Prediction of water quality in lakes and reservoirs. Part I — Model description

A one-dimensional water quality model (DYRESM Water Quality) is described which combines a process based hydrodynamic model (DYRESM) with numerical descriptions of phytoplankton production, nutrient cycling, the oxygen budget and particle dynamics. The hydrodynamic component is free from calibration, which ensures that it is readily transferable to other lakes and reservoirs. This improves water quality predictions derived for different hydrodynamic forcing events. It also allows for identification of the specific hydrodynamic processes that influence water quality. The water quality component consists of 13 state variables which may include up to three algal groups, BOD, dissolved oxygen and four components of the dissolved oxygen budget (inflows, biochemical processes, surface aeration and oxygen present in the reservoir at the start of a simulation), nutrients (PO4P, NO3N, NH4N, TP and TN) and inorganic particles. The particle model simulates settling and flocculation/deflocculation of up to seven different size classes of particles. The hydrodynamic, water quality and particle models interact on a sub-daily time step. Forcing data for the model are entered as daily-averaged values. The ecological component requires calibration for each new application through adjustment of several different biological and chemical parameters. Literature ranges for these parameters are wide, but provided the process description is correct, many of the parameters can be validated with measured data.