Rainfall and river flow trends for the Western Cape Province, South Africa

Rakhee Lakhraj-Govender,Stefan W Grab

doi:10.17159/sajs.2019/6028

Abstract

Climate change has the potential to alter the spatio-temporal distribution of rainfall, subsequently affecting the supply and demand of water resources. In a water-stressed country such as South Africa, this effect has significant consequences. To this end, we investigated annual and winter rainfall and river flow trends for the Western Cape Province over two periods: 1987–2017 and 1960–2017. Annual rainfall for the most recent 30-year period shows decreasing trends, with the largest magnitude of decrease at the SA Astronomical Observatory rainfall station (-54.38 mm/decade). With the exception of the significant decreasing winter rainfall trend at Langewens (-34.88 mm/decade), the trends vary between stations for the period 1960–2017. For the period 1987–2017, statistically significant decreasing winter trends were found at four of the seven stations, and range from -6.8 mm/decade at Cape Columbine to -34.88 mm/decade at Langewens. Similarly, the magnitudes of decreasing winter river flow at Bree@Ceres and Berg@Franschoek are greater for the more recent 30-year period than for 1960–2017. Correlation coefficients for Vilij@Voeliv rainfall and four river flow stations Berg@Franschoek, Bree@Ceres, Wit River@Drosterkloof and Little Berg@Nieuwkloof) are stronger for shorter periods (i.e. 1987–2017 and 2007–2017) than that for the longer period, 1960–2017. The Intergovernmental Panel on Climate Change emphasises the importance of studies to assist with model prediction uncertainties. To this end, our study expands the understanding of regional hydrological responses to rainfall change in the water stressed region of the Western Cape Province. Significance:  Historical trend analyses provide the basis for future rainfall and river flow projections and also improve our understanding of hydrological responses to rainfall change, which is important for water resource planning and management in light of recent rainfall shortages experienced in the Western Cape region. Anthropogenic and natural influences on river systems must be taken into account when assessing the impact of rainfall changes on river flow because these factors ultimately affect water resource management. We identified decreasing winter rainfall trends across the Western Cape Province for the most recent three decades.

Highlights

Anthropogenic activities such as an increasing human population, economic development and urbanisation, place enormous pressure on water resources across the globe.[1]
For the period 1987–2017, a statistically significant annual decreasing trend of 54.38 mm/decade was recorded at the SA Astronomical Observatory (Table 2; Figure 2)
The Cape Point station recorded 85 mm of rainfall on 13 November 2013, which corresponds with the flash flood experienced in parts of the Western Cape on that day. This flash flood subsequently affected the annual rainfall trend at Cape Point, which shows a decline in rainfall from 2013 (741.80 mm) to 2017 (325 mm)

Summary

Introduction

Anthropogenic activities such as an increasing human population, economic development and urbanisation, place enormous pressure on water resources across the globe.[1] Climate change significantly impacts the supply and demand of such water resources.[2] The Intergovernmental Panel on Climate Change provides robust evidence that current climate change is projected to decrease renewable surface water.[3] One of the most significant consequences of climate change is the change in magnitude and frequency of precipitation.[4] Rising temperatures increase evaporation levels, spatio-temporal run-off distribution and water resource storage.[5] It is anticipated that global warming will intensify the hydrological cycle, resulting in extreme high/low river flows, depending on the region.[6] In a global analysis of 200 rivers, significant trends were recorded for one third of these rivers – 45 decreasing and 19 increasing.[1]

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Conclusion