5 METHODOLOGY
5.4 Validation of Climate and Streamflow Simulations
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1. The period 1971 to 1990 was cropped from the historical period 1950 to 1999.
2. The daily rainfall values were seperated into monthly aggregates, also taking into account leap years.
3. The daily rainfall values occurring during each month were then summed to obtain a monthly rainfall value.
4. Rainfall values for each individual month were then summed for the 20 year period.
5. The sorted rainfall outputs from each of the five GCMs for the present period were then tabulated with rainfall output cropped from the historical period.
6. This facilitated a graph of the historical vs simulated accumulated rainfall.
A similar procedure for the relative error analysis was repeated in order to compute the accumulated streamflow and sediment yield values at the exits of the 10 selected catchments for the same period, the results of which are shown in the following sub-section.
5.4.2 Validation of Rainfall, Streamflow and Sediment Yield, by Relative Error Analysis A comparison between historical and simulated present rainfall, streamflow and sediment yield values was undertaken for each of the 10 selected catchments. In this sub-section, Figure 5.4.2.2 highlights the accuracy of rainfall, streamflow sediment yield outputs from three of the 10 selected catchments. These three catchments cover three major climatic regions, with the Groen representing the winter rainfall region, the Krom the all year and the Thukela the summer rainfall region. Results from the other seven catchments are given in Appendix A. Figure 5.4.2.2 illustrates the tendency of four of the five GCMs to under- estimate rainfall, the exception being the GISS GCM which is known to over-estimate rainfall in the eastern regions of South Africa (Schulze, 2010a). These errors, expressed as ratios and then converted to percentage values, illustrated in rainfall estimations are amplified in higher order hydrological responses, such as streamflow (cf. also the results in Appendix A). These errors are still further amplified by the estimation of sediment yield, the reason being that both peak discharge and stormflows are utilised in the MUSLE calculation of event-by-event sediment yield (cf. Sub-section 5.3.3). In addition, it is illustrated that certain GCMs offer more accurate simulations in different regions.
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Based on this validation for each of the 10 selected catchments it was assumed that simulations of future periods, i.e. for the intermediate (2046 – 2065) and distant (2081 – 2100) future, would have similar levels of accuracy. From this validation study it is illustrated that the output from most GCMs is reasonably accurate. In order to confirm that this is the case for higher order hydrological variables, an additional validation of streamflow by regression analysis has been undertaken and is described in the following section.
GROEN CATCHMENT
Historical vs Simulated Present Climate Scenarios; Baseline Land Cover; Multiple GCMs Comparison of Mean Annual Rainfall, Streamflow and Sediment Yields
0 10 20 30 40 50 60 70 80 90 100 110
CCC CRM ECH IPS GISS
GCM
Relative Error (%)
RFL SFL SEDYLD
KROM CATCHMENT
Historical vs Simulated Present Climate Scenarios; Baseline Land Cover; Multiple GCMs Comparison of Mean Annual Rainfall, Streamflow and Sediment Yields
0 20 40 60 80 100 120 140
CCC CRM ECH IPS GISS
GCM
Relative Error (%)
RFL SFL SEDYLD
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THUKELA CATCHMENT
Historical vs Simulated Present Climate Scenarios; Baseline Land Cover; Multiple GCMs Comparison of Mean Annual Rainfall, Streamflow and Sediment Yields
0 50 100 150 200 250
CCC CRM ECH IPS GISS
GCM
Relative Error (%)
RFL SFL SEDYLD
Figure 5.4.2.2: Relative errors (%), in GCM derived rainfall, streamflow and sediment yield for selected estuaries in the winter rainfall region (Groen; semi-arid), the all year rainfall region (Krom; sub-humid) and the summer rainfall region (Thukela, sub-humid), with the
values derived from the historical climate data for the same period used as the reference
5.4.3 Validation of Streamflows by Regression Analysis
The validation of streamflow output at the exit Quinary of each of the 10 selected catchments is imperative in estuary studies as it demonstrates firstly, the integrity and accuracy of the simulations undertaken with output from GCMs and secondly, the value of the results for management decisions.
The median monthly streamflow values for the exit Quinary of each of the 10 selected catchments were obtained for both the GCM derived present period and the same historical period. Median values mute the effects of extreme outlier values. Using these monthly streamflow values, scatter plots were created and regressions and the equations for each of the five GCMs for each of the 10 selected catchments were derived, as shown in Figure 5.4.3.1.
The scatter plots making up this Figure 5.4.3.1 illustrate that:
• streamflows are all under-simulated in the western regions (Berg, Buffels, Groen, Olifants),
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• streamflows are both under- and over-simulated in the southern regions (Breede, Klein, Krom), and
• streamflows are over-simulated in the eastern regions (Mdloti, Mlalazi, Thukela).
Additionally, these figures illustrate the signifcant over-simulation of streamflows when climate output from the GISS GCM is used in the eastern catchment where it is known to over-simulate rainfall (eg. Mdloti, Mlalazi and Thukela). As a consequence of this over- simulation, GISS was excluded from the more detailed simulations of the Klein catchment.
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Figure 5.4.3.1: Scatter plots of streamflows at the exit Quinaries of the 10 selected catchments, with derived using outputs from 5 GCMs for the present period (1971 – 1990)
and output using historical data for the same period
This validation study shows that the GCM output when used to generate streamflows show marked regional variation in accuracy. However, as the impact assessments in estuarine inflows in this study are all ratio based, many of the apparent errors will be self-correcting if it is assumed that GCM errors for the present period are transferred into future scenarios.
Whether or not that is so remains untested.