Using lattice-gas cellular automata we simulated fluid flow through a straight tube with a sinusoidally varying cross-section, i.e., what we call a varicose tube. We found that the hydraulic conductance of such a constricted tube was bounded between the long-wavelength values predicted by Reynolds' equation and the so-called stagnant limit at short wavelengths. In particular, we conclude that pore space associated with roughness at wavelengths shorter than about 15% of the pore radius, should not contribute to hydraulic conductivity and need not be resolved for flow simulation purposes. ¿ 2000 American Geophysical Union