Water resources management with dynamic optimization strategies and integrated models of lakes and artificial wetlands

Abstract

In this work, we propose an integrated mechanistic model for a freshwater eutrophic reservoir and an artificial wetland and within a dynamic optimization framework. A partial differential equation system results from dynamic mass balances for the main phytoplankton groups; two zooplankton groups and two size classes of zooplanktivorous fish in the reservoir, as well as dissolved oxygen and main nutrients in both the reservoir and wetland. Algebraic equations stand for forcing functions profiles, such as temperature, solar radiation, river inflows and concentrations, etc. The model is formulated as an optimal control problem within a control vector parameterization approach in gPROMS. Optimization variables are the fraction of nutrient rich water stream that is derived through the wetland and fish removal rates from the reservoir. The objective function is the minimization of a weighted sum of integrals along the entire time horizon: the quadratic difference between phytoplankton concentration and a desired value below eutrophication limits and the quadratic difference between phosphate concentration in the wetland outflows and a desired value. Numerical results provide optimal profiles for restoration actions and their effects on the studied ecosystem.