2. CONCEPTS 1: A BRIEF OVERVIEW OF CLIMATE CHANGE AND THE
2.3 Modelling the Impacts of Climate Change
2.3.3 Modelling impacts of climate change on hydrological responses over
review of research up to 2005
The majority of research in South Africa on impacts of projected changes in hydrological responses due to climate change in the period 1996 – 2005 was summarised in reports on the South African Country Study on climate change
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(Schulze and Perks, 2000), a PhD thesis by Perks (2001), in the 20 papers making up the so-called “Thukela Dialogue” edited by Schulze (2003b) and in a major report on scenarios, impacts, vulnerabilities and adaptation to the Water Research Commission by a multi-institutional team, also edited by Schulze (2005a).
According to C-CAM regional climate scenarios (Engelbrecht, 2005), MAP for South Africa was projected to decrease between 5 and 10% of the present, with reductions of up to 25% in the already water stressed areas of the lower Orange River Catchment. Furthermore, the number of dry days was projected to increase over most of the Orange River Catchment, while the number of days producing heavy rainfall, i.e. ≥ 25 mm, was projected to increase in the upper reaches of the catchment; amplifying the risk of increases in sediment yields – and the indirect consequences for water quality (Gleick, 2000; Dennis et al., 2003; DWAF, 2004e;
UNDP-GEF, 2008) – as well as posing heightened flood risks (cf. Section 2.2). This was surmised to be exacerbated by an expected increase in an already high inter- annual variability over most of South Africa (Schulze et al., 2005a).
Changes in the above-mentioned rainfall parameters were modelled to change soil moisture storage, runoff processes and groundwater recharge which, in turn, was expected to affect the amount of water available to the various water-using sectors, viz. domestic, environment, industry, agriculture and recreation (Schulze et al., 2005a).
Generally, changing patterns in runoff were consistent with those identified for precipitation (IPCC, 2001b). This was consistent with the results of Schulze and Perks (2000) who found that the mean accumulated streamflows over most of the Orange River Catchment would decrease in a future climate, with the worst effects felt in the lower reaches of the catchment (Figure 2.5). However, more recent results published by Schulze (2005c) indicated that the situation was not necessarily going to be so dire, with some areas in the lower Orange River Catchment projected to experience an increase in mean annual accumulated streamflows of up to 4 times the present amount. Furthermore, mean annual stormflow – the water that contributes to streamflows, which is generated from a specific rainfall event, either at or near the surface, i.e. does not include baseflow – was projected to increase in
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many parts of the Orange River Catchment. This was considered important, as it is largely from stormflow events that reservoirs are filled (Schulze, 2005c).
Figure 2.5 Simulated relative changes in mean annual accumulated streamflows in the Orange River Catchment, using an older GCM (Schulze and Perks, 2000)
Future projections of the global climate indicated that precipitation patterns were changing, thus leaving the future probability of deep percolation to recharge aquifers an area of marked concern for some of the country (Cavé et al., 2003). Shallow, unconfined aquifers along floodplains in semi-arid and arid regions are recharged by seasonal streamflows and can be depleted directly by evaporation (Appleton et al., 2003). Projected increases in evaporation and potential increases in the concentration of streamflow in many of the rivers in the Orange River Catchment (Schulze et al., 2005a) could thus lead to reduced groundwater recharge. This is particularly concerning for the lower Orange River Catchment where a large proportion of the domestic and agricultural water supply is derived from groundwater (cf. Section 1.2.2).
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This chapter has provided an overview of climate change by describing the concept of climate change and by alerting the reader to the projected impacts thereof.
Furthermore, approaches adopted for climate change impacts studies have been discussed, with particular reference to results from such approaches that have been conducted in South Africa and the Orange River Catchment.
What is clear from the assessment of potential climate change impacts studies on hydrological responses in South Africa is that most climate change research in South Africa has been focussed on water resources and agriculture, with very little reference to hydro-climatic risk management, particularly with regard to modelled studies of floods and droughts. Where reference has been made to changes in design rainfall, for example WRC Report 1430/1/05 (Schulze, 2005a), very little was said about the projections. Furthermore, in that same study, references to changes in design floods were based on shifts in climate zones rather than simulated floods.
This study, which focuses on modelling the impacts of climate change on hydro- climatic hazards, therefore fills an important knowledge gap.
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3. CONCEPTS 2: RISK MANAGEMENT – HAZARD, VULNERABILITY, RISK AND UNCERTAINTY
The Orange River Catchment has been shown already to display characteristics such as low rainfall in places, high evaporative demand, and overall high climatic variability, which render it a high risk environment (cf. Section 1.2). Furthermore, it has been indicated in Chapter 2 that this situation may be exacerbated by climate change. This section provides an overview of the concepts of risk and risk management, as well as their respective components.
A plethora of definitions of risk may be found in the literature, and a selection is shown in Box 3.1. Imbedded within many of the definitions of risk are the terms hazard and vulnerability. Cardona’s (2003) definition of risk introduces the mathematical concept of convolution, which, in this case, refers to the concomitance and mutual conditioning of hazard and vulnerability. If there is no hazard it is not feasible to be vulnerable, when seen from the perspective of the potential damage or loss due to the occurrence of an event. In the same way, there is no situation of hazard for an element, or system, if it is not exposed, or vulnerable, to the potential event (Cardona, 2003). In order to understand the concept of risk it is necessary to understand the components of risk. Therefore, the terms hazard and vulnerability are described next, before returning to broader issues surrounding risk and risk management.