Application of Lavsoe, an Eutrophication Model, to Bloemhof Dam

SUMMARY In 1985 the Department of Water Affairs introduced an effluent phosphate standard of 1 mg l−1 (as P) in seven potentially sensitive catchments. Although this is an uniform standard, permits can be granted for effluents to exceed the standard in cases where it can be shown that the impact of the effluent on the water environment will be negligible. This policy requires the ability to predict the trophic response of waterbodies to different P loads. Two types of models were evaluated for this purpose; i.e., simple mass balance models and complex ecological models. This paper reports the results of a study undertaken to evaluate one of the complex ecological models. The LAVSOE model, an eutrophication model for shallow lakes which was developed in Denmark, was evaluated as a management tool for the control of eutrophication at Bloemhof Dam. To ensure that the basic underlying principles of LAVSOE were well understood and the assumptions not violated, the evaluation of LAVSOE followed the same procedure as model development; i.e., model construction check, a priori sensitivity analysis, calibration and verification. The aim of the model construction check was to assure that the model did what it was designed to do after it was transferred onto the Water Affairs mainframe. During this phase two additional features were added to LAVSOE to accommodate South African conditions. The aim of the a priori sensitivity analysis was to rank the parameters in order of sensitivity. The model was calibrated by means of a trial-and-error procedure because it was simple and had heuristic value. LAVSOE could not be calibrated successfully for simulating the observed nutrient dynamics and phytoplankton growth In Bloemhof Dam. In this paper, the main structure and the major processes of the LAVSOE model are introduced. The results of the sensitivity analysis and the reason for the unsuccessful calibration of LAVSOE on Bloemhof Dam are discussed. Modifications to LAVSOE to make it more applicable to turbid systems is proposed.