Geology Reference
In-Depth Information
7.4.1.2 ASCE Penman
-
Monteith (ASCE-PM) Model
The manner and form of the standardized ASCE-PM equation is very much akin to
that of the FAO-56-PM equation. The standardization of this equation assumes that
weather parameters are at a height of 0.1
0.2 m above the grass for both grass and
alfalfa reference applications. The ASCE standardized reference equation (ASCE-
PM) is
-
C
n
0
:
408
D
ð
R
n
G
Þþc
T
hr
þ
273
u
2
ð
e
s
e
a
Þ
ET
0
¼
ð
7
:
17
Þ
D
þ cð
1
þ
C
d
u
2
Þ
where ET
0
is the standardized reference crop evapotranspiration for short surfaces
(mm day
−
1
) with daily time steps or mm h
−
1
for hourly time steps, R
n
the calculated
net radiation at the crop surface (MJ m
−
2
day
−
1
for daily time steps or MJ m
−
2
h
−
1
for hourly time steps), G the soil heat
flux density at the soil surface (MJ m
−
2
day
−
1
for daily time steps or MJ m
−
2
h
−
1
for hourly time steps), T the mean daily or
hourly air temperature at 1.5- to 2.5-m height (
C), u
2
the mean daily or hourly
wind speed at 2-m height (m s
−
1
), e
s
the saturation vapor pressure at 1.5- to 2.5-m
height (kPa), calculated for daily time steps as the average of saturation vapor
pressure at maximum and minimum air temperature, e
a
the mean actual vapor
pressure at 1.5- to 2.5-m height (kPa),
°
Δ
the slope of the saturation vapor pressure-
C
−
1
). C
d
is
also known as bulk surface resistance and aerodynamic resistance coef
C
−
1
), and
temperature curve (kPa
°
ʳ
the psychrometric constant (kPa
°
cient, the
value of which varies with day and night. Variation of C
n
and C
d
are detailed in
Table
7.2
.
7.4.1.3 CIMIS Penman
Monteith (CIMIS-PM) Model (Hourly Time
-
Step)
This model is a modi
cation of the Penman equation to represent hourly evapo-
transpiration from a hypothetical grass, ET
0
(mm h
−
1
), and is known as the CIMIS
PM equation [
9
,
22
,
23
]:
Table 7.2 Variation of C
d
and C
n
Time step
Reference crop
(short)
Reference crop
(tall)
Units of ET
0
C
d
C
n
C
d
C
n
mm day
−
1
Daily
0.34
900
0.38
1,600
mm h
−
1
Hourly (daytime)
0.24
37
0.25
66
mm h
−
1
Hourly (nighttime)
0.96
37
1.7
66
