Em4885 irrigation management practices to protect ground water

If a cutback flow is going to be used, the equation is modified as:
where:
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CHAPTER 4
(GPM 1 x HOURS 1 x 96.3) + (GPM 2 x HOURS 2 x 96.3)
INCHES = ................................................................................................. x AE
FURROW LENGTH x FURROW SPACING
INCHES = net amount of water infiltrated in inches
GPM 1 = initial furrow flow rate in gallons per minute
HOURS 1 = time that initial flow in place
GPM 2 = cutback furrow flow rate in gallons per minute
HOURS 2 = time that cutback flow in place
FURROW LENGTH = furrow length in feet
FURROW SPACING = furrow spacing in feet
AE = expected application efficiency as a decimal (i.e., 70% efficiency = .7)
The above equation can also be used for border strips by substituting BORDER LENGTH
and BORDER WIDTH for FURROW LENGTH and FURROW SPACING.
Sprinkle - Hand-line and side-roll sprinklers have an application rate that is measured in
terms of inches of water applied per hour. Knowing this application rate, expected
application efficiency, and the soil water depletion at irrigation allows an easy computation
of indicated set time using the equation:
where:
SET TIME = SWD / (AE x AR)
SET TIME = the indicated set time in hours
SWD = soil water deficit (including leaching requirements) in inches
AE = expected irrigation efficiency as a decimal
AR = application rate of the system in inches per hour
Example: assume the following:
• application rate (AR) = .2 inch/hour
• overall application efficiency (AE) expected is 75%
• soil water depletion (SWD) at irrigation is 3 inches
SET TIME = SWD / (AE x AR)
= 3 in / (.75 x .2 in/hr)
= 20 hours
It is often desirable for sprinkler sets to be multiples of 12 or 24 hours, making it convenient
for labor to change sets. In the above example, if the set was started at 6:00 A.M., then the
set would have to be changed at 2:00 A.M. Another way to use the equation is to first
assume a 12- or 24-hour set, or whatever is desirable, and then determine the soil water
depletion that the set will satisfy.