Geoscience Reference
In-Depth Information
see whether their characteristics conform to the respective climatic zones
by plotting average monthly rainfall against calendar months; and com-
paring whether the resulting pattern is similar for all stations in the same
climatic zone. The station with the longest record and least missing data,
which is the most reliable station, was chosen for the analysis.
4
Masindi
Meteorological station was chosen for Masindi district, where as Kalisizo
Meteorological station was chosen for Rakai district.
The missing data records were in-filled using the Normal Ratio method
and the data were extended using the Markov Generation Techniques.
5
The data were then checked for homogeneity using the Double Mass Curve
which revealed consistency in the sets of data. The climate trends of the two
regions were established from the rainfall and temperature patterns of the
regions. Drought analysis was done using two methods namely Standardized
Precipitation Index (SPI) method and the Drought Volumes Method.
3.1.
Standard precipitation index method
6
The Standard Precipitation Index (SPI) value is defined as the ratio of the
difference between the measured rainfall and the long-term mean to the
standard deviation for any month (as shown in Eq. (1)).
SPI = (
R
t
−
M
t
)
/σ
t
,
(1)
where
σ
t
is the standard deviation for the month,
R
t
and
M
t
are the mea-
sured monthly and long-term mean monthly rainfall respectively.
Positive SPI values indicate greater than mean precipitation, and nega-
tive values indicate less than mean precipitation thus a deficit. The drought
magnitudes were calculated as the positive sums of the SPI values that are
continuously negative over consecutive months.
3.2.
Drought volumes method
7
Drought volumes were calculated by cumulating consecutive rainfall deficits
(Eq. (2)). Rainfall deficits were the differences between effective precipita-
tion and the mean monthly rainfall. The drought volumes were then used
to calculate the respective drought intensities (Eq. (5)), severity indices
(Eq. (7)) and return periods (Eq. (8)).
D
V
=
(
E
t
−
M
t
)
i
,
(2)
i
=1
E
t
=(
R
t−
1
−
M
t−
1
)
∗
W
t
+
R
t
,
(3)
