REFERENCE CALCULATION
OUTPUT
CROP WATER REQUIREMENTS.
From
CROPWAT
ETo=5.13mm
Kc=1.15
ETc=5.13*1.15=5.8995mm/day
FAO
manual 9;
Table 1.
ETc=5.38995mm/
Day.
ETcrop-loc=ETc*Kr
Taking Ground cover as 40% Kr=0.4
ET crop-loc=5.8995*0.4=2.3598mm/day
D gross=ET crop-loc/Ea
ET crop-loc=
2.3598mm/day.
Ea=KS*EU;
Where Ks : Average water stored in root volume / Average water
applied.
EU: Coefficient reflecting the uniformity of application.
Ea=0.91*0.9=0.82
D gross=2.3598/0.82=2.878mm/day.
Irrigation Requirements
IR n=2.3598-R + LR
LR ; Leaching requirement.
R; water received by plant other than irrigation(rainfall).
Dgross=2.878
mm/day.
N/B; R will be taken as zero assuming worst case scenario ie no
rainfall.
FAO maual
9; Table 2.
Where ; LR=LRt*(IRn/E a)
LRt= EC w / (2*(MAX ECe)
LR=2/ (2*10) =0.1
LR=0.1
LR=0.1*(2.3598/0.82) =0.288mm/day
IR g= (2.3598/0.82)-0+0.2888=3.1659mm/day
Percentage wetted area.
Pw = 100%
FAO
manual 9;
table 3.
IRg
=3.1659mm/day.
Pw=100%
Area wetted by one emitter.
Aw = 1.5m2
Emitters per plant and spacing.
Potato Spacing 1.5m by 1.5m
Emitters per plant = (1.5*1.5*1) / 1.5 = 1.5 (take 2 emitters per
plant).
Emitter spacing = Sp / Np = 1.5 / 2 = 0.75m.
Where Sp; Distance between plants in a row.
Np; Number of emitters per plant.
Aw = 1.5m2
Emitters per
plant =2 emitters
Emitter spacing
=0.75m.
IRRIGATION FREQUENCY AND DURATION.
Effective rooting depth=0.6m
Available soil moisture=140mm/m
Moisture Depletion for drip system=20%
Net Irrigation Requirement (IR n) =2.3598mm/day
Potato Spacing 1.5m by 1.5m
Net Irrigation Per Potato
= (2.3598/600) *1.5*1.5=8.85*10600) *1.5*1.5=8.85*103m3/day/potato or 8.85L/day/potato
Area to Wetted Soil=SP*SR*Pw
=1.5*1.5*0.87=1.9575m2
Available Soil Moisture Per Potato
= (140/1000) *1.9575=0.2741m3 OR 274.05L/Potato
Net irrigation per
potato
=8.85L/day/potato.
Wetted area=
1.9575m2
Moisture to be replenished per potato by drip irrigation
=274.05*(20/100) =54.81L/Potato
Irrigation Frequency= (54.81/8.85) =6.2days
Adapt 6 Days
Net amount of water to be applied
=6*8.85=53.1L/Potato
Irrigation
Frequency
=6 days
DURATION OF IRRIGATION PER DAY.
Ta= IR g/ (Np*q)
IR g=3.1659mm/day
= (3.1659*1.5*1.5) / 1000=7.123*103m3 OR 7.123 L/Potato/Day
IRg=3.1659mm/day
Taking a 2Lph emitter
Ta=7.123/ (2*2) =1.78 hours=2 hours
Farm will be divided into 6 sections
Lateral spacing=1.5m
Ta=2hours
Number of laterals per section
98/1.5=65.333=66 laterals
132 laterals per
section
66*2=132 laterals
Length of lateral=170/2=85m
Number of potatoes per lateral=85/1.5=56.7=57 potatoes
Number of emitters per lateral= 57 * 2 = 114
114*132=15048 emitter per section
emitters per
lateral= 114.
15048 emitter per
section
Length of manifold=98meters
Length of main line =500 meters
Discharge for each potato with spacing of 1.5m by 1.5m
= (2.3598/600) *1.5*1.5=8.85*10^-3m3/day/potato
In 24 hours (8.85*10^-3/24) =3.688*10^-4m3/hr. (24 hours a day)
For 2 hours of operation per day, discharge required
=3.688*10^-3*24/2=4.425*10^-3m3/hr.
=4.425L/hr.=1.23*10^-3L/s
With application efficiency of 82% required per potato
= (1.23*10^-3)/0.182=1.5*10^-3L/S
The discharge per emitter with 2 emitters per potato is then,
= (1.5*10^-3)/2=7.5*10^-4L/S
Discharge of each line
LINE
No. of
potatoes
HALF LATERAL 57
HALF
3762
MANIFOLD
FULL
7524
MANIFOLD
MANIFOLD TO 45144
PUMP
Lateral length=85m
No. of
emitters
114
7524
Required
Discharge
0.0855
5.643
15048
11.286
90288
67.716
Manifold
Length=98m
Mainline
Length=500m
Selecting emitter with k=4.0, x=0.48, q=7.5*10^-4L/S
Substituting in q=K h ^x with average discharge of 7.50*10^-4L/S
=Log q= log k+ log h
=Log h= (Log q-Log k)/X
= (log 7.50*10^-4) -(log 4.0*10^-4)
0.48
=Log h =0.5687
=h=3.7m
This is the average operating head
Operating
Head=3.7m
Total allowable pressure loss of 15% in both lateral and manifold
=3.7*0.15=0.55m of which 0.55*0.55=0.30m for lateral and
0.45*0.55=0.25m for manifold
From Hf=K*L*Q^1.75*D^-4.75 to select the required diameters
LINE
Q(L/S)
F
0.0855
PIPE
L(M)
DIAMETER
25mm
85
HALF
LATERAL
HALF
MANIFOLD
FULL
MANIFOLD
MANIFOLD
TO PUMP
5.643
120mm
98
0.357
11.286
120mm
98
1
67.716
207.8mm
425
1
Calculating F=(1/M+1) + (1/2N)
Where M=1.852, N=114
Half Lateral
F= (1/2.852) + (1/228) =0.36
Half Manifold
=F= (1/2.852) + (1/15048) =0.351
=Hf(m)=(KL(Q/C) ^1.852)/D^4.87
HALF LATERAL.
Hf(M)
0.36
0.21
=Hf(m)=0.30m= (7.89*10^5) *(85) *(0.0855/150) ^1.852
=D^-4.75=3.309*10^-7
=Log 3.309*10^-7=-4.75
=log (3.309*10^-7) =-4.75
Log D
=-6.480 =-4.75
Log D
=-4.75Log D =-6.480
=Log D=1.364=10^1.364=23.12mm=25mm
HALF MANIFOLD.
=0.25= (7.89*10^5) *98*(5.643) ^1.75*D^-4.75
=D^-4.75=0.25=
1.565*10^10
(1597517656)
=Log 1.565*10^10=-4.75
Log D
=Log D 1.565*10^-10=-4.75
=-9.805=-4.75
Log D
=Log D=1
-9.805 -4.75
=Log D =-9.805=2.064
-4.75
=10^2.064=112.8mm
=120mm was adopted Hf(m)=0.21m
MAIN /SUBMAINS.
=Hf(m)=K*L*Q^1.75*D^-4.75
= (7.89*10^5) *(425) *(67.716) ^1.75*D^-4.75
=Q=AV
=A=Q/V
=A= (0.067716)/2=0.0339m2
=A=3.142*D^2
4
=D^2=0.0432
=D=0.2078m=207.8mm
Half Lateral
D=25mm
Half Manifold
D=120mm
Hf(m)=0.21m
Main/Submains
=207.8mm