Chapter 11: Sprinkle Irrigation - NRCS Irrigation ToolBox Home Page

The desired I depends on time per set, net depth to
be applied per irrigation, and application efficiency.
It is practical to change periodic-move laterals only
once or twice per day unless they are automated.
For one change per day, the time per set will be 24
hr minus the length of time required to change the
lateral position. leaving a total of 23 to 23.5 hr. For
two changes per day, set times will range between
11 and 11.5 hr.
Figure 11-28 shows a copy of figure 11-13 completed
for a sample field of alfalfa and potatoes.
Sample calculations 11-3 and 11-4 illustrate the
procedure for determining the desired application
rate (I) and related average sprinkle discharge (q)
for the alfalfa field and the potato field, respectively.
Sample calculation 11-3.-Determine the net depth
per irrigation, irrigation interval, irrigation efficiency,
application rate, and sprinkler discharge requirement.
Given:
The information in parts I and I1 of figure 11-28
for alfalfa
. - - " -
An average wind of 4-10 mph
Assume:
The soil moisture depletion is MAD = 50%
There will be one change per day
The sprinkler spacing is 40 X 60 ft
Calculation:
For a MAD = 50%" the allowable soil water depletion
is 50 percent of the total available waterholding
capacity of the root zone which in this case
is:
5 U
6 ft X 2.0 inlft X - = 6.0 in
100
The maximum allowable irrigation interval during
the peak use period is
allowable depletion (in) 3-- - 20 days
water use rate (inlday) 0.30
-- " - - - .
These are the maximum allowable depletion and
corresponding maximum interval during the peak
use period that will give the desired level of productivity.
To fit the final system design, lesser net applications
and correspondingly shorter intervals
may be used.
The application efficiency can be estimated from
the effective portion of the applied rate (R,) and the
uniformity of application. Assuming the spray will
be midway between coarse and fine (from fig.
11-18) for a potential evapotranspiration rate of 0.3
(0.97 + 0.91) _
inlday the effective portion, Re = -
0.94.
Because alfalfa is a relatively low value crop, an
applied efficiency (Eh) based on the average low-half
depth is appropriate, i.e., use CU. Assuming an Eh
of 75 percent, the gross application would be:
Assuming it will take 1 hour to change the position
of a hand-move lateral, the time per set with
one change per day will be 23 hr. Thus the preliminary
application rate is:
From table 11-10 (4-10 mph winds) the anticipated
CU = 84% on a 40- x 60-ft spacing and 0.34
iph. A more specific estimate of CU can often be obtained
directly from a supplier, The expected application
efficiency can now be estimated by equation
8:
The required gross application can now be more accurately
computed as:
atld the required application rate is:
I=-- 7'6 in - 0.33 iph
23 hr
The required sprinkler discharge can now be calcw
lated by equation 2:
2