Irrigation & Water Management
PRINCIPLES OF SPRINKLER IRRIGATION DESIGN 1
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Sprinkler Irrigation Design Exercise ď ŹA
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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Fig. 1
432m 108
312m
107
106
Dam elev 100m
105 800m
4 Prev. wind
1. Irrigation water requirements Dnet
= AWC * Rz * MAD = mm
Dgross
= Dnet / System Eff •= mm
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2. Irrigation cycles Etc
= Kc * Eto=
Irrigation
=
mm/d
interval = Dnet / Etc days. 6
3. Sprinkler selection 1. Application rate < infiltration rate to avoid runoff 2. Choose pressures 2.5 â&#x20AC;&#x201C; 4 bars, as long as CU>85% 3. Consider effect of wind on max sprinkler spacing 7
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Cont’ For
infiltration rate of 6mm/hr, selecting 4mm nozzle @ 350kPa, 12*18 spacing
Wetted
dia = 30.5m, Qspr = 1.16m3/hr. 10
Acceptability test 40%
of 30.5m = 12.2m > 12m 60% of 30.5m = 18.3m > 18m Application
rate = 5.37mm/h <
6mm/h Therefore ACCEPTABLE! 11
Set time Set =
time = Dgross / Ppt rate hrs.
System
can do
sets
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System layout ď Ź 1.
Laterals placed across prevailing wind direction and perpendicular to predominant slope for uniform head losses.
ď Ź 2.
Mainline to run through the middle for reduced frictional losses. 13
Contâ&#x20AC;&#x2122; No. of sprinklers per lateral = Lateral length / Spr. Spacing = spr / lateral No. of lateral positions = 2 { (Field length / Lateral spacing)} = positions. 14
cont No. of hydrants = 1/6* lateral posns ď Ź= hydrants. No. of laterals per posn = No. of shifts * Irrig Interval = moves. No. of laterals = No. posn / No. moves = laterals 15
System capacity ď Ź Qsystem
Qspr =
= Spr/lat * No. laterals * m3/h
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Pipe size determination Friction
loss in lateral to be kept at minimum. Hfriction + Hstatic <= 20% operating pressure ‘F’ friction reduction used for multiple outlets 17
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Lateral & Mainline friction losses Qlat =
No. spr/lat * Qspr =
Using
3” Ali, HL= Header line loss=
m3/h m m
Mainline Using
125/6, HL=
m
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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 selection Power
(kW)
=
Qsystem * TDH 360*Pump Efficiency
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Bill of materials Item
Qty
Sprinklers
c/w riser Ali pipe c/w riser T/O Ali pipe plain Hydrant control valve Hydrants 24
BOM cont End
plugs PVC pipe Pump Ali 90deg bend Fittings etc 25