1. Brady S. McElroy, P.E. USDA-NRCS Lamar, Colorado
Water Measurement
Brady S. McElroy, P.E.
USDA-NRCS
Lamar, Colorado

2. 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
Work some example problems

3. 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

4. Why is water measurement important?
Some measurement may have a negative connotation
Regulatory (mandated by state, etc.)
Billing

5. KANSAS

6. Why is water measurement important?
Water is one of the most precious resources in the West
Increased competition among water users

7. “Whiskey is for drinking. Water is for fighting over.”
Mark Twain

8. References
Primary reference for NRCS is Chapter 9 of Part 623 (Irrigation) of the National Engineering Handbook
States that NRCS’ reference shall be the Bureau of Reclamation’s Water Measurement Manual, 3rd edition, published in 1997
Available online at

9. References Other useful references Other NRCS documents
Irrigator’s Guides
Extension publications
Hydraulic texts
King’s Handbook of Hydraulics

10. Definitions
Volume: length3 Flow Rate (Q): volume/time Velocity: length/time Area: length2

11. Definitions
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

12. Definitions
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)
h=p/γ
(multiply psi x 2.31 for feet of H20)

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

14. Units Flow rate units expressed in volume/time Open channel flow
Cubic feet per second (cfs)
second-feet
Pipe flow
Gallons per minute (gpm)

15. Handy Conversion Factor
Units
Handy Conversion Factor
1 cfs = gpm or
1 cfs ≈ 450 gpm

16. Units 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.)

17. A share is not a share is not a share
Units
A share is not a share is not a share
Canal Allocation/share (cfs)
Bessemer
Colorado
Rocky Ford Highline
Oxford
Otero
Holbrook
Catlin
Rocky Ford
Fort Lyon
Amity cfs at 0.6 hr/share
Lamar

18. Units
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

19. Handy Conversion Factor
Units
Handy Conversion Factor
1 cfs for 24 hours ≈
2 acre-feet or
1 cfs ≈ 1 ac-in/hr

20. Water Measurement Mathematics

21. Water Measurement Mathematics

22. Water Measurement Mathematics
Continuity Equation
Q=vA
Irrigator’s Equation
Qt=Ad

23. Continuity Equation Q=vA Q = flow rate v = velocity A = area Qin Qout

24. Continuity Equation
Q=vA v=Q/A A=Q/v

25. Continuity Equation Given: d=12 inches v=2.5 ft/s Find: Q in cfs

26. Continuity Equation Solution: Q = vA A = 0.785 ft2
v=2.5 ft/s
12 in.
Solution: Q = vA
A = ft2
Q = 2.5 ft/s x ft2 = 1.96 ft3/s

27. Irrigator’s Equation
Qt = Ad
Q = flow rate
t = time
A = area
D = depth

28. d = Qt/A Q = Ad/t t = Ad/Q A = Qt/d
Irrigator’s Equation
d = Qt/A Q = Ad/t t = Ad/Q A = Qt/d

29. Irrigator’s Equation
Given: d = 3 inches A = 50 acres Q = 2 cfs Find: Time required to apply d

30. Irrigator’s Equation Solution: t = dA/Q 1 cfs ≈ 1 ac-in/hr
t = 75 hours

31. Irrigator’s Equation
Given: t = 36 hours A = 20 acres Q = 2 cfs Find: Depth of applied water, d

32. Irrigator’s Equation Solution: d = Qt/A 1 cfs ≈ 1 ac-in/hr
d = 3.6 inches

33. 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

34. Water Measurement Devices
Devices that sample or sense velocity
Current meters
Propeller meters
Vane deflection meters
Float and stopwatch

35. 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

36. Open Channel Devices
Weirs
Flumes
Submerged Orifices
Other devices

37. Weirs
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

38. Weirs Usually named for the shape of the overflow opening
Rectangular
Triangular
Cipolletti
Lowest elevation on overflow is zero reference elevation for measuring h

39. Weirs 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

40. Weirs

41. Weirs
Cipolletti Weir

42. Weir Box Turnout with Cipolletti Weir
Weirs
Weir Box Turnout with Cipolletti Weir

43. 90 degree triangular and suppressed rectangular
Weirs
Compound Weir
90 degree triangular and suppressed rectangular

44. Weirs Advantages Simple to construct Fairly good at passing trash
1 head measurement
Disadvantages
High head loss
Susceptible to sedimentation problems
Sensitive to approach and exit conditions

45. Weirs 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

46. Weirs
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

47. Weirs
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

48. Weirs

49. Weirs 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

50. Poor approach condition
Weirs
Poor approach condition

51. Sediment in approach pool
Weirs
Sediment in approach pool

52. Flumes 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

53. Flumes 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

54. 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

55. 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. DEDICATED BY THE AMERICAN SOCIETY OF AGRICULTURAL ENGINEERS 1985

56. Parshall Flumes

57. 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

58. Parshall Flumes Flow occurs under two conditions Free flow
Downstream water surface does not reduce discharge
Requires only 1 head reading (Ha)

59. 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 Ha Hb

60. Parshall Flumes Advantages
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

61. Parshall Flumes Disadvantages Complicated geometry for construction
Tight construction tolerances
Aren’t amenable to fluid flow analysis
BoR does not recommend for new construction

62. Parshall Flumes

63. 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

64. Parshall Flumes

65. Parshall Flumes
Given: 1 foot Parshall Flume Ha = 1 ft Hb = 0.8 ft Find: Q, in cfs

66. Parshall Flumes
Solution: Determine if submergence exceeds 70% (Hb/Ha) 0.8/1.0=0.8>0.7 Therefore, must correct for submergence

67. Parshall Flumes
Solution: From table A8-12, Q=3.95 cfs Find correction factor Use Figure 8-16

68. Parshall Flumes

69. 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

70. Broad-crested Weirs Also called ramp flumes, Replogle flumes
Long throated flume where only the bottom is raised. No side contractions
Also called ramp flumes, Replogle flumes

71. Broad-crested Weirs

72. Long throated flume (broad-crested weir) under construction)
Broad-crested Weirs
Long throated flume (broad-crested weir) under construction)

73. Long throated flume (broad-crested weir) Q = 1200 cfs
Broad-crested Weirs
Long throated flume (broad-crested weir) Q = 1200 cfs

74. Broad-crested Weirs Advantages
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

75. Broad-crested Weirs Disadvantages
Some state laws or compacts may preclude use
Not readily accepted by some water users
Not what they’re used to using

76. Other Flumes Several other types of flumes are used H-flumes
Cutthroat flumes
Palmer-Bowles

77. Other Flumes

78. Flumes Inspection of Existing Structures Approach flow Turbulence
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

79. 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

80. Submerged Orifices

81. Submerged Orifices Advantages Less head required than for weirs
Used where space limitations prevent weir or flume
Disadvantages
Sediment and debris accumulation will prevent accurate measuring
Typically not used if conditions permit flumes which handle trash better

82. 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

83. Current Meters Types of current meters Anemometer Propeller
Electromagnetic
Doppler
Optical strobe
Anemometer and propeller are most common for irrigation work

84. Anemometer type current meter
Current Meters
Anemometer type current meter

85. 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

86. 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

87. Float Method

88. 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

89. 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

90. 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

91. Venturi Meter

92. 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

93. Nozzle Meter

94. 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

95. Orifice Meter

96. Elbow Meters
Measure pressure difference between inside and outside of an elbow

97. 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

98. Propeller Meters

99. Saddle type propeller meter
Propeller Meters
Saddle type propeller meter

100. Propeller Meters

101. 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

102. Propeller Meters Advantages Commercially available Totalizing meter
Can achieve good accuracy

103. Propeller Meters Disadvantages
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

104. 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

105. Pitot Tube Velocity

106. Other Conduit Devices Magnetic Flowmeters Deflection Meters
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

107. 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

108. Trajectory Method Vertical Pipe Horizontal 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

109. Trajectory Method

110. Trajectory Method

111. Trajectory Methods

112. 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

113. 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

114. Siphon Tubes

115. Siphon Tubes

116. Summary 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

117. Summary Open channel devices Pressurized conduit devices Flumes Weirs
Submerged orifices
Pressurized conduit devices
Propeller meters
Differential head meters

118. Summary Installation requirements Other opportunities for measurement
Examine existing structures
Other opportunities for measurement
Canal gates
Float method
Power consumption coefficient
Pipe trajectory
Siphon tubes

119. Questions?