Agriculture Reference
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and plant growth ceases; and
W
max
is extractable water that usually equals
W
f
W
w
. It has been sufficient (but not necessary) to use the simplest
linear forms of these equations. Water availability is defined as (Jupp et
al., 1997):
−
m
a
(
WB
)
=
max
{
0
,
min [1
,β(
W
t
−
W
w
)/
W
max
]
}
[28.5]
Yang and Tian (1991) developed a drought index (
D
) following a water-
balancing approach:
D
=
(
W
t
−
W
w
)/
W
r
[28.6]
where
W
r
is the amount of water required by the vegetation for its normal
growth and respiration. Derivation of
D
, which is closely related to
m
a
,
needs adequate meteorological data, spatial information about the hydro-
logical properties of soils, and the nature of the land cover. If
m
a
or
D
can be determined during various periods by remote sensing techniques
(chapters 7 and 8), it is possible to validate water-balancing models.
[359
Line
——
-1.4
——
Norm
PgEn
R
emote Sensing
Remote sensing provides physical measurements of components of the en-
ergy balance—daily input of solar radiation and its various forms, such as
convection, evaporation, and storage in the earth or oceans. In particular,
the Advanced Very High Resolution Radiometer (AVHRR) data provide
information on shortwave absorption and the condition of the cover and
surface temperature.
If
R
n
denotes net wavelength radiation (
W
/
m
2
) and
G
denotes the heat
flux into the soil (
W
/
m
2
), the net energy available at the earth's surface will
be
R
n
−
[359
G
, which can be partitioned as:
A
=
R
n
−
G
=
λ
E
+
H
[28.7]
w
here
E
is the evapotranspiration flux (
m
/
sec
) of water vapor;
λ
is latent
he
at of vaporization of water (
J
/
m
3
),
H
is sensible heat flux (
W
/
m
2
)orthe
en
ergy involved in the movement of the air and its transfer to other objects
(s
uch as trees, grass, etc.), and
λ
E
is the energy required to transform water
fr
om liquid into vapor and can be computed as follows (Monteith and
U
nsworth, 1990):
λ
E
=
(
R
n
−
G
)
−
H
[28.8]
The above components may be computed in various ways using resis-
tance models (Monteith and Unsworth, 1990) of differing levels of com-
plexity introduced by land surface structure and cover. In each case, the
sensible heat flux is driven by the differences between the distribution of
temperatures among the surface components and the air temperature. For
simple (one-layer) or more complex models (Jupp, 1990), the remotely
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