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Climate change is an additional threat that puts increased pressure on already stressed hydrological systems and water resources. The impacts of climate change are already visible given that temperature and rainfall variabilities have increased and intensified over the last two decades. Recent studies indicate the extreme vulnerability of Southern Africa to the impacts of climate change and recommend that appropriate adaptation measures be put in place. Rainwater harvesting (RWH) is listed among the specific adaptation measures that the water sector in Africa needs to undertake to cope with future climate change. At present, there is limited application of RWH, despite its high potential for alleviating the impacts of climate change on water security in many areas of Africa.

The paper presents a methodology that enables water managers to incorporate the climate change component during the design phase of domestic RWH (DRWH) systems. The Roof model was used to calculate the optimum size of the RWH tank and appraise its water security (percentage of demand satisfied) under different scenarios (with and without climate change). Results indicate that the optimum RWH tank volume is 0.5 m(3) in the four quaternary catchments (QCs) studied with the humid QC, as expected, providing the highest water security of about 30%. On the basis of forecasted rainfall down-scaled from six global circulation models, the ranges of water security attained by a 0.5 m(3) RWH tank are 10-15% in the arid QC, 15-20% in both the semi-arid and dry sub humid QCs and 30-40% in the humid QC. Furthermore, the high value of the water security climate induced index (gamma > 0.6) confirms the high susceptibility of South Africa to climate change and, therefore, the need for appropriate adaptation measures. (C) 2010 Elsevier Ltd. All rights reserved.