Measurement of Irrigation Water

Comprehensive vocabulary flashcards covering the metrics, methods, and devices used for measuring irrigation water in both open channels and closed pipes.

Cubic Foot (at rest)

The volume of water that would be held in a container one foot wide by one foot long by one foot deep.

Acre-Inch

The volume of water that would cover one acre ($43,560$ square feet) one inch deep.

Acre-Foot

The volume of water that would cover one acre one foot deep.

Cubic foot per second (cfs)

The quantity of water equivalent to a stream one foot wide by one foot deep flowing with a velocity of one foot per second.

Gallon per minute (gpm)

The quantity of water equivalent to a stream which will fill a gallon measure once each minute.

Volume Method

A measurement method suitable for small irrigation streams where water is collected in a container of known volume and the time to fill it is recorded.

Area-velocity Method

A method of determining flow rate in an open channel by multiplying the cross-sectional area by the average velocity of water.

Float Method

An inexpensive and simple method that measures surface velocity by timing how long an object takes to float a specified distance downstream.

Current Meter

A device consisting of a revolving wheel or vane turned by moving water, where the speed of the propeller is proportional to the average velocity of flow.

Weir

A barrier across the width of a river that alters flow characteristics and usually results in a change in the height of the river level.

Weir Crest

The edge over which the water flows in a weir structure.

Nappe

The sheet of water which overflows a weir.

Sharp Crested Weir

A weir with a thin-edged crest such that the overflowing sheet of water has minimum surface contact with the crest, such as a V-notch or Cipolletti weir.

Broad-crested weir

A weir having a horizontal or nearly horizontal crest sufficiently long in the direction of flow so that streamlines become parallel to the crest invert.

Suppressed Weir

A rectangular weir whose notch sides are coincident with the sides of the approach channel, meaning lateral flow contraction is eliminated.

Cipolletti Weir

A trapezoidal weir with side slopes of one horizontal to four vertical ($1:4$).

Flume

A human-made open declined gravity chute with walls raised above surrounding terrain used to measure water flow when head drop is very small.

Froude number (Fr)

A dimensionless value used to describe flow states; in flume converging sections, flow must enter in a sub-critical state typically with a value no more than $0.5$.

Parshall Flume

A specific type of short-throated flume widely used in irrigation, though prone to clogging in sizes below $1\,ft$.

Orifice

An opening of any size or shape in a pipe or container wall through which fluid is discharged.

Vena contracta

The point of maximum contraction of a fluid jet as it passes through an orifice where velocity and pressure change.

Pipe

A closed conduit used for carrying fluids under pressure where the fluid completely fills the cross-section and has no free surface.

Venturi meter

A device consisting of a converging part, a throat, and a diverging part used to measure the rate of flow through a pipe via pressure difference.

Pitot tube

A device consisting of a glass tube bent at right angles used to measure the fluid flow velocity at any specific point in a pipe or channel.

Water meter

A scientific instrument for the accurate measurement of the quantity of water distributed to consumers, requiring the pipe to flow full at all times.

Propeller meter

A mechanical flow measuring device used in full-pressure conduits that uses rotating blades geared to a revolution counter to record flow rate and cumulative total.