5. METHODS: MODELLING IMPACTS OF CLIMATE CHANGE ON THE
5.2 Model Input
7.1.7 Results D: Uncertainty analysis of projected changes in long
futures
The general consensus from the selected GCMs is one of concurrence with the hypothesis that long duration design rainfall will increase in the intermediate and more distant futures, as shown in Figures 7.8 and 7.9, respectively. Furthermore, it appears that there is more confidence in the hypothesis for the more distant future climate scenario than that for the intermediate future. It should be noted, however, that the area displaying medium confidence (shaded blue in the maps) in the distant future may be misleading (cf. Section 7.1.6) as this is the result of only two out of four GCMs in agreement with the hypothesis, compared to the three out of five required for medium high confidence in the intermediate future scenarios.
For the intermediate future, Figure 7.8 shows that there is an increase in confidence, in a westward direction, that one day duration design rainfalls will increase in magnitude. This observation is no longer evident for the three day duration, and appears to reverse for the seven day duration, for which an increase in concurrence in an eastward direction is shown. For the more distant future, although lacking the apparent westwards increase in concurrence at the one day duration, which was evident for the intermediate future, a similar spatial trend seems to follow with an increase in concurrence in an eastward direction as the event duration is increased (Figure 7.9).
Within a particular scenario there are two trends that stand out. Firstly, the area of the Orange River Catchment with medium (or higher than medium) confidence generally increases from the one day duration through to the seven day duration, i.e.
the longer the duration of the design rainfalls, the greater the confidence that they will increase. Secondly, the area of the Orange River Catchment with medium (or higher) confidence appears to decrease as the return period increases, i.e. the higher the return period, the lower the levels of confidence that the long duration design rainfall will increase. This is confirmed by the statistics in Table 7.5, where it is also shown that these trends are present for the percentage area with high confidence (or
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Figure 7.8Levels of confidence in the hypothesis that long duration design rainfalls of increasing duration (left to right) and increasing return period (top to bottom) will increase in the intermediate future, derived from downscaled daily rainfall output from multiple GCMs
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Figure 7.9Levels of confidence in the hypothesis that long duration design rainfalls of increasing duration (left to right) and increasing return period (top to bottom) will increase in the more distant future, derived from downscaled daily rainfall output from multiple GCMs
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greater). An analysis of the results in Table 7.6, which shows the projected direction of change based on the agreement of three or more GCMs, indicates that while confidence in the hypothesis increases with increasing duration and decreasing return period, the opposite occurs for projected decreases in design rainfall, while the area that has no projected changes is little affected.
Table 7.5 Percentage area of the Orange River Catchment with medium (or greater) and high (or greater) confidence in the hypothesis that long duration design rainfalls will increase in the intermediate and more distant futures, derived from computations using downscaled daily rainfall output from the five GCMs used in this study
Attribute Percentage Area of the Orange River Catchment Intermediate Future More Distant Future Confidence Return Period One
Day
Three Day
Seven Day
One Day
Three Day
Seven Day
Medium + 2 Year 85.8 90.5 99.4 98.3 98.1 99.9
Medium + 10 Year 75.0 83.5 99.7 86.4 90.5 99.6
Medium + 50 Year 63.4 69.3 95.9 76.7 81.7 97.3
High + 2 Year 57.6 57.6 81.0 84.6 85.1 93.3
High + 10 Year 38.1 50.0 80.7 63.9 61.1 85.9
High + 50 Year 24.2 33.2 74.4 41.2 47.3 77.4
Table 7.6 Percentage area of the Orange River Catchment projected by > 50% of the GCMs used in this study to experience an increase, decrease, or no change in long duration design rainfalls in the intermediate and more distant futures
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
Decrease 4.1 3.0 0.0 1.0 0.7 0.0
Increase 85.8 90.5 99.4 84.6 85.1 93.3
Same 0.6 0.3 0.0 0.1 0.1 0.0
Inconclusive 9.5 6.2 0.6 14.3 14.1 6.7
10 Year
Decrease 12.4 10.1 0.1 5.6 2.5 0.1
Increase 75.0 83.5 99.7 63.9 61.1 85.9
Same 1.1 0.3 0.0 0.1 0.1 0.0
Inconclusive 11.5 6.1 0.2 30.4 36.3 14.0
50 Year
Decrease 23.6 17.9 1.8 17.2 11.7 1.5
Increase 63.4 69.3 95.9 41.2 47.3 77.4
Same 0.4 0.1 0.0 0.1 0.0 0.0
Inconclusive 12.6 12.7 2.3 41.5 41.0 21.1
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Since it was decided that more than half of the GCMs – i.e. at least three GCMs for the intermediate and more distant future scenarios – had to agree in order to allocate a direction of change, the limitations of only having four GCMs for the more distant future climate scenario become immediately apparent in Table 7.6, where it appears that more of the Orange River Catchment is projected to experience increases in design rainfalls in the intermediate future than in the more distant future. This is contrary to the findings using output from the ECHAM5/MPI-OM GCM (cf. Figures 7.3 and 7.4). Furthermore, it is evident that the percentage area of the Orange River Catchment for which a direction of change is inconclusive is shown to increase for the more distant future scenario. This is likely to be the result of only having four GCMs for the more distant future analysis.
It was shown above that the level of concurrence with the hypothesis, viz. that design rainfalls would increase in the future, decreases with return period. Table 7.7 shows that, regardless of the direction of projected change, there is less agreement between the GCMs when the return period is increased. This can also be seen diagrammatically in Figures 7.8 and 7.9, which show that the spatial variation of confidence becomes increasingly erratic with increasing return period.
Table 7.7 Percentage area of the Orange River Catchment with various levels of GCM agreement on projecting a direction of change in long duration design rainfalls in the intermediate and more distant futures*
Attribute Percentage Area of the Orange River Catchment Intermediate Future More Distant Future Return Period Agreement of GCMs One
Day
Three Day
Seven Day
One Day
Three Day
Seven Day
2 Year
5 GCMs 15.9 17.0 35.5
4 GCMs 41.9 40.8 45.4 40.7 36.9 55.9
3 GCMs 32.6 36.0 18.5 44.9 49.1 37.4
High Confidence + 57.8 57.8 81.0 85.6 85.9 93.3
10 Year
5 GCMs 5.9 9.3 34.0
4 GCMs 33.7 42.6 46.7 23.9 23.5 50.8
3 GCMs 49.0 42.0 19.1 45.8 40.2 35.2
High Confidence + 39.6 51.9 80.7 69.6 63.8 86.0
50 Year
5 GCMs 3.1 5.0 29.9
4 GCMs 27.0 32.2 44.5 9.9 17.1 36.0
3 GCMs 57.4 50.2 23.3 48.6 41.9 42.9
High Confidence + 30.1 37.2 74.4 58.5 59.0 78.9
* Shaded area implies non-applicable case
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These findings may be indicative of the uncertainty surrounding the ability of the GCMs to simulate extreme events. Furthermore, these results indicate that the 20 year simulation period may be too short for design rainfall estimations to display high confidence, particularly when extrapolating to design rainfalls for return periods higher than 20 years.
There is, however, an increase in agreement between the GCMs as the duration of the design rainfalls increase beyond one day. This is hypothesised to be related to the GCMs’ reduced ability to simulate daily events, particularly for extremes, a problem which largely self-corrects when rainfalls are accumulated over a number of days.
7.1.8 Results E: Uncertainty analysis of projected changes in short duration