Chapter 11: Sprinkle Irrigation - NRCS Irrigation ToolBox Home Page

The leading manufacturers of sprinklers are mntinually
field testing their products, and data are
available on several sprinklers operating under various
field conditions. When planning sprinkle irrigation
systems, request such data from the distributors
or manufacturers. If available, the data should
be used as a basis for selecting the combination of
spacing, discharge, nozzle size, and operating pressure
that will result in the highest practical uniformity
coefficient for the existing operating conditions.
Spacing.--The basic criterion governing the selection
of spacing for any given sprinkler nozzlepressure
and wind combination is the uniformity of distribution
desired. Tn general, a CU of about 85 percent
is recommended for delicate and shallow-rooted
crops such as potatoes and most other vegetables.
A CU above 75 percent is generally adequate for
deep-rooted field crops such as alfalfa, corn. cotton,
and sugar beets. Tree and vine crops that have deep
spreading root systems can be adequately irrigated
if the CU is above 70 percent. When applying chemicals
through the system, however, a CU above 80
percent is recommended. When systems have low
CUs due to wind, chemicals should be applied only
during calm periods,
Table 11-7 gives a more useful meaning to the
concept of CU. From table 11-7, if a sprinkle
system has a CU of 86 percent, for each inch of
gross application received by the crop or soil, 80
percent of the area would receive at least 0.85 in. If
the CU were only 70 percent, 80 percent of the area
would receive at least 0.68 in. To apply a net application
of 1.0 in to at least 80 percent of the area
with a system having a CU of 86 percent, a gross of
1.0 divided by 0.85 = 1.18 in plus wind drift and
evaporation losses must be applied. With a CU of
only 70 percent, a gross after drift and evaporation
losses of 1.0 divided by 0.68 = 1.47 in would be required.
Figure 11-27 illustrates the relation between rainfall
area and depth of water applied at the CU
values discussed above. Both 70 and 86 percent CU
values leave 20 percent of the area underirrigated,
and 80 percent of the area adequately or overirrigated.
However, this requires a gross application of
approximately 25 percent more water with the 70
percent CU than with the 86 percent CU. Data for
constructing figure 11-27 were taken from table
11-7.
Table IL-7.--Minimum depth of water applied per 1.0 in
gross application for various values of CU and percentages
of land area adequately irrigated
Percent of area adequately irrigated
CU
percent 95 90 85 80 75 70 65 60
(inch)
90 0.79 0.84 0.87 0.89 0.92 0.93 0.95 0.97
86 .71 .78 .82 .85 .88 .91 .93 .96
82 .63 .71 .77 .81 .85 38 .91 .94
78 .55 .65 .71 .77 .81 .86 -89 .93
74 .46 .58 .66 .73 .78 .83 .88 .92
70 .38 .52 .61 .68 .75 .80 .86 .91
AREA RECEIVING LESS INDICATED DEPTH . PEnCENT
Figure 11-27.-Relationships between surface area and depth of
water applied for CU values of 70 and 86 percent when 20 percent
of the area is underirrigated and the remaining 80 percent
of the area is adequately (or over) irrigated.
When any given CU value is used as the irrigation
application efficiency, the area adequately
irrigated will be approximately 80 percent, i.e., note
that the values under the 80 percent adequacy
column correspond almost perfectly with the values
under the CU column.
When three or more adjacent laterals are operated
simultaneously in a fixed or block-move system, the
wind drift and evaporation losses are minimized
and essentially all of the water is applied effectively.
Therefore, table 11-7 can be used to approximate
overall irrigation efficiency for "block
system" layouts.
Table 11-8 gives a better understanding of CU
and shows the relative productivity, especially
when dealing with shallow-rooted vegatative crops