2. CONCEPTS 1: A BRIEF OVERVIEW OF CLIMATE CHANGE AND THE
2.2 Impacts of Climate Change on the Hydrological Cycle
Spatial and temporal changes in hydrological responses are determined by changes in temperature, evaporation and precipitation; the latter considered to be the most important as it is precipitation that generally induces the critical changes in catchment responses (Chiew, 2007). Increasing global temperatures will have profound effects on evaporation, which in turn affects atmospheric water storage and hence magnitudes, frequencies and intensities of rainfall events as well as the seasonal and geographic distribution of rainfall and its inter-annual variability (Kabat et al., 2003).
Some of these climate change impacts on hydrological processes may have already been observed (IPCC, 2007a). Figure 2.4 shows that, although the number of disasters reported that are associated with geophysical events, such as earthquakes and volcanic eruptions, have remained remarkably constant, those associated with hydro-meteorological events, particularly storms and floods, have increased significantly.
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Figure 2.3 Comparison between global mean surface temperature anomalies (°C) from observations (black) and GCM simulations forced with (a) both anthropogenic and natural forcings, with the multi-model ensemble mean shown as a thick red curve and individual simulations shown as thin yellow curves; and (b) natural forcings only, with the multi-model ensemble mean shown as a thick blue curve and individual simulations shown as thin blue curves. Vertical grey lines indicate the timing of major volcanic events (IPCC, 2007b)
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Figure 2.4 A schematic of globally reported hydro-meteorological and geophysical disasters from 1900 - 2008 (EM-DAT, 2009)
Climate change impacts may also have already been observed at a more local scale.
Some examples are given below:
• Mason et al. (1999) identified significant increases in the intensity of extreme rainfall between 1931 - 1960 and 1961 - 1990 over approximately 70% of South Africa. The intensity of the highest rainfall in 10 years has increased by over 10% over vast areas of the country, except in parts of the northeast, northwest and in the winter rainfall region of the southwest of South Africa (Mason et al., 1999). Percentage increases in the intensity of high rainfall
Droughts Earthquakes Floods Mass Earth Movements (Dry/Wet)
Storms Volcanic Eruptions
Number of Natural Disasters Reported (1900 – 2008)
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events were found to be largest for the most extreme events (Mason et al., 1999).
• In agreement with the above findings, New et al. (2006) analysed climate data from 14 south and west African countries over the period 1961 - 2000 and found strong trends in the regional climate, most notably with respect to changes in secondary attributes of rainfall such as intensity and frequency, but also with respect to dry spell duration.
• Furthermore, an analysis of historical precipitation trends by Hewitson et al.
(2005c), which used robust regression with an interpolated 0.1º gridded precipitation data set that draws on over 3 000 station records across South Africa, identified the following broad regional characteristics:
o Increases in the late summer dry spell duration for much of the summer rainfall region.
o Arid zones, in general, receiving more days on which rain fell.
o Contrasting changes in the winter rainfall region, with mountainous regions receiving more rain days per month and increased totals, while the neighbouring coastal plain regions displayed the reverse.
• On the hemispheric scale Hewitson and Crane (2006) note that there are also indications that the El Niño-Southern Oscillation teleconnection to southern Africa may be weakening (Landman and Mason, 1999; Sewell and Landman, 2001). Since this teleconnection may not exist in future climates its use for climate prediction should be avoided (Hewitson and Crane, 2006).
Although climate change is expected to affect many of the natural and man-made sectors of the environment (Ringius et al., 1996), change in water availability is considered to be one of the most critical factors associated with climate change impacts (MacIver, 1998; Hardy, 2003). There is a sensitive, non-linear relationship between rainfall and runoff, where a small change in rainfall causes considerable effects in runoff (IPCC, 2001b; Schulze, 2003a). Arid and semi-arid regions, such as the Orange River Catchment, are particularly sensitive to changes in rainfall owing to the small fraction of water that runs off or percolates to the groundwater (Schulze, 1997a).
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Potential impacts of climate change on surface water supply include (Schulze, 2003a):
• Changes in the seasonality of streamflows, which might affect supply and demand for various sectors, e.g. irrigation or domestic, with knock-on effects on water pricing/licensing and effects on water resources infrastructure, including sizing of reservoirs, curtailment rules, or reservoir maintenance.
• Changes in streamflow variability at inter- and intra-annual time scales, including effects on vegetation dynamics and resultant hydrological responses, the regional amplification of variability and persistent sequences of flows above or below selected thresholds.
• Changes in magnitudes and frequencies of extreme events related to both floods and droughts.
• Effects of land use on water availability, mainly through alterations in the partitioning of rainfall into stormflow and baseflow.
Potential impacts of climate change on higher order hydrological responses, such as changes in water quality, also need to be considered (Schulze, 2003a). Water quality changes that might be expected are alterations in the physical, chemical and biological status of the water systems (Ashton, 1996). All soil erosion studies have suggested that increased rainfall intensities and flooding will result in greater rates of erosion (IPCC, 2007b) and, therefore, higher sediment yields. Erosion rates are expected to change primarily due to changes in rainfall and streamflow erosivity, but also due to the knock-on effects of the impacts of climate change on land cover and land use (Tucker and Slingerland, 1997; IPCC, 2007b). Increases in sediment yields may have repercussions for water quality (Gleick, 2000; Dennis et al., 2003; DWAF, 2004e; UNDP-GEF, 2008) as sediments have the capacity to bind with nutrient chemicals (e.g. phosphates) and industrial toxins (Newson, 2009). Furthermore, increases in sediment yields may increase the rate of sedimentation of river channels and reservoirs (Takeuchi, 2002; Newson, 2009), leading not only to a reduction in water storage capacity, and hence water supply, but also to increased flood risks.
Climate perturbations through the hydrological system may result in potential changes in transboundary water interests and conflicts, where rivers form international boundaries, or especially where rivers discharge downstream from one
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country to another (Schulze, 2003a), as is the situation in the Orange River Catchment (cf. Chapter 1). Furthermore, there may also be changes in water issues to the poor, who often live either on floodplains, which may become more prone to flooding in the future, or alternatively live along watersheds, where presently ephemeral streams may become even more so in future (Schulze, 2003a).
In order to project future climate trends, or to validate various assumptions on the potential impacts of climate change, such as those presented above, scientists employ the use of complex atmospheric models. The following section, therefore, provides an overview of modelling the impacts of climate change.