The main aim of this work was to construct and validate a mathematical water quality model of the Dianchi lake, so that by altering input total phosphate (TP) loads the projected changes in the lake water TP concentrations could be estimated. Historical information had indicated deteriorating lake water quality with increasing TP concentrations. The model was based on a simple annual mass balance, relying on 3 years (wet, average and dry) data with all TP loads quantified, 7 years of lake water quality, and 36 years of flow data. All lake processes were considered within a single variable, R. Planning TP removal at STWs and within fertilizer plants, coupled with interventions to reduce non-point TP loads from all land run-off by 50%, suggested future lake water TP concentrations could be stabilised at about 0.3 mg TP/l, i.e. the estimated limit for producing algal concentrations that would cause major problems in water treatment plants. The TP load reductions envisaged as realistic would only stabilise the lake water quality by about the year 2008; interventions, unfortunately, could not return the lake to its former pristine condition. The accuracy of the predictions was ± 0.1 mg TP/1, so collection of better data was needed.

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