3. Uncertainties in the hillslope model parameter values had greater impact on the predictions than uncertainties in the channel parameters. For hillslopes, the three most influential parameters were soil saturated hydraulic conductivity, the soil volumetric rock fraction and the soil surface roughness. For channels, most influential were soil surface roughness and the Woolhiser coefficient. The latter parameter reduces the effective wetted perimeter for infiltration during low flows when a trapezoidal channel simplification introduces significant error