A Basic Water Quality Model for the River Neckar: Part 1 – model development, parameter sensitivity and identifiability, calibration and validation

In order to simulate and forecast dissolved oxygen (DO) in the central part of River Neckar, we developed a Basic Water Quality Model (BWQM). The present paper gives a comprehensive description of model development, model structure along with the governing equations, sensitivity and identifiability analyses as well as calibration and validation. The development of BWQM was guided by two major restrictions: it has to account for all relevant processes affecting DO, and it should only be based on readily available data and system knowledge. These restrictions led to a model, calculating nine state variables, including DO, phytoplankton, nutrients, and biochemical oxygen demand. However, due to limited data, the representation of phytoplankton is quite simple. A local sensitivity analysis led to a subset of five parameters, which are much more sensitive than the 22 other parameters. These five parameters are all closely connected with phytoplankton dynamics, highlighting the importance of phytoplankton for the DO budget. On the basis of an a priori identifiability analysis, four out of the five most sensitive parameters were chosen for model calibration. The calibrated model performed well in simulating DO and phytoplankton. Given the data availability and the present state of system knowledge, BWQM comes close to representing the optimal model with respect to predicting DO and phytoplankton.