Evaluation of potential changes in hydrologically relevant statistics of rainfall in Southern Africa under conditions of climate change&lt;sup&gt;#&lt;/sup&gt;

Tg Lumsden

doi:10.4314/wsa.v35i5.49190

Abstract

Scenarios of present, intermediate and future climates for Southern Africa were analysed to evaluate potential changes in hydrologically relevant statistics of rainfall that could be observed this century as a result of climate change. These climate scenarios were developed in previous studies by applying empirical downscaling techniques to relatively coarse-scale climate scenarios simulated by general circulation models (GCMs) as part of the Intergovernmental Panel on Climate Change 3rd and 4th Assessment Reports (TAR and AR4, respectively). The regional climate scenarios were available at a daily time-step and for a spatial grid resolution of 0.25° over Southern Africa, comprising South Africa, Lesotho and Swaziland. In the study, the regional climate scenarios were related to the 1946 quaternary catchments in the region since the possible hydrological impacts of climate change will ultimately be assessed explicitly by applying the regional climate scenarios in a daily time-step hydrological model. The analysis of potential changes in hydrologically relevant rainfall statistics was qualitative in nature and focused on determining where convergence exists amongst the different climate models with respect to changes in rainfall, and what the likely hydrological implications would be for the region. According to all of the GCMs evaluated in the study, more rainfall is projected for the east of the region. The greater rainfall projected for the east would be in the form of more rain days and more days with bigger rainfalls. If these scenarios are correct, the combination of wetter antecedent conditions and larger rainfall events would result in more runoff being generated and this would have implications for, inter alia, filling of dams and water quality. According to all of the GCMs evaluated, less rainfall is projected along the west coast and the adjacent interior, with the possibility of a slight increase in inter-annual variability. If correct, this would result in a decrease in flows and an increase in flow variability, since changes in precipitation are amplified in the hydrological cycle. As convergence in climate-change scenarios becomes apparent, there is now an arguable basis for developing appropriate response strategies for incorporation into adaptation policy. Perhaps one of the greatest challenges in this regard is now to explore the issues of uncertainty and probability in order to develop a more rigorous basis to enable proactive responses.

Highlights

A focus on potential impacts of climate change on the water sector of Southern Africa was triggered by a series of activities and events in the first few years of the new millennium which included the South African Country Study on Climate Change, the World Summit on Sustainable Development, the Intergovernmental Panel on Climate Change (IPCC) reports in 2001, the 3rd and 4th World Water Forums, as well as active South African participation in the International Geosphere-Biosphere Programme and the International Dialogue on Water and Climate, among others
The general circulation models (GCMs) used to develop the global climate scenarios which were downscaled to a quarter degree resolution spatial grid for Southern Africa for application in this research, included 3 from the IPCC 3rd Assessment Report (IPCC, 2001), and 3 from the World Climate Research Programme’s Coupled Model Intercomparison Project Phase 3 (CMIP3) multi-model dataset used in the IPCC 4th Assessment Report (IPCC, 2007) – the CMIP 3 Archive
Two methods are commonly employed in downscaling global climate-change scenarios to produce regional scenarios; downscaling with regional climate models (RCMs) embedded within the low resolution GCM fields, and empirical downscaling forced by the GCM fields (Hewitson et al, 2005b)

Summary

Introduction

A focus on potential impacts of climate change on the water sector of Southern Africa (i.e. the Republic of South Africa together with Lesotho and Swaziland) was triggered by a series of activities and events in the first few years of the new millennium which included the South African Country Study on Climate Change, the World Summit on Sustainable Development, the Intergovernmental Panel on Climate Change (IPCC) reports in 2001, the 3rd and 4th World Water Forums, as well as active South African participation in the International Geosphere-Biosphere Programme and the International Dialogue on Water and Climate, among others. # This paper ( modified) was originally presented at the 13th SANCIAHS Symposium, 6-7 September 2007, Cape Town, South Africa system and that if climate changes were to manifest themselves in the manner which international science was projecting, it would add a further layer of concern to the management of Southern Africa’s already high-risk and stressed water sector, with potential implications to the entire region’s socio-economic well-being, but that of the poor. Long-term changes in observed rainfall in South Africa have been noted in a number of studies. Some of these studies were focused on localised areas while others were focused at a national level. At a national level, Richard et al (2001) and Fauchereau et al (2003) noted no overall wetting or drying, but did report an increase in inter-annual rainfall variability during the 20th century. Warburton and Schulze (2005) reported that over the latter half of the 20th century, median annual rainfall has decreased markedly over the Limpopo, Available on website http://www.wrc.org.za

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Conclusion