Field Measurement Techniques

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,NTRODUCTTON
Before proceeding to the procedures for evaluating the hydraulic performance of
surface-irrigated fields, it is helpful to review the various field measurement techniques that are commonly employed. The descriptions ofvarious field measurements presented in this chapter are not intended to be exhaustive. Supplementary

material can be found in a wide variety of technical publications, including textbooks, handbooks, and manuals.
Generally, measuring surface hydrologic parameters is much easier than measuring subsurface hydraulic and hydrologic parameters. The most accurate measurements when evaluatingsurface-irrigated fields are inflow discharge and tailwater
runoff because standardized flow-measuring devices are available that can easily
be installed. In contrast, the measurement of infiltration, while easily measured,
is complicated by its variability from one location to another in the same field and
its variability from one irrigation event to the next during the irrigation season.

FLOWMEASUREIYTENT
There are many standardized flow-measuring devices available for measuring surface water in irrigation systems. Weirs were the first devices, developed more
than a century ago. Various flumes have been developed during the twentieth
century and have become the most commonly used devices in irrigation systems.
Another very common device in irrigation systems is the free orsubmerged orifice
lhap. 3

47

generally associated with gates for controlling deliveries

to individual farms or

fields.

Free FIow and Submerged Flow
The two most significant flow regimes under which any open channel constriction
may operate are free flow and submerged flow. The distinguishing difference
between the two flow conditions is the occurrence of critical velocity in thevicinity
of the constriction (usually, a very short distance upstream from the narrowest
portion of the constriction). When this critical flow control occurs, the flow is
uniquely related to the flow depth or "head" upstream of the critical section. Thus
measurement of a flow depth at some specified location upstream, ft,, from the
point of the critical condition is all that is necessary to obtainthe free-flow discharge.
Thus

Q¡

:

(4.1)

f(h")

When the flow conditions are such that the downstream flow depth is raised
to the extent that the flow velocity at every point through the constricti,on becomes
less than the critical value, the constriction is operating under submerged-flow
conditions. With this flow regime an increase in tailwater flow depth, Aftr, will
increasethe head upstream of the constriction by A,h, (Aft" will be less than Lh).
A constriction operating under submerged-flow conditions requires that both upstream depth, hu, and downstream depth, ho, be measured. The definition given

$

for submergence, S, is

F

t:ro

(4.2)

and may be represented in percent. The submerged-flow discharge, Q,, is a function of h, and h¿, generallywritten as a relation between flow rate, head loss
(h, - h), and submergence:

Q,

:

f(h",

h) : f(h" -

hr,

(4.3)

S)

Often, constrictions designed initially to operate under free-flow conditions
become submerged in response to unusual operating conditions or the accumulation
of moss and vegetation in the open channel. Care should always be taken to note
the operating conditionof the constriction in order to determine which rating should
be used. The value of submergence marking the change from free flow to submerged flow, or vice versa, is referred to as the transition submergence, Sr. At
this condition, the discharge given by the free-flow equation is exactly the same
as that given by the submerged-flow equation. Hence, if discharge equations are
known for both...