Chapter 11: Sprinkle Irrigation - NRCS Irrigation ToolBox Home Page

11-45
and to maintain a nearly constant load on the
pumping plant. Because no variation will be needed
in a rectangular area, farmers should be encouraged
to relocate fences, drainage ditches, roads, and
other field boundaries, where practicable, to obtain
a rectangular area.
Pipe lengths are generally standardized, and
sprinklers on portable systems are normally spaced
at 30-, 40-, and 60-ft intervals on the laterals. Furthermore,
the spacing between laterals is usually at
40-, 50-, 60-, and 80-ft intervals along the mainline.
Since whole laterals must be operated simultaneously,
the preliminary system capacity determined by
equation 1 may be lower than the required capacity
even on rectangular fields. However, the depth per
irrigation (d) or the length of actual operating time
per irrigation (f X T) can usually be adjusted to
optimize the fit.
On odd-shaped fields where it is sometimes necessary
to operate less than the average required number
of sprinklers for one or more lateral settings,
the engine is throttled down to reduce the discharge.
Where two or more laterals (each containing
different numbers of sprinklers) are operating simultaneously,
valves in the lateral lines must be used
to control the pressure at the sprinklers. For most
odd-shaped tracts, the number of sprinklers needed
will exceed the theoretical minimum number computed,
and extra equipment will be necessary to
serve parts of the tract most distant from its tenter.
If the design area is subdivided, the number of
sprinklers required for each subdivision must be
computed separately.
Number of Lateral Settings.-The number of settings
required for each lateral depends on the number
of allowable sets per day and on the maximum
number of days allowed for completing one irrigation
during the peak-use period (f). The requited
number of settings per lateral must not exceed the
product of these two factors.
If the system layout provides for at least the
theoretical minimum number of sprinklers required,
then the number of settings required per lateral will
not exceed this allowable limit. Long, narrow, or irregularly
shaped parts of a tract, however, may require
additional lateral settings. Thus, more equip*
ment is necessary if such areas are to be served
within the allowable time period.
Lateral Layout.-Figure 11-30 shows the effects
of topography on lateral layout. To obtain near-uni-
form application of water throughout the length of
a lateral, the line must be of a pipe diameter and
length and follow an alignment that will result in a
minimum variation in the discharge of individual
sprinklers along the line. Normally this variation in
discharge should not exceed 10 percent unless long
term economic justification exists. Therefore, either
pressure (or flow) regulation must be provided for
each sprinkler or laterals must be located and pipe
sizes selected so that the total losses in the line,
due to both friction head and static head, will not
exceed 20 percent of the average design operating
pressure for the sprinklers (P,).
To meet this pressure-variation criteria, it is
usually necessary to lay laterals across prominsnt
land slopes (fig. 11-30A and B). Laid on level land
or on the contour, a lateral of a given pipe size with
a fixed average sprinkler-discharge rate (q,) will
thus be limited only to that length in which 20 percent
of P, is lost as a result of friction.
Running laterals uphill should be avoided wherever
possible. Where they must be used, they need
to be materially shortened unless pressure or flow
regulators are used. Such a lateral of a given pipe
size and fixed q, is limited to that length in which
the loss due to friction is equal to the difference between
20 percent of Pa and the loss due to static
head. For example, if the static head caused by the
difference in elevation between ends of the lateral
amounts to 12 percent of Pa, then the line is limited
to the length in which only 8 percent of Pa is lost
because of friction.
Running laterals downslope is often a distinct advantage,
provided the slope is fairly constant and
not too steep (see fig. 11-30C, D, E, and F). Be- ,
cause the difference in elevation between the two
ends of the line causes a gain in head, laterals running
downslope may be longer than lines laid on
level ground.
While downslopes may permit longer laterals for
a given pipe size or smaller pipe for a given length
of lateral, such a layout does not usually permit
split-line layout and lateral rotation about the mainline
or submain. Thus labor costs may be higher.
When the slope of the ground along the lateral is
about equal to the slope of the friction loss, the
pressure along the lateral is nearly constant. When
the slope along the lateral increases for successive
settings, intermediate control valves may be required
to avoid building up excessive pressures and
exceeding the variation limit,