Chapter 11: Sprinkle Irrigation - NRCS Irrigation ToolBox Home Page

Chapter 1 has a discussion of intake rates and the
effects of slope, vegetation, and soil condition. The
rate of application should be planned so it is no
higher by the end of an irrigation than the capacity
of the soil to absorb water. Ideally intake versus
time of application information should be developed
by applying water at the expected sprinkling intensity
of the system selected on crops, soils, and
slopes similar to the expected site conditions. This
information can often be obtained by examining an
existing system, but it is difficult to set up an experiment
to observe it.
On bare soils drop impact causes surface sealing
and reduces infiltration. The kinetic energy of a falling
drop is the product of one-half its mass and the
square of its velocity, Drop sizes range from 0.5 to
5.0 millimeters (mm) and have terminal falling
velocities ranging from about 6 to 30 ftls, respectively.
With a typical fall distance equivalent to
about 10 to 20 ft, most drops come close to reaching
their respective terminal velocities. Table 11-3
presents terminal velocities and kinetic energies asF
sociated with different drop sizes,
Drop size is reduced as pressure increases (fig,
11-15), or as nozzle size decreases. Drop sizes can
also be reduced by using means other than high
pressures to cause jet breakup.
Table 11-3, Terminal velocities and kinetic energies associated
with different size raindrops
Kinetic energy values
Terminal in relation per inch
Drop size Volume velocity to a of rain,
(mm) (mm3) (ftls) 1.0-mrn drop ft-lblft
and structure, amount and type of crop cover, and
the application rate. Figure 11-16 shows the general
relation between drop size and reduction in infiltration
rate on three different bare soils with an
application rate of approximately 0.5 iph. The reduction
in infiltration rate on the freshly tilled,
heavy-textured soil approached the maximum level
about 20 rnin after the beginning of the application.
DISTANCE FROM SPRINKLER - FT
Figure Il-15.-Drop sizes at various distances from a standard
5132-in nozzle operating at 20 and 60 psi.
CLAY LOAM /
/ SANDY LOAM, .-
Some such devices are the use of pins penetrating
the jet near the nozzle orifice; using sharp orifices
instead of tapered nozzles; using triangular, rectangular,
or oval orifices; and using impinging jets,
Because of escalating energy costs the interest in
obtaining small drops without high pressures has
been accelerated.
The surface sealing and reduction in infiltration
caused by drop impact depends on the soil texture
DROP DIAMETER +
Figure 11-16.-Relation of infiltration rate reduction due to
sprinkling three different soils at an application rate of appraximately
0.5 iph.
mm