Geoscience Reference
In-Depth Information
W
t
=0
.
1
1+
12
M
t
MAR
,
(4)
where
E
t
is the effective precipitation defined by Eq. (3),
R
t
is the monthly
rainfall,
M
t
the monthly mean rainfall and
W
t
the weighting factor, which
allows for carry-over from one month to the next (Eq. (4)) and
MAR
is the
mean annual rainfall.
D
is the duration of drought.
The drought intensity is given by
Y
=
i
=1
[(
E
t
−
M
t
)
−
(
MMD
)
t
]
i
=1
(
MMD
)
t
,
(5)
where (
MMD
)
t
is the monthly mean deficit (
t
=1
,
2
, ...,
12) evaluated
from the
N
years of record and is given by
N
(
M
t
−
R
t
)
i
.
(
MMD
)
t
=
(6)
N
i
=1
The sum
(
MMD
)
t
gives the mean annual deficit
MAD
.
A severity index of drought =
YD
.
(7)
Return period
T
r
is given by
T
r
=(
n
+1)
/m,
(8)
where
m
is the rank and
n
is the number of events.
4. Analysis and Discussion
4.1.
Standard precipitation index method
Table 1 shows the most severe droughts for both areas. The plots for the
drought magnitudes against return periods are shown in Figs. 2 and 3. From
the plots, it is clearly seen that there is a linear relationship between the
drought magnitudes and the return periods associated with them.
Furthermore, large magnitudes are associated with very large return
periods.
4.2.
Drought volumes method
The drought volumes calculated using Eq. (2) were used to determine the
drought intensity and severity. The drought volumes were divided into a
series of class intervals to develop a duration curve shown in Fig. 4 to
illustrate the frequency of occurrence of droughts.
