Chapter 11: Sprinkle Irrigation - NRCS Irrigation ToolBox Home Page

Table 11-4,-Suggested maximum application rates for
sprinklers for average soil, slope, and tilth
0-5% 5-8% 8-12% 12-16%
Soil texture Slope Slope Slope Slope
and profile
(inlhr) (inlhr) (inlhr) (inlhr)
1. Coarse sandy soil
to 6 ft 2.0 1.5 1.0 0.50
2. Coarse sandy soils
over more compact
soils 1.5 1.0 0.75 0.40
3. Light sandy loam
to 6 ft 1.0 0.80 0.60 0.40
4. Light sandy loams
over more compact
soils 0.75 0.50 0.40 0.30
5. Silt laams to 6 ft 0.50 0.40 0.30 0.20
6. Silt loams over
more compact
soils 0.30 0.25 0.15 0.10
7. Heavy-textured
clays or clay
loams 0 0.10 0.08 0.06
Continuous-Move Systems
Traveling sprinklers, Like periodic-move systems,
are usually managed to apply relatively deep irrigations.
Furthermore, drop sizes tend to be large so
values from Table 11-4 should be reduced by 25
percent for use as guides to selecting maximum
application rates for traveling sprinklers.
It is practical to apply frequent, light applica*
tions with center-pivot and linear-move systems.
With light applications, up to 0.5 in of the applied
water can be stored in small depressions on the soil
surface. Because of this, the peak application rates
near the end of center-pivot or linear-move laterals
operating on a 1- or 2-day cycle can often be more
than 100 percent greater than specified in Table
11-4 and not cause runoff on slopes of less than 8
percent. This is fortunate because it is difficult to
nozzle centerpivot systems that have a maximum
application rate of much less than 1.0 iph.
Sprinkle Irrigation Efficiency
Irrigation efficiency is a concept that is used extensively
in system design and management, It can
be divided into two components, uniformity of
application and losses. If either uniformity is poor
or losses are large, efficiency will be low. Several
factors affect the water-application efficiency of
sprinkle irrigation systems:
1. Variation of individual sprinkler discharge
along lateral lines can be held to a minimum by
proper lateral design.
2. Variation in moisture distribution within the
sprinkler-spacing area is caused primarily by wind
movement. For periodic move, fixed, and traveling
sprinklers this can be partially overcome by closely
spacing sprinklers or tow paths to meet adverse
wind conditions. In addition to the variation caused
by wind, there is always a variability in the distribution
pattern of individual sprinklers. The extent
of this variability depends on sprinkler design, operating
pressure, and sprinkler rotation. For centerpivot
and linear-move systems wind distortion is
not a serious problem because the sprinklers are
spaced close together along the lateral, and the lab
eral is continuously moving.
3. Loss of water by direct evaporation from the
spray increases as temperature and wind velocities
increase and as drop size and application rate decrease.
4. Evaporation from the soil surface before the
water reaches the plants decreases proportionally as
greater depths of water are applied.
Uniformity
Distribution uniformity (DU) is a useful term for
placing a numerical value on the uniformity of
application. The DU indicates the uniformity of infiltration
throughout the field.
DU =
Average low-quarter depth
of water received
Average depth of water received
X 100
The average low-quarter depth of water received is
the average of the lowest one-quarter of the measured
values where each value represents an equal
area.
Another parameter that is used continuously to
evaluate sprinkle irrigation uniformity is the uniformity
coefficient developed by Christiansen :
where
X * absolute deviation of the individual
observations from the mean (in)
' Chn'stianserr, J. E. 1942. Irrigation by sprinkling. University
of California. Bull. No. 670.