An analysis of Muskingum diffusion model, in which routing parameters are calculated based on channel and grid characteristics, is carried out. Numerical experiments are performed to throw additional light onto the criterion setting an upper limit to the space step if accuracy is to be preserved. Computational experience indicates that for very large values of the space step, there is a tendency for physically unrealistic negative outflows to occur. The results of the numerical experiments point to the condition C2 m ζ if accuracy is to be preserved, in which C2 is one of the routing coefficients and ζ is a real number. A value of ζ = 0.33 is recommended for practical applications. The spatial resolution criterion is expressed in terms of Courant and cell Reynolds numbers.