoR does not recommend for new construction
parshall flumes63
Parshall Flumes

Given: 1 foot throat Parshall Flume

Free flow

Ha = 0.40 feet

Find: Q, in cfs

Solution: Refer to Table A8-12 in BoR Water Measurement Manual, 3rd edition

parshall flumes65
Parshall Flumes

Given: 1 foot Parshall Flume

Ha = 1 ft

Hb = 0.8 ft

Find: Q, in cfs

parshall flumes66
Parshall Flumes

Solution: Determine if submergence exceeds 70% (Hb/Ha)


Therefore, must correct for submergence

parshall flumes67
Parshall Flumes

Solution: From table A8-12, Q=3.95 cfs

Find correction factor

Use Figure 8-16

parshall flumes69
Parshall Flumes

Correction=0.35 ft3/s

Actual Q =(free flow Q) – (correction)

=3.95 ft3/s – 0.35 ft3/s

=3.6 ft3/s

broad crested weirs
Broad-crested Weirs

Long throated flume where only the bottom is raised. No side contractions

  • Also called ramp flumes, Replogle flumes
broad crested weirs72
Broad-crested Weirs

Long throated flume (broad-crested weir) under construction)

broad crested weirs73
Broad-crested Weirs

Long throated flume (broad-crested weir) Q = 1200 cfs

broad crested weirs74
Broad-crested Weirs


  • Easily constructed, especially in existing concrete lined channels
  • WinFlume software available to quickly design and rate structures
  • Less expensive construction
  • Low head loss
  • Handle trash and sediment well
broad crested weirs75
Broad-crested Weirs


  • Some state laws or compacts may preclude use
  • Not readily accepted by some water users
    • Not what they’re used to using
other flumes
Other Flumes

Several other types of flumes are used

  • H-flumes
  • Cutthroat flumes
  • Palmer-Bowles

Inspection of Existing Structures

  • Approach flow
    • Flumes are in-line structures
    • Should have smooth flow across width and depth of cross section
    • Length of straight approach varies depending on control width, channel width, and velocity
  • Turbulence
  • Level both along and perpendicular to flow
  • Excessive submergence
  • Exit flow conditions
submerged orifices
Submerged Orifices

A well defined sharp-edged opening in a wall or bulkhead through which flow occurs

  • When size and shape of the orifice and the heads acting on it are known, flow measurement is possible
  • Orifices are typically circular or rectangular in shape
  • Can be used to regulate and measure water in a turnout structure
  • Radial gates can act as submerged orifices
submerged orifices81
Submerged Orifices


  • Less head required than for weirs
  • Used where space limitations prevent weir or flume


  • Sediment and debris accumulation will prevent accurate measuring
  • Typically not used if conditions permit flumes which handle trash better
current meters
Current Meters

Velocity measuring devices

  • Sample velocity at one point
    • Point sample isn’t representative of average velocity in flow are
      • Develop relationship between observed and average velocity, or
      • Take multiple velocity readings
  • Use continuity equation (Q=vA) to compute discharge
current meters83
Current Meters

Types of current meters

  • Anemometer
  • Propeller
  • Electromagnetic
  • Doppler
  • Optical strobe

Anemometer and propeller are most common for irrigation work

current meters84
Current Meters

Anemometer type current meter

other open channel methods
Other Open Channel Methods

Slope-Area Method

  • Slope of water surface and average cross-sectional area used with Manning’s equation
  • Difficult to estimate “n”
  • Can only approximate Q
float method
Float Method

Similar in concept to current meters

  • Velocity is estimated by timing how long a floating object takes to travel a pre-determined distance
  • Observed velocity is adjusted by some factor to estimate average velocity
  • Determine cross-sectional flow area
  • Use continuity equation to estimate Q
  • Provides only a rough estimate
pressurized conduit devices
Pressurized Conduit Devices

Pipeline devices are usually classified by their basic operation

  • Calibrated velocity sensing meters
  • Differential head meters
  • Positive volume displacement summing meters (municipal water)
  • Measured proportional or calibrated bypass meters
  • Acoustic meters
differential head meters
Differential Head Meters

Include venturi, nozzle, and orifice meters

  • When properly installed, accuracy ±1%
    • Some irrigation operating conditions probably limit accuracy to ±3-5%
  • No moving parts
    • Uses principle of accelerating flow through a constriction
    • Resulting pressure difference is related to discharge using tables or curves, or a suitable coefficient and the proper equation
venturi meter
Venturi Meter

Common differential head meter

  • Minimal head loss
  • Full pipe flow required
  • Also used to inject chemicals into an irrigation system
    • Pressure reduction is used to pull chemicals into the system
  • Examples of venturi meters constructed of standard plastic pipe fittings
nozzle meter
Nozzle Meter

Simplified form of venturi meter

  • Gradual downstream expansion of venturi is eliminated
  • Higher head loss than venturi
  • Full pipe flow required
  • Not used extensively in irrigation
orifice meter
Orifice Meter

Another differential pressure meter

  • Often used for measuring well discharge
  • Also used to measure chemical injections
    • Typically small meters with details provided by manufacturer
  • Requires long straight pipe lengths
  • Full pipe flow required
  • Limited discharge ratio
elbow meters
Elbow Meters

Measure pressure difference between inside and outside of an elbow

propeller meters
Propeller Meters

Used at end of pipes and in conduits flowing full

  • Multiple blades that rotate on horizontal axle
  • Must have full pipe flow
  • Basically operate on Q=vA principle
  • Usually have totalizer plus instantaneous discharge display
  • Accuracy can be ±2-5% of actual flow
propeller meters99
Propeller Meters

Saddle type propeller meter

propeller meters101
Propeller Meters
  • Should be selected to operate near middle of design discharge range
    • If system has oversized pipes, some sections may need replaced with smaller pipes to provide correct velocity and approach
  • Must be installed to manufacturer’s specifications for accurate measurement
  • Must have full pipe flow
propeller meters102
Propeller Meters


  • Commercially available
  • Totalizing meter
  • Can achieve good accuracy
propeller meters103
Propeller Meters


  • Operating conditions different from manufacturer’s calibration conditions will affect accuracy
  • Only tolerate small amount of weeds and debris
  • Moving parts operating underwater
  • Can require a good deal of maintenance and inspection
other conduit devices
Other Conduit Devices

Pitot Tube Velocity Measurements

  • Piezometer
    • Straight tube attached flush to wall and perpendicular
    • Senses pressure head in pipe
  • Pitot Tube
    • Right angle bend inserted with horizontal leg pointed upstream and parallel to flow
    • Senses both velocity and pressure head
  • Velocity head, flow area, and coefficient can then be used to calculate flow rate
other conduit devices106
Other Conduit Devices
  • Magnetic Flowmeters
    • Use the principle that voltage is induced in an electrical conductor moving through a magnetic field. Conductor is flowing water
    • For a given field strength, the magnitude of the induced voltage is proportional to velocity
  • Deflection Meters
    • Vane or plate projecting into flow and a sensing element to measure deflection
    • Calibrated to indicate flow in desired units
  • Vortex Flowmeters
    • Obstructions in flow generate vortex shedding trails
      • Properly shaped obstructions create vortices that can be sensed and related to velocity
other conduit methods
Other Conduit Methods

Trajectory Method

  • Measure the horizontal and vertical coordinates of a point in the jet of water issuing from the end of a pipe
  • Accurate ±15%
  • Coordinates can be difficult to accurately measure
trajectory method
Trajectory Method
  • Vertical Pipe
      • Two kinds of flow occur, depending on how high water rises
        • <0.37d, circular weir
        • Transistional region between
        • >1.4d, jet flow
  • Horizontal Pipe
    • Pipe must be truly horizontal; slope will skew results
      • Vertical component can be difficult to measure
other conduit methods112
Other Conduit Methods

Power Consumption Coefficients

  • Volume discharged from wells can be estimated using power consumption records
    • Wells must be analyzed to determine the energy needed to pump a certain volume of water
    • Relationship can then be used to estimate discharge volume
    • Only certified well testers can perform the tests and develop the power consumption coefficient
    • Must recalibrate every 4 years, or more often depending on conditions
other conduit methods113
Other Conduit Methods

Siphon Tubes

  • Estimate discharge based on head, diameter, and length of siphon tubes
  • Accuracy ±10-15%
  • Provides an in-field method of estimating flow
  • Information also available in irrigator’s guides and NRCS Engineering Field Manual, Chapter 15
  • Water measurement is an important component of IWM
  • BoR Water Measurement Manual
  • Continuity equation
    • Q=vA
  • Irrigator’s equation
    • Qt=dA
  • 1 cfs≈450 gpm
  • 1 cfs≈1 ac-in/hr
  • Open channel devices
    • Flumes
    • Weirs
    • Submerged orifices
  • Pressurized conduit devices
    • Propeller meters
    • Differential head meters
  • Installation requirements
    • Examine existing structures
  • Other opportunities for measurement
    • Canal gates
    • Float method
    • Power consumption coefficient
    • Pipe trajectory
    • Siphon tubes