Long-Term Water Quality Modeling of a Shallow Eutrophic Lagoon with Limited Forcing Data

Water quality modeling can be an important tool for lake management. However, if the simulation period is small and the forcing data is limited, the result uncertainty may diminish its usefulness. This study was conceived with the aim of implementing a long-term water quality simulation of a shallow eutrophic lagoon, with limited forcing data, to evaluate result uncertainty and the advantages of such an approach for the water management process. The lagoon water quality was simulated, over a period of 19 years (2000 to 2019), with the CE-QUAL-W2 model, and the watershed with the SWAT model. Two different scenarios regarding the lagoon inflow water quality characterization were considered and evaluated. The CE-QUAL-W2 model was used to simulate the lagoon water column and sediment biogeochemical fluxes using the following seventeen constituents as a calibration reference: water temperature; dissolved oxygen; orthophosphates; total phosphorus; ammonia; nitrate plus nitrite; total nitrogen; carbonaceous biochemical oxygen demand; total dissolved solids; pH; six algae biomass groups, and chlorophyll-a. The results show that despite the high level of uncertainty in the lagoon's daily constituents' variation, the long-term perspective on the biogeochemical fluxes, as described by the interannual constituent's evolution, enabled the identification of ecological thresholds, the convergence and divergence of the system, and constituent trajectories. These results can represent a step forward into the establishment of cause-and-effect relationships between pressures and water body status, thus allowing an effectiveness improvement on the integrated water resource planning and management under the European Water Framework Directive.