Abstract
The ongoing drought in the Western Cape of South Africa (2014 to present) has called for an urgent need to improve our understanding of water resources in the area. Rivers within the Western Cape are known to surge rapidly after rainfall events. Such storm-flow in natural river catchments in the Jonkershoek mountains has previously been shown to be driven by displaced groundwater, with less than 5% of rainfall appearing in the storm-flow. However, the origin of storm-flow surges within urban rivers in the region remains unknown. In this study, we used stable isotopes in water to illustrate that at least 90% of water in the Liesbeek River during a storm event was rainwater. There was a strong correlation between storm-flow and rainfall rates (P < 0.001, Pearson’s r = 0.86), as well as between the δ18O and δ2H values of river-water and rainwater (δ18O: Pearson’s r = 0.741 (P = 0.001), δ2H: Pearson’s r = 0.775 (P < 0.001)). Storm-flow within this urban river therefore appears to be driven by overland-flow over the hardened urban catchment, rather than piston-flow as seen in natural catchments. Our results support studies suggesting the Liesbeek River could be a target for stormwater harvesting to augment water resources in the city of Cape Town.
Highlights
Water resources in the Western Cape of South Africa are scarce and are predicted to become increasingly so in the future (Otieno and Ochieng, 2004)
Rain gauges were placed at the river sampling point in Mowbray (33° 56′ 48.23′′ S 18° 28′ 39.69′′ E, 11 m amsl), at the University of Cape Town’s Upper Campus (UCT, 33° 57′ 24.30′′ S 18° 27′ 40.00′′ E, 120 m amsl) and at the top of Skeleton Gorge on Table Mountain
41 375.8 m3 flowed down the river during the storm event
Summary
Water resources in the Western Cape of South Africa are scarce and are predicted to become increasingly so in the future (Otieno and Ochieng, 2004). The cities within this predominately winter-rainfall region rely on surrounding dams for their water supply, which are generally replenished through winter streamflow. Extended below-average rainfall in the region since 2014 led to severe water shortages in the City of Cape Town and in 2017 threatened the potential collapse of the city’s water infrastructure (so-called ‘DayZero’). In order to secure future water supply, cities in the Western Cape will have to adapt to the likelihood of reduced rainfall in the future (New, 2002; Ziervogel et al, 2011), as well as augment their water supply from alternative sources such as aquifers and stormwater amongst others (New, 2002; Fisher-Jeffes et al, 2017). As such, understanding the storm-flow dynamics of rivers in this region and their relationship to rainfall is important, to properly manage the area’s water resources
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