Climate influences on Vaal River flow

Mark R Jury

doi:10.4314/wsa.v42i2.07

Abstract

A study of climatic influences on Vaal River discharge, near Johannesburg, South Africa, finds that peak summer flows in the period 1979–2014 coincide with ocean–atmosphere interaction in the east Atlantic. The analysis has three parts: interannual influences by correlation of summer discharge with climate fields, atmosphere and ocean composites of 14 peak flow months, and a case study flood in January 2010 and its regional scale forcing. Inter-annual links are established with low pressure over the east Atlantic and an eastward equatorial ocean current and suppressed upwelling in the northern Benguela. During the January 2010 flood in the Vaal River, flow increased to 2 801 m 3 /s. There was a low salinity plume and warm sea temperatures off Angola > 29°C. A terrestrial vegetation fraction > 0.6 and corresponding latent heat fluxes enriched NW-cloud bands over the Vaal River catchment, during the flood case study of January 2010. Comparison of (Pacific) Southern Oscillation and east Atlantic influence on Vaal River discharge reveals the former drives evaporative losses while the latter provides an advance warning of flow variability. Keywords : Vaal River, hydro-meteorology, climate influence

Highlights

The Vaal River flows west near Johannesburg, South Africa (Appendix A1), eventually joining the Orange River and draining into the Atlantic Ocean
Vaal River discharge at 27°S, 26.7°E from 1979–2014 (Fig 2a) has a 2% up-trend due to imported base flows
Vaal River discharge was earlier related to Atlantic sea-level pressure (Fig. 3f), a signal that is part of a secondary mode of global climate variability, after the Pacific Southern Oscillation zonal dipole

Summary

Introduction

The Vaal River flows west near Johannesburg, South Africa (Appendix A1), eventually joining the Orange River and draining into the Atlantic Ocean. Dry season base flow is maintained by transfers from the south (ORASECOM, 2015). Seasonal river discharges and rainfall fluctuate from year to year (CV = 0.3), a constraint to production and resource development (Tadross et al, 2005). Variations in the flow of Africa’s larger rivers, such as the Congo, have been linked to east Atlantic sea surface temperatures (SST) and continental-scale climate anomalies (Hirst and Hastenrath, 1983a; Jury and Engert, 1999; Jury et al, 2000; Camberlin et al, 2001; Rouault et al, 2003; Todd and Washington, 2004). NW-cloud bands form with a cut-off low (Taljaard, 1985) and bring much-needed rains by linking moist air flows from the Congo basin (Jury, 2010) and east Atlantic Ocean

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Conclusion