Agriculture Reference
In-Depth Information
where
net incoming shortwave radiation [MJ/m
2
/d]
R
ns
=
net outgoing long-wave radiation [MJ/m
2
/d]
R
nl
=
Net shortwave radiation is the radiation received effectively by the crop canopy
taking into account losses due to reflection:
R
ns
=
(1
−
α
)
R
s
≈
0.77
R
s
(10.14)
where
α
=
albedo or canopy reflection coefficient
=
0.23 overall average for grass
incoming solar radiation [MJ/m
2
/d]
R
s
=
Shortwave radiation can be estimated from measured sunshine hours according to
the following empirical relationship:
a
s
+
R
a
n
N
R
s
=
b
s
(10.15)
where
a
s
=
fraction of extraterrestrial radiation (
R
a
) on overcast days
≈
0.25 for
average climate
a
s
+
b
s
=
fraction of radiation on clear days
≈
0.75
b
s
≈
0.50 for average climate
n/N
=
relative sunshine fraction
n
bright sunshine hours per day (h)
R
a
=
=
extra-terrestrial radiation, MJ/m
2
/d
Available local radiation data can be used to carry out a regression analysis to
determine the Angstrom coefficients
a
s
and
b
s
.
The extraterrestrial radiation can be calculated as
24
×
60
R
a
=
G
sc
·
d
r
(
ω
s
sin
ψ
sin
δ
+
cos
ψ
cos
δ
sin
ω
s
)
(10.16)
π
where
extraterrestrial radiation (MJ/m
2
/d)
G
sc
=
R
a
=
solar constant (MJ/m
2
/d)
=
0.0820
d
r
=
relative distance of earth and sun
ψ
=
latitude (rad)
δ
=
solar declination
ω
s
=
sunset hour angle (rad)
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