sprinkler irrigation design exercise (by zechariah hatinanyika)

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Irrigation & Water ManagementIrrigation & Water Management

PRINCIPLES OF

SPRINKLER IRRIGATION

DESIGN

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Sprinkler Irrigation Design Sprinkler Irrigation Design ExerciseExercise

A semi-portable sprinkler system is to be designed for a farm in Mvurwi where the maximum Eto is 6mm/d. The soils are loamy with AWC of 100mm/m, and average infiltration rate of 6mm/h. Wheat, with a Kc value of 1.1 and root depth of 0.8m is to be grown. Wind speed max is 11km/h. MAD is 50%.

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108

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432m

312m

800m Prev. wind

Dam elev 100m

Fig. 1

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1. Irrigation water 1. Irrigation water requirementsrequirements

Dnet = AWC * Rz * MAD = mm

Dgross = Dnet / System Eff• = mm

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2. Irrigation cycles2. Irrigation cycles

Etc = Kc * Eto= mm/d

Irrigation interval = Dnet / Etc = days.

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3. Sprinkler selection3. Sprinkler selection

1. Application rate < infiltration rate to avoid runoff

2. Choose pressures 2.5 – 4 bars, as long as CU>85%

3. Consider effect of wind on max sprinkler spacing

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Cont’Cont’

For infiltration rate of 6mm/hr, selecting 4mm nozzle @ 350kPa, 12*18 spacing

Wetted dia = 30.5m, Qspr = 1.16m3/hr.

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Acceptability testAcceptability test

40% of 30.5m = 12.2m > 12m60% of 30.5m = 18.3m > 18m

Application rate = 5.37mm/h < 6mm/h

Therefore ACCEPTABLE!

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Set timeSet time

Set time = Dgross / Ppt rate= hrs.

System can do sets

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System layoutSystem layout

1. Laterals placed across prevailing wind direction and perpendicular to predominant slope for uniform head losses.

2. Mainline to run through the middle for reduced frictional losses.

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Cont’Cont’

No. of sprinklers per lateral = Lateral length / Spr. Spacing

= spr / lateral

No. of lateral positions = 2 { (Field length / Lateral spacing)}

= positions.

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contcont

No. of hydrants = 1/6* lateral posns= hydrants.No. of laterals per posn = No. of

shifts * Irrig Interval = moves.No. of laterals = No. posn / No.

moves = laterals

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System capacitySystem capacity

Qsystem = Spr/lat * No. laterals * Qspr = m3/h

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Pipe size determinationPipe size determination

Friction loss in lateral to be kept at minimum.

Hfriction + Hstatic <= 20% operating pressure

‘F’ friction reduction used for multiple outlets

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Lateral & Mainline friction Lateral & Mainline friction losseslosses

Qlat = No. spr/lat * Qspr = m3/h

Using 3” Ali, HL= mHeader line loss= m

MainlineUsing 125/6, HL= m

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Total Dynamic Head (TDH)Total Dynamic Head (TDH)Static head =Operating presssure = Riser height =Ali friction losses =PVC friction losses =Suction lift =Total =+ 10% loss fittings =TDH =

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Pump selectionPump selection

Power (kW)

= Qsystem * TDH 360*Pump Efficiency

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Bill of materialsBill of materials

Item QtySprinklers c/w riserAli pipe c/w riser T/OAli pipe plainHydrant control valveHydrants

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BOM contBOM cont

End plugsPVC pipePumpAli 90deg bendFittingsetc