Water Resource Evaluation

Surface water evaluation

Groundwater evaluation

Soil Water evaluation

Precipitation evaluation
Measurement for evaluation
Why is water measurement important to IWM?

Explain some of the mathematics of water

measurement

Discuss some of the common measuring devices

Discuss other opportunities for measurement

Work some example problems

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
Definitions
Volume: length3

Flow Rate (Q): volume/time

Velocity: length/time

Area: length2
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
Water Measurement Mathematics

Continuity Equation

Q=vA
Irrigator’s Equation

Qt=Ad
Irrigator’s Equation

Qt = Ad

Q = flow rate
t = time
A = area
D = depth
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 Devices
Devices that sample or sense velocity

 Current meters
 Propeller meters
 Vane deflection meters
 Float and stopwatch
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

Weirs

Flumes

Submerged Orifices

CTD, Other devices

 Weirs
• 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

• A weir is an overflow structure installed perpendicular to open

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

Weirs
Weirs

Cipolletti Weir
Weirs

Weir Box Turnout with Cipolletti Weir

Weirs

Compound Weir
90 degree triangular and suppressed rectangular
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
 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
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
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
Weirs

Poor approach condition

Weirs

Sediment in approach pool

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
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
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
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.
Parshall Flumes
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 Flumes
Flow occurs under two conditions
 Free flow
 Downstream water surface does not reduce discharge
 Requires only 1 head reading (Ha)
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
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
Parshall Flumes
Disadvantages
Complicated geometry for construction
Tight construction tolerances
Aren’t amenable to fluid flow analysis
BoR does not recommend for new
construction
Broad-crested Weirs
Long throated flume where only the bottom is
raised. No side contractions
• Also called ramp flumes, Replogle flumes
Broad-crested Weirs
Broad-crested Weirs

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

Broad-crested Weirs

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

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
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
Other Flumes
Several other types of flumes are used
H-flumes
Cutthroat flumes
Palmer-Bowles
Other Flumes
Flumes
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
• 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 Orifices
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
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 Meters
Types of current meters
Anemometer
Propeller
Electromagnetic
Doppler
Optical strobe

Anemometer and propeller are most common

for irrigation work
Current Meters

Anemometer type current meter

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
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
Float Method
 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
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
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
Venturi 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
Nozzle 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
Orifice Meter
Elbow Meters
Measure pressure difference between inside and
outside of an elbow
 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 Meters
Propeller Meters

Saddle type propeller meter

Propeller Meters
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 Meters
Advantages
Commercially available
Totalizing meter
Can achieve good accuracy
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
 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
Pitot Tube Velocity
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
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
 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
Trajectory Method
Trajectory Method
 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 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

Siphon Tubes
Siphon Tubes
Summary
Open channel devices
Flumes
Weirs
Submerged orifices
Pressurized conduit devices
Propeller meters
Differential head meters
Summary
Installation requirements
Examine existing structures
Other opportunities for measurement
Canal gates
Float method
Power consumption coefficient
Pipe trajectory
Siphon tubes