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Excessive phosphate loading in the upper Olifants River, South Africa hasresulted in widespread eutrophication and associated ecosystem healthimpacts. The majority of sewage treatment works (STWs) are operating poorlyand are likely an important source of bioavailable ortho-phosphate (OP) inthe catchment. The Soil Water Assessment Tool (SWAT) was used to identifyimportant sources of OP loading in the catchment and to predict changes inthe trophic status of four reservoirs associated with three STW effluent OPstandards (the current average of 4 mg L−1 and standards of 1 and0.1 mg L−1). Nash–Sutcliffe and PBIAS statistics showed good modelperformance for simulations of flow and OP loading using a multi-sitecalibration and validation approach. Key sub-catchments responsible for highOP loading were identified. Three of the four reservoirs are heavily impactedby OP loading originating from STWs. Two reservoirs show mean annual OPconcentrations indicative of eutrophic conditions and a 1 mg L−1effluent standard would reduce concentrations to well within the mesotrophicrange. There was little difference in OP loads and concentrations associatedwith the 1 and 0.1 mg L−1 effluent standards, indicating thatattention to nonpoint sources would be required to realise any benefitassociated with the strictest effluent standard. Regression analyses andassociated 95% prediction limits between simulated OP loads andconcentrations for all effluent treatment scenarios from 2002 to 2010 allowedfor the quantification of OP loading that would ensure a specific trophicstatus in each reservoir. This study is one of the first to apply SWAT insimulating OP loading and concentrations in large reservoirs, and itsapplication in South Africa provides further support for its utilitythroughout a wide geographical area

Applying SWAT to predict ortho-phosphate loads and trophic status in four reservoirs in the upper Olifants catchment, South Africa