Errors are unavoidable in the development of simulation models dealing with natural processes.

They cannot be eliminated completely, but can be reduced. It is therefore necessary to study the significance of these errors on overall output response of the models.

Uncertainty and sensitivity analysis addresses this issue and can be done before and/or after model calibration.

In this study, an uncertainty and sensitivity analysis was conducted on Yarra River (Victoria, Australia) water quality model (YRWQM) prior to its model calibration. Although there can be many different forms of errors that are present in model development, only the errors related to estimation of reaction parameters of water quality constituents were considered in this paper.