MODELING EFFECTS OF CURTAIN METHOD ON ALGAL BLOOMING IN RESERVOIRS

The Terauchi Dam Reservoir, one of largest reservoirs in Kyushu, Japan has been supplying water for paddy field irrigation and domestic cons umption. During recent years it has shown serious deterioration of wat er quality, especially during the summer season. Destruction of the th ermocline has been prohibited in order to maintain desirable temperatu re downstream of the reservoir for paddy field irrigation, thus, most of the existing control devices cannot be applied to the reservoir. As one of the means of reducing algal blooming in the reservoir, two ver tical curtains, having sufficient depth to cover the epilimnion thickn ess, were installed across the reservoir in order to curtail the nutri ent supply from nutrient-rich inflow to the downstream epilimnion of t he reservoir. Withdrawal level was also regulated to keep the downstre am epilimnion off the nutrient supply. The physical and biological pro cess in the reservoir ecosystem were modelled, by assuming a stratifie d layered structure at each zone of the reservoir, to predict the wate r quality and algal species composition in the reservoir. The state va riables, temperature, four types of phytoplankton as chlorophyll-a, so luble phosphorus, nitrate, ammonium, dissolved oxygen, biochemical oxy gen demand, internal nitrogen, internal phosphorus, and when diatoms a re modelled explicitly silica, are considered in the model. The model is calibrated with the measurements taken in the field. The effects of different curtain depths on the algal concentration at downstream zon es were also studied. (C) 1997 Elsevier Science B.V.