12In this work, a three-dimensional PEM fuel cell model has been developed and is used to investigate the 13 effects of water flooding on cell performance parameters. The presence of liquid water in the cathode gas diffusion 14 layer (GDL) limits the flow of reactants to the cathode catalyst layer, thereby reducing the overall reaction rate and concentration of reactants/products and cell current densities. It is determined that the gas diffusion layer 23 permeability has little or no effect on the current densities due to the diffusion dominated nature of the gas flow. 24However, through the incorporation of percolation theory based effective diffusivity model; a marked reduction in 25 the cell performance is observed which closely resembles published experimental observations. This is a reasonable 26 approximation for effects of water flooding which has been inherently used for further parametric studies. 27