Advancing Three-Dimensional Coupled Water Quality Model of Marine Ranches: Model Development, Global Sensitivity Analysis, and Optimization Based on Observation System

Abstract

Marine ranching is a stock enhancement project that has been an important part of aquaculture in China. Due to the lack of scientific management, disasters have occurred, resulting in millions of economic losses. Based on the observation system of marine ranches, a three-dimensional water quality model of marine ranches was developed to study the spatio-temporal variation of key ecological indicators, including the concentrations of chlorophyll-a, zooplankton, detritus, nutrients, and dissolved oxygen (DO). The model is coupled offline with the residual current, temperature, and salinity simulated by a regional oceanic modeling system (ROMS). The conservative characteristic finite difference (C-CFD) scheme is introduced to solve the equations, which guarantees model stability and mass conservation and allows for a larger time step compared to traditional difference schemes. In state-of-the-art water quality models, the biogeochemical processes are parameterized. Due to the complexity of the water quality model, a combination of global sensitivity analysis (GSA) and the adjoint method is introduced as the methodology to optimize the model parameters. Morris’ sampling method is implemented as the GSA method to find out the key factors of the water quality model. The optimization of sensitive parameters with the adjoint method significantly improves the model precision, while the other parameters can be set as empirical values. The results indicate that the combination of GSA and the adjoint method is efficient in the parameter optimization of the water quality model. The model is applicable in marine ranches.