Chapter 11: Sprinkle Irrigation - NRCS Irrigation ToolBox Home Page

sign computations should be based on field or test
facility data. Field evaluation techniques for estimating
the uniformity of periodic-move, traveling,
and center-pivot sprinklers are presented in the following
sections. However, when test data are not
available in general planning for the most common
periodic-move sprinkler spacings, tables 11-5
through 11-8 can be used to obtain estimated
values of CU for various wind conditions and application
rates.
The average uniformity of the catch data of two
irrigations is always higher than the average uniformities
of the two irrigations measured individually,
because of changes in wind and water jets.
Uniformity can be further improved by positioning
the laterals midway between the previous settings
for alternate irrigations. This practice is called alternate
sets, and it halves the lateral spacing for
the pair of irrigations. The uniformity of a pair of
irrigations using alternate sets can be approximated
by:
cu, = lo m
or
For gun or boom sprinklers CU values of 60 to 75
percent are typical for low and moderate wind conditions.
These sprinklers are not recommended for
use in high winds. By using alternate sets along the
lateral or between laterals when practical, CU,
values of about 80 percent can be obtained in the
central portion of a field.
For traveling sprinklers the effective spacing
along the tow path that corresponds to the lateral
is zero. Thus, the expected CU in the central portion
of the field and in low to moderate winds
should be similar or slightly better than the CU, of
80 percent for periodic-move gun or boom sprinklers.
Center-pivot and linear-move systems produce
high uniformities because the sprinklers are usually
relatively close together on the moving laterals.
With proper nozzling CU $ 94%, DU > 90% can be
expected in the area under the system hardware in
relatively level fields. The same high uniformities
can be maintained even on steep, undulating fields
if flow control nozzle sprinklers or other means af
countering elevation effects by regulating pressure,
flow, or system speed are used. When large end gun
sprinklers are used on center-pivots, the average
CU of the whole irrigated area drops about 1 percent
for each 1 percent of area covered past the end
of the hardware.
Water Loss
Although efforts are often concentrated on evaluating
systems by dealing with uniformity problems,
loss of water also reduces system efficiency. Frequently,
designers assume that systems will be perfectly
managed and losses will almost be elirninated,
but this is seldom the case. Overwatering is
perhaps the greatest cause of loss in any irrigation
system. Other major causes of losses associated
with sprinkle irrigation are:
1. Direct qvaporation from droplets and from wet
soil surfaces and transpiration from unwanted vegetation.
2. Wind drift,
3. Leaks and system drainage.
Wind drift and evaporation losses may be less
than 5 percent when irrigating a crop with a full
vegetative canopy in low winds. More commonly,
wind drift and evaporation losses range between 5
and 10 percent. However, under very severe conditions
they can be considerably greater. Figure
11-17 has been developed as a guide for estimating
the effective portion of the water applied that
reaches the soil-plant surface (R,). The values given
for effectiveness for different potential evapotranspiration
rates are based on an assumed full plant
canopy and 24-hr applications. The fine-spray
curves are based on 3116-in nozzles operating at 60
psi in a 40- x 60-ft spacing. The coarse spray is for
3116-in nozzles operating at 30 psi in a 30- x 60-ft
spacing.
TO use fimre 11-17, it; is necessary to know
whether the spray from a sprinkler is coarse, fine,
or somewhere in between. To make this determination
a coarseness index (CI) is used. This index can
be calculated by the following method:
where
P = Nozzle operating pressure (psi)
B = Nozzle size (64th~ of an inch)