Long‐range, low‐frequency sound propagation over varying terrain conditions is an important problem with both civilian and military applications. This work considers the modeling of cw acoustic signals as they propagate over variable‐impedance topography. The NASA implicit finite‐difference (NIFD) implementation of the parabolic approximation sound propagation model that incorporates variable‐impedance ground surfaces was used. The accuracy of this numerical model is demonstrated with a pair of test problems. An application of the results is used to predict excess attenuation of pure‐tone signals whose propagation path includes portions of a flat lake surface.