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© Irrigation Association
Analyze the Resource Inventory
© Irrigation Association
Calculate Flow Requirements
• System capacity formula
© Irrigation Association
Basic form
QT=AD
– Q = flow rate (cfs)– T = time (hrs)– A = area (ac)– D = depth (in)
Let’s look at the manual for calculation
of GIR
© Irrigation Association
Etc is the average daily ET
• For shallow rooted crops use 5 day period
• For medium rooted crops use 10 day period
• For deep rooted crops use 20 day period
• For sandy soils these values are halved– Example: beans are medium rooted
crop 10 days /2 = 5 days period
© Irrigation Association
ET During Growing Season
May
June
July
August
Septe
mber
0.00
0.10
0.20
0.30
0.40
Cro
p E
T (
in/d
ay
)Wheat Corn Soybean
© Irrigation Association
Daily ET graph
1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586878889909192939495969798991001011021031041051061071081091101111121131141151161171181191201211221231241251261271281291301311321331341351361371381391401411421431441451461471481491501511521531541551561571581591601611621631641651661671681691701711721731741751761771781791801811821831841851861871881891901911921931941951961971981992002012022032042052062072082090
0.1
0.2
0.3
0.4
0.5
Alfalfa
Growing Day
ET
(in
/da
y)
© Irrigation Association
Adjustment Factor for frequency of wetting
• kf
© Irrigation Association
© Irrigation Association
Calculate Irrigation Frequency
• Maximum is related to MAD x AW x RZ
• Precipitation reservoir• Safety buffer for downtime• Balancing frequent irrigations,
intake rates and reserve storage
© Irrigation Association
Suggested starting intervals
• 2.5 days for sandier soils• 3.5 to 4.5 days for medium
textured soils• See Appendix G for more
information
Let’s look at the manual for calculation
of Qs
© Irrigation Association
Fraction of time operated
• t =the fraction of time that the system is turned on for a typical day during the peak period.
• Often in design, t = 0.9 is used to account for some down time.
© Irrigation Association
Limited water supply
• Limits on water– Insufficient water rights– Inadequate well or pump capacity– Limited seasonal water supply
© Irrigation Association
Deficit Irrigation
• What is the responsibility of the designer?
© Irrigation Association
0 5 10 15 20 25 300
50
100
150
200Wheat Soybean Corn
Evapotranspiration (inches)
Yie
ld (
bu
/ac)
Yield vs Evapotranspiration
© Irrigation Association
Practice Session #1 - Calculating the design flow
rate
© Irrigation Association
Practice Session #2 - Design project flow rate