5. METHODS: MODELLING IMPACTS OF CLIMATE CHANGE ON THE
5.2 Model Input
7.1.3 Results B: Projected changes in long duration design rainfalls with
From the five climate models available for this study, the ECHAM5/MPI-OM GCM has been selected to develop techniques for the analysis of projected changes in long duration design rainfall, for intermediate and distant future climate scenarios.
The ECHAM5/MPI-OM GCM provides “middle of the road” estimates of future climate projections relative to the other four available GCMs (CSAG, 2008), as already mentioned in Section 6.1. Figures 7.2 and 7.3, respectively, show the projected ratio changes for the intermediate and distant futures in long duration design rainfalls for selected return periods and durations, from the ECHAM5/MPI-OM GCM.
The spatial distribution of the projected changes for the one day, 2 year return period in the intermediate future (Figure 7.2: top left) shows the eastern parts of the Orange River Catchment tending towards a slight increase in design rainfalls. This increase forms part of a band of projected increases that extends down the east coast of South Africa. However, extending from the northeast of South Africa, across a vast area of the Vaal component of the Orange River Catchment (i.e. the northeastern region) and the southern regions of the catchment, to the southwest of South Africa, is a band of predominantly decreasing design rainfalls. West of this band, in the semi-arid parts of the Orange River Catchment, a mixed picture of increasing and decreasing design values is displayed, but with the increases greater in magnitude than the decreases.
Moving from the one to three day duration, i.e. from the top left to the top centre map of Figure 7.2, indicates that the spatial pattern of design rainfalls changes with event
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Figure 7.2Ratios of intermediatefuturetopresent designrainfalls for selectedreturnperiods anddurations, derivedfrom downscaled daily rainfall output from the ECHAM5/MPI-OM GCM
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Figure 7.3Ratios of moredistant futuretopresent designrainfalls for selectedreturnperiods anddurations,derivedfrom downscaled daily rainfall output from the ECHAM5/MPI-OM GCM
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duration. The central band of decreasing design rainfalls that was apparent for the one day duration is less distinct, with many of the Quinary Catchments that were showing a decrease now showing no change or even a slight increase in design rainfalls. This is an intensification of an increasing trend projected in the east of South Africa, which seems to be expanding westwards. This trend continues on to the seven day duration, for which most of the catchment shows increasing design rainfalls, however, with a significant portion of the Orange River Catchment west of the Vaal Catchment still showing decreasing design rainfalls. This trend of projected increases in design rainfalls, in terms of magnitude and spatial extent, is present for all return periods.
Moving down the left-hand side of Figure 7.2, i.e. from the 2 to the 10 and 50 year return periods for the one day duration, it can be seen that, while the area that will experience increases in design rainfalls is shrinking, the spatial distribution of design rainfalls becomes increasingly less distinct. Furthermore, the shades of reds and blues are getting darker with increasing return period, indicating that the magnitude of change is increasing, both positively and negatively. Although these trends are apparent for all design durations, they are most prominent for the lower return periods.
Shifting the focus to the more distant future (Figure 7.3), it is immediately noticeable that a greater area of the Orange River Catchment is projected to experience increasing design rainfalls when compared to the intermediate future projection. As was apparent for the intermediate future, the cumulative area projected to experience higher design rainfalls decreases with increasing return period, while the magnitudes of projected changes increase.
One trend that was apparent for the intermediate future (Figure 7.2), but is not as clear for the more distant future scenario (Figure 7.3), is the increase in the area projected to experience higher design rainfalls with increases in the duration of the design event (i.e. one to three to seven day flood producing rainfalls). The ratio change between the one day and three day duration is barely noticeable in Figure 7.3; however, the increase in the area projected to experience greater design rainfalls does become apparent between the three and seven day durations.
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These findings drawn from Figures 7.2 and 7.3 are confirmed by results presented in Table 7.1, where it is also shown that, although the area of the Orange River Catchment that is projected to experience greater design rainfall decreases with increasing return period, the area projected to experience changes in design rainfalls in excess of 30% increases.
Table 7.1 Percentage area of the Orange River Catchment projected to experience an increase, decrease, or no change in long duration design rainfalls in the intermediate and more distant futures, derived from computations using downscaled daily rainfall output from the ECHAM5/MPI-OM GCM
Attribute Percentage Area of the Orange River Catchment Intermediate Future More Distant Future Return
Period Direction of Change One Day
Three Day
Seven Day
One Day
Three Day
Seven Day
2 Year
No Change 11.3 11.1 8.8 6.7 4.0 3.5
Decrease 42.2 31.0 24.5 16.1 18.0 13.9
Increase 46.5 57.9 66.7 77.2 78.0 82.6
Decrease > 30% 0.0 0.0 0.0 0.0 0.0 0.0
Increase > 30% 0.5 1.6 1.6 3.9 3.9 6.8
10 Year
No Change 7.1 7.9 7.1 5.5 7.1 4.6
Decrease 48.6 42.9 24.3 29.3 32.9 13.9
Increase 44.3 49.2 68.6 65.2 60.0 81.5
Decrease > 30% 2.1 1.0 0.3 0.0 1.2 0.4
Increase > 30% 1.4 1.5 7.7 9.1 9.9 15.0
50 Year
No Change 6.8 5.2 4.3 6.6 4.0 3.9
Decrease 47.4 51.9 29.0 37.1 39.5 20.9
Increase 45.8 42.9 66.7 56.3 56.5 75.2
Decrease > 30% 7.9 6.6 2.4 4.1 4.7 2.4
Increase > 30% 7.6 8.3 26.8 14.7 16.5 29.1
Of the above results, it is the increases that occur in the eastern parts of the Orange River Catchment that are most concerning to water managers in the future as this is where most of the rainfall occurs (cf. Figure 6.4) and where the current design values are already the highest in the catchment (cf. Figure 7.1). Even where there are increases in design rainfalls projected in the west, they are only of the order of 10 - 20%, which when considered with the relatively small design values calculated from the historically observed data, results in small increases in the design values.
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