Agriculture Reference
In-Depth Information
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Fig
ure 4.6
The variation in drought probabilities according to the number of subareas.
[51],
Line
——
1.4
——
Cust
PgEn
Drought Parameters
The global assessment of model performances can
be achieved on the basis of drought parameters such as averages (i.e., expec-
tations and variances), but for drought predictions, the PDF expressions as
derived above are significant. The expected (i.e., average) number of SMDs,
E
i
(
A
d
), over a region of
m
subareas during time interval,
i
t,
is defined as
m
kP
i
t
A
d
=
k
E
i
(
A
d
)
=
[4.20]
[51],
k
=
0
Si
milarly, the variance,
V
i
(
A
d
), of a drought-affected area is given by defi-
ni
tion as
m
k
2
P
i
t
A
d
=
k
−
E
i
(
A
d
)
V
i
(
A
d
)
=
[4.21]
=
k
0
Th
e drought-stricken average area within the whole region can be deter-
m
ined as:
i
−
1
q
r
E
i
(
A
d
)
=
mp
r
[4.22]
k
=
0
or
, succinctly,
m
1
q
r
E
i
(
A
d
)
=
−
[4.23]
Furthermore, the percentage of agricultural drought area,
P
i
A
, can be
ca
lculated by dividing both sides by the total number of subareas,
m
,
le
ading to
P
i
A
=
(
1
q
r
)
−
[4.24]
Fi
gure 4.7 shows the variation in a drought-affected area with the number
of SMD subareas,
i
, for given SMD probability,
q
r
.
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