Chapter 11: Sprinkle Irrigation - NRCS Irrigation ToolBox Home Page

Alfalfa
453 X 50 X 4.3 = 609
Q =
10 X 16
Total for July = 898 gpm
Although only two of the three crops are being
irrigated, the maximum capacity requirement of
the system is in July.
The quality of most water is good enough that no
extra system capacity is required for leaching during
the peak use period. Leaching requirements can
usually be adequately satisfied before and after the
peak use-period.
If highly saline irrigation water is to be used on
salt sensitive crops (when the conductivity of the irrigation
water is more than half the allowable conductivity
of the drainage water), it is advisable to
provide a portion of the annual leaching requirement
at each irrigation. Thus, the system capacity
should be increased by an amount equal to the annual
leaching requirement divided by the number of
irrigations per year. The procedure for determining
leaching requirements is presented in Technical Release
No. 21.
It is not wise to irrigate under extremely windy
conditions, because of poor uniformity and excessive
drift and evaporation losses. This is especially
true with periodic-move systems on low infiltration
soils that require low application rates. When these
conditions exist, system capacities must be increased
proportionately to offset the reduced number
of sprinkling hours per day.
In water-short areas, it is sometimes practical to
purposely underirrigate to conserve water at the expense
of some reduction in potential yields. Yields
per unit of water applied often are optimum with
system capacities about 20 percent lower than are
specified for conventional periodic-move systems in
the same area. Underirrigation is best achieved by
using a longer interval between irrigations than is
normally recommended for optimum yields.
Fixed Systems
Fixed systems can be used for ordinary irrigation,
high frequency irrigation, crop cooling, and frost
protection. Special consideration is required when
estimating the system capacity required by each of
these uses. All fixed systems are ideal for applying
water-soluble fertilizers and other chemicals.
Ordhary Irrigation*- Some fixed syeterns are installed
irl permanent crops, and relatively long irrigation
intervals are used. The capacity of such systems
can be 5 to 10 percent less than conventional
periodic-move systems in the same area because
there is no down time during lateral moves. The capacity
should be sufficient to apply the peak "net"
crop water requirements for low frequency (1- or 2-
week interval) irrigations when the system is operated
on a 24-hr day, 7-day week bssis. These syatems
may be used to apply ferkilizers and other
chemicals and can be controlled by hand valves.
High Frequency.-If the system is designed to
apply irrigations once or twice a day to control soil
temperatures and to hold the soil moisture content
within a narrow band, a greater system capacity is
required. The net system capacity should be increased
hy 10 to 20 percent over a conveiltiond
periodic-move system because the crop will dways
be consuming water at the peak potential evapotranspiration
rate. By contrast, under lower fr9-
quency irrigation, as the soil moisture decreases the
consumptive use rate falls below the peak pdtential
rate. A major purpose for such a system is to keep
the crop performing at a peak rate to incresse quelity
and yield. Clearly, crops that do not respond
favorably to uniform high soil moisture conditi~~s
are not particularly good candidates for solid set
systems. High frequency systems can be hand
valve operated. However, automatic valve systems
can be used to apply fertilizers and chemicals.
Crop Cooling.-Very high frequency systems
used for foliar cooling must have antomatic valving,
use high qudity water, and have uy: to double the
capacity of ordinary high frequency systems. Foliar
cooling systems are sequenced so that the leaves
ate kept wet. Water is applied until the leaf surfaces
are saturated, shut off untii they are nearly
dry, then reapplied. This generally requires having
114 to 116 of the systam in operation simultaneously
and cycling the system once every 15 to 40 min depending
on system capacity, crop size, and climatic
conditions. For example, a system for ccoling trees
might be operated 6 ou!: of every 30 min so that 115
of the area is being sprinkled at any one time,
Foliar cooling systems must have sufficient capacity
to satisfy the evaporation demand on a minuteby-minute
basis throughout the peak use hours during
the peak use days. To accori~plish this, the system
capacity must be 1.5 to 2.5 times as great as is
required for a conventional periodic-move system.