Chapter 11: Sprinkle Irrigation - NRCS Irrigation ToolBox Home Page

Sample calculation ll-4.-Determine irrigation efficiency
and application rate.
Given:
The information in parts I and I1 of figure 11-28
for potatoes.
An average wind of 10-15 rnph
Assume:
The soil moisture depletion is MAD = 60%
Side-roll laterals will be used and two changes per
day will be made.
The sprinkler spacing is 40 x 50 ft
Calculation:
Determine Re = 0.92 from figure 11-17 for ET =
0.25 inlday for 10-15 mph wind and average spray.
Because potatoes are a relatively high-value, shallow-rooted
crop, an application efficiency (Eq) based
on the average low-quarter depth, is appropriate, so
use DU as the measure of uniformity. This will
leave approximately 10 percent of the area underwatered.
Assuming an E, of 67 percent the gross
application would be:
-- 2b0
- 3.0 in
671100
Assuming it will take 30 min to change the posib
tion of a side-roll lateral, the time per set with two
changes per day will be 11.5 hr. Thus the preliminary
application rate is:
I' = 3.0 in = 0.26 iph
11.5 hr
From table 11-11, (10-15 rnph winds) the antic&
pated CU =78%. If alternate sets are used the improved
CU, can be estimated by equation 6a as:
These are two processes that can be used to develop
the expected Eq. An estimated DU, can be
determined by equation 5b as:
and from
-
equation 9:
Eq 81 X 0.92 = 75%
The other method is to use table 11-7 with CU,
= 88% and find that for 90 percent of the area adequately
irrigated 0.81 in is the minimum depth of
water applied per 1.0 in of effective application so:
Obviously, if alternate sets had not been used the
efficiency would have been much lower, i.e., about
60 percent for 90 percent adequacy. Also, if an efficiency
of 75 percent is assumed and alternate sets
are not used, the area adequately irrigated will only
be 75 percent. This was determined by noting that
0.81 in is the minimum depth of water applied per
1.0 in of effective application with a CU of 78 percent
and 75 percent adequacy in table 11-7.
The required gross application, assuming Eq =
759'0, can now be determined as:
and the required application rate is;
The required sprinkler discharge can now be computed
by equation 2 as:
The production value of having 90 percent adequacy
by using alternate sets vs. 75 percent adequacy
can be demonstrated, assuming overwatering
does not reduce yields. Table 11-8 gives relative
percentages of optimum production for different CU
and adequacy values. With a CU = 78% and 75
percent adequacy, the relative production is 95 percent
and for a CU = 88% and 90 percent adequacy,
it is 99 percent. Thus the use of alternate sets can
be expected to improve yields by at least 4%. If,
however, uneven watering decreases production or
quality (due to leaching of fertilizer or waterlogging),
the gross income differences may be considerably
larger than 4 percent.
Nozzle Size and Pressure.--Table 11-13 is a list
of the expected discharge and wetted diameters in
conditions of no wind from typical 112- and 314-in
bearing impact sprinklers with angles of trajectory
between 22" and 28" and having standard nozzles