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Analyze the Resource Inventory

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Calculate Flow Requirements

• System capacity formula

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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

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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

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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

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Daily ET graph

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0.1

0.2

0.3

0.4

0.5

Alfalfa

Growing Day

ET

(in

/da

y)

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Adjustment Factor for frequency of wetting

• kf

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© 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

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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

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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.

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Limited water supply

• Limits on water– Insufficient water rights– Inadequate well or pump capacity– Limited seasonal water supply

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Deficit Irrigation

• What is the responsibility of the designer?

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0 5 10 15 20 25 300

50

100

150

200Wheat Soybean Corn

Evapotranspiration (inches)

Yie

ld (

bu

/ac)

Yield vs Evapotranspiration

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Practice Session #1 - Calculating the design flow

rate

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Practice Session #2 - Design project flow rate