3.3 The Relationship Between Water Supply and Demand
4.2.3 The major factors influencing the risk associated with environmental hazards in South Africa
In many areas throughout the world the risks associated with environmental hazards are increasing. This is a result of escalation in the probability of occurrence of extreme events as well as in an increase in the losses associated with such events (Figure 4.2). These changes in risk can either be attributed to anthropogenically induced changes to the physical system, natural changes in the physical system or to changes in the vulnerability of the human environment interacting with the physical system.
4.2.3.1 Changes occurring within the physical system
Changes in the physical system tend to influence the probability of occurrence of extreme events. This implies that they either shift the mean, or change the variability, of events associated with hazard-causing phenomena.
In South Africa climate change and anticipated escalated temperatures due to the enhanced greenhouse effect could result in higher rainfall variability, increased evaporative demand and less rainfall than at present (Shackleton et al., 1996; Schulze and Perks, 2000). In drier areas the higher evaporative demand due to increased temperatures could result in enhanced desiccation, leading to greater aridity in such areas. Increases in global mean temperatures are expected to augment the water holding capacity of the air. This, in turn, could theoretically increase average amount of precipitation and result in a greater number of extreme flood producing rainfall events (Schulze, 1997a). Increases in the average SSTs could result in an intensification of the severity and a greater number of tropical cyclones. Increases in the number of tropical . cyclones passing down the Mozambican channel could also significantly increase the number of flood producing rainfall events in the eastern part of South Africa. Increases in the SSTs could also result in higher average sea levels, increasing the number of coastal flooding events. These factors combine to produce more flood hazards ona global scale (IPCC, 1996b). In South Africa an increase in the number of both dry and wet extreme
events could thus increasing exposure to both flood and drought hazards (Schulze, 1997a; Perks, 2001).
In South Africa population pressure and industrial development are changing the physical catchment characteristics in many areas. Land use changes resulting from development change the physical aspects of an area, which result in changes in the physical response to different hazard causing phenomenon (Kienzle et al., 1997). South Africa is experiencing a high population growth rate, 2.4% between 1970 and 1995 (Anon, 1996), coupled with rapid urbanisation. The urban population expanded from 13.7 million in 1980 to 21.3 million in 1995 at a rate of over 3% per annum (Calitz, 1991; Calitz and Grove, 1991). These two factors are combining to cause land use changes in many areas throughout South Africa.
Urbanisation and resulting development increases imperviousness and decreases the hydrograph response times of catchments, resulting in the quicker onset and increased magnitude of flood events. Hence, in many areas of South Africa rapid urban development is increasing the probability of occurrence of greater magnitude flood events. Van Beek (1993) estimated that approximately 1 100 informal houses had been established below the 1:50 year flood plain near the Jukskei river in the Alexandra township, as a result of high population pressure and a high rate of urbanisation. In this area alone, it was estimated that more than 6000 people are vulnerable to a flood hazard.
In rural areas, population pressures are placing increased demand on the natural resources with many marginal areas are now being cultivated, resulting in land denudation which is exacerbated by overgrazing and poor farming practices (Kienzle et al., 1997).
The decreased carrying capacity of the land increases the severity of damages associated with drought events. Vegetation removal can also increase the hydraulic smoothness of the catchment, by removing flow impeding vegetation and increasing the soil surface compaction, decreasing the time of concentration and increasing flood .
. Commercial farming in South Africa is also under pressure to increase yields to support the growing population. Irrigation agriculture and forestry are major users of water resources in South Africa (Kienzle et al., 1997; Schulze et al., 1990). Afforestation tends to reduce runoff as trees enhance the infiltrability of the catchment by retarding water surface flows and intercepting rainfall as well as trees usually having higher transpiration rates than the natural vegetation they replace (Schulze, 1995). Irrigation, on the other
hand, reduces the streamflow by direct extraction from the source (Schulze et al., 1998).
This could result in increased hydrological drought occurrence to downstream users.
4.2.3.2 Changes in vulnerability
Perhaps the major factor that is causing increased economic and life losses, as a result of hazards is the change in the vulnerability of the human environment. Vulnerabil ity is essentially a combination of resilience to hazard, which is the ability to cope with hazard threats, and reliability of structures, Le. the ability of structures to cope with disaster (UNEP, 2000). Changes in the vulnerability in South Africa can be attributed to changes in resilience as a result of changes in the population dynamics of the country. Poverty, education, culture and age all impact the vulnerability of an individual (S mith, 1996). Large rural to urban migrations that are currently taking place in South Africa are changing the population structures in many communities. Hence, the poverty, education, culture and age structures in such communities are also changing, resulting in the vulnerability changes.
Losses of life due to hazard threats are age dependent, especially in the case of drought and flood disasters. The majority of the loss of life is restricted to those people who lack the physical strength to withstand the initial disaster onset, e.g. swimming in the case of floods, or the physical robustness to withstand the results of the disaster, e.g. the ability to fight off post-event disease (Smith, 1996). The population in South Africa grew at a rate of 2.06% per annum between '1991 and 1995 (Anon., 1996). High growth rates tend to create a population distribution of predominantly young people (Figure 4.3). Over 10% of the population is below the age of 5, while more than 40% of the population is below the age of 20 (Orkin, 1999). With a predominantly young population a large number of people in the vulnerable category are exposed to hazard threats. This means that the vulnerability to hazardous events in the country as a whole increases. Whiteside and Sunter (2000) performed a study in KwaZulu-Natal having a look at the impact of AIDS. Population projections showed that without the AIDS epidemic population would grow at an average of 1.76% between 1996 and 2010. However, if the impact of the AIDS epidemic was considered the population in KwaZulu-Natal would grow at an average rate of 0.58%, with negative growth being experienced in the years 2008 to 2010 (Whiteside and Sunter, 2000). Assuming these trends could be extended to the rest of the country the population growth could slow considerably from those shown in previous census data.
The AIDS epidemic also has the potential to affect the population dynamic influencing the