Abstract
A personal computer-based COD model was developed in order to make eutrophication programs effective for Toyano Lagoon which has a short retention time (1.8 days) and is a shallow and eutrophic lake. Toyano Lagoon is mainly used for irrigation water through the pumping stations at the inlet and outlet where connect Kurinoki River and Shinano River, respectively. This means that flow control such as dilution and discharging at pumping stations is feasible as eutrophication control.
First the model was calibrated using observed data in 1988 FY. Then the calibrated model was verified by observed data in 1985 FY (i.e., before eutrophication control measures). The sensitivity analysis for parameters and forcing functions and water pollution analysis were performed using the verified model. It was found in both the calculated values and the observed data that COD concentrations increased steeply in April, July and September when the inflow and outflow decreased. The model calculations showed that the increase of autochthonous COD due to primary production corresponds to the peak of COD concentrations in these months. The production rate of autochthonous COD and the external COD loading rate are 228 t·y-1 and 1,946 t·y-1, respectively. The control of inflow and outflow being increased by 150% and kept constant is very effective for prevention of the peak of autochthonous COD, though reduction control of external nutrient load not. The COD concentration in Toyano Lagoon decreases by about 1.5g·m-3 per 1,000kg·d-1 reduction of the external COD load.
