CROP WATER REQUIREMENT CALCULATION

Cwr=ETa+ ( S∧P ) field

ETa=ETo∗kc

REFERENCE EVAPOTRANSPIRATION CALCULATION

(Using Penman-Monteith Method)

Climatic Data Requirements of Tanay, Rizal (based on PAG-ASA average values)

ITEM SYMBOL UNIT-VALUE

Initial Data Required:
Maximum Temperature Tmax 25.6 oC
Minimum Temperature Tmin 19.9 oC
Elevation above sea level z 646.05 m
Wind speed uz 4 m/s
Dewpoint Temperature Tdew 20.8 Oc
Relative Humidity RH 90%
Latitude Latitude 14 34’53.22” N
Actual duration of sunshine n 12 hours

Parameters used in Calculations:

Mean daylight hours N 11.3 hours
Slope of vapour pressure curve ∆ kPa/oc
Psychrometric constant γ kPa/oc
Saturation vapor pressure es kPa
Saturation vapor pressure at Tmax e^o (Tmax) kPa
Saturation vapor pressure at Tmin e^o (Tmin) kPa
Actual vapor pressure ea kPa
Extraterrestial radiation Ra MJ/m^2/day
Solar radiation Rs MJ/m^2/day
Clear-sky solar radiation Rso MJ/m^2/day
Net solar radiation Rns MJ/m^2/day
Net longwave radiation Rnl MJ/m^2/day
Net radiation Rn MJ/m^2/day
Soil heat flux G MJ/m^2/day
Reference evapotranspiration ETo Mm/day

Parameters:
Tmax+Tmin 25.6+19.9
Air Mean Temperature Tmean= =
2 2
Tmean=22.75 oC
Slope of Vapor Pressure ∆=0.17 kPa/o C (Table B.4)

Value of Psychrometric Constant γ =0.062kPa /oC (Table B.5)

o
Saturation Vapor Pressure e (Tmax )=¿ 3.361 kPa/ oC (Table B.6)
o
e ( Tmin )=¿2.338 kPa/ oC

o o
e ( Tmax ) +e (Tmin ) 3.361+2.338
es= =
2 2

es=2.8495 kPa

o
Actual Vapor Pressure ea=e (Tdew) (Table B.6)

ea=2.487 kPa

MJ
Extraterrestrial Radiation Ra=30.6 2
/day (Table B.7)
m

Solar Radiation (
Rs= 0.25+ 0.50
n
N )
Ra

(
¿ 0.25+0.50
12 h
11.3h )
MJ
30.6 2 /day
m
MJ
Rs=23.90 2
/day
m
Altitude
Clear-sky Radiation Rso=0.75+2 Ra
100,000
646.05 MJ
¿ 0.75+2 (30.6 2 /day)
100,000 m
MJ
Rso=23.0112 2
/day
m

Net Solar Radiation Rns=0.77∗Rs

 MJ
¿ 0.77 (23.90 2
/day)
m
MJ
Rns=18.403 2
/day
m

4 MJ
Net Longwave Radiation σTmax k =39.27 2
/day (Table B.9)
m
4 MJ
σTmin k =36.21 2
/day
m

4 4
σTmax k + σTmin k Rs
Rnl= (0.34−0.14 √ ea)(1.35 −0.35)
2 Rso
39.27+36.21 23.90
Rnl= (0.34−0.14 √ 2.487)(1.35 −0.35)
2 23.0112
MJ
Rnl=4.73 2
/day
m

Net Radiation Rn=Rns−Rnl

¿ 18.403−4.73
MJ
Rn=13.673 2
/day
m

Soil Heat flux ¿ can be neglected ,G=0

Reference Evapotranspiration
900
0.408 ∆ ( Rn−G ) +γ uz (es−ea)
Tmean+273
ETo=
∆+ γ (1+ 0.34 uz)
900
0.408∗0.17 (13.673−0 )+ 0.062 ∗4 (2.8495−2.487)
22.75+273
ETo=
0.17+ 0.062(1+0.34∗4)
0.9484+0.18867 (1.45)
ETo=
0.31632
 mm
ETo=3.86
day

Actual Evapotranspiration

ETa=ETo∗kc (ornamental plants)

mm
ETa=(3.86 ∗0.90)
day
mm
ETa=3.474
day
Crop Water Requirement

Cwr=ETa+ ( S∧P ) (field−clay loam)

¿ 3.474+ 1.75
mm
Cwr=5.224
day
FRONT
L L
Application Rate q=30 ¿ 0.008
h s
ΣSc (0.34 +0.3+0.36)m
Sc ave= = =0.333 m
3 3
ΣSm (0.5+ 0.44+0.54)m
Smave= = =0.493 m
3 3
L 3
∗1 m
( )
3
h 1000 mm
30 ∗
q 1000 L 1m
r= =
( )
Sc∗Sm 1000 mm 2
0.333 m∗0.493 m∗
1m
mm
r =182.74
hr

dgross 59.55 mm
Set Time ts= =
r 182.74 mm/hr
hr
ts=0.33
set

System Capacity Q=Nc∗Ns∗q q=Sl∗Sm∗r

Nc ave=2
(50+ 63+42)
Ns ave= =51.67∨52
3
 mm
∗1 m
hr
Q=2∗52∗(0.333 m∗0.493 m∗182.74 )
1000 mm
3
3.12 m
∗1000 L
hr
3
∗1hr
1m
Q=
3600 s
L
Q=0.87
s