Geography Reference
In-Depth Information
Table 11.12. Reservoir surface areas (ha) classified from ENVISAT ASAR images
Reservoir size (ha)
Date
1
2
3
4
5
6
7
8
21 May 2005
5.5
2.9
3.5
6.6
6.6
9.4
4.3
3.6
6 June 2005
5.8
3.7
3.1
7.8
3.3
10.2
25.0
4.1
24 June2005
6.1
4.7
3.7
10.0
12.6
12.9
26.3
4.8
11 July 2005
8.2
9.0
4.8
9.8
13.4
14.6
28.2
8.2
29 July 2005
8.8
10.9
7.2
10.2
17.0
14.1
29.5
8.6
15 August 2005
8.3
11.2
5.6
12.9
17.6
15.0
29.4
8.8
13 June2006
4.9
2.2
5.4
9.9
6.0
4.0
1.8
4.1
29 June 2006
5.1
2.3
5.3
9.9
6.6
3.9
2.4
4.3
11 July 2006
5.0
2.4
5.5
10.1
7.0
4.1
1.8
4.6
30 July 2006
8.0
5.9
7.2
13.7
10.9
12.6
21.3
7.6
3 August 2006
8.5
6.1
5.5
14.4
11.5
13.1
21.6
7.8
15 August 2006
8.7
7.4
8.9
14.8
14.1
15.1
27.0
8.6
The locations of the reservoirs are given in Figure 11.61
.
surface areas, storage volumes over time were esti-
mated. The resulting time series of reservoir storage
volumes formed the basis for the calibration of the run-
off model.
when reservoir levels rose for the first time. Initially, two
S
max
values were determined with the Thornthwaite
Mather
procedure. The maximum S
max
value was determined
assuming that the first percolation occurred on the date that
the first rise in the reservoir was observed at the time of the
satellite image acquisition. The minimum S
max
values were
calculated assuming that the rise occurred at the preceeding
satellite image acquisition date.
-
Rainfall
The method was applied for the time period extending
from the dry season into the rainy season in 2005 and
2006. In 2005, the rainy season was segmented into three
to five wet periods with distinct dry spells in between. In
2006, there were only three wet periods, which were sep-
arated by only short dry spells. The
In the Thornthwaite
-
Mather model the mean S
max
value was used.
The shape factor value, a, was obtained by visually fitting
predicted reservoir inflow to the observed inflow based on
volume increases determined with satellite images. The
method does not apply once the reservoir is full, because
inflows are then routed through the reservoirs over their
spillways and do not result in an increase in storage.
For each catchment, the calibrated minimum, maximum
and mean values of S
max
and the shape factor value, a, are
reported in
Table 11.13
. The catchments
analysis was
used as an indicator of the strength of the rainy season,
where a day is considered wet when the rainfall in the 10
preceeding days exceeds 34 mm, and rainfall exceeds
potential evaporation on that day. Although the rainfall in
2006 was, on average, only 3% below the rainfall in 2005,
the rainfall patterns were distinctly different. Overall, the
2006 rainy season was shorter and occurred later than the
rainy season in 2005. In 2006 there were 67
'
wet days
'
.
Fifty-nine per cent of rain fell on those days. In 2005, on
average, 51% of the total precipitation fell on 63 wet days.
'
wet day
'
mean S
max
values
range from 25 to 45 mm, and the a values range from 0.01
to 0.08 day/mm. It is remarkable that, for these catchments
that are spread over the northern part of Ghana and western
part of Togo, similar S
max
and a values can describe how
and when the reservoirs fill up.
Figure 11.63
gives an
example of how information is extracted from the time
series of storage volumes.
'
Calibration
The records for the eight reservoirs are divided into two
years. The parameters S
max
and a were calibrated using the
2005 data, and then used in 2006 for validation. In calibra-
tion, the S
max
determines mainly the time of the first
catchment contribution to the reservoir, while a is related
to the total amount of effective rainfall that reaches the
reservoir.
S
max
was determined on the basis of the storage volume
time series, specifically by taking into account the moment
Validation
The 2006 season was used to validate model results.
Figure
11.64
compares the observed reservoir volumes with the
predicted cumulative quickflow (Qf) for the eight catch-
ments. In both years, the reservoirs were spilling water in
August and therefore images taken on 15 August 2005 and
15 August 2006 were omitted from the analysis, because
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