The quasi-dynamic wetness index, in its original development, was computed by calculating the travel time along all the possible upslope flow paths on a contour-based terrain network. In more recent applications the same approach has been extended to gridded digital elevation models with single-flow-direction algorithms. Multiple-flow-direction algorithms, although more effective in representing flow paths, have not been used because they are not practicable with the established methodology. We propose an alternative method for computing the quasi-dynamic wetness index based on the numerical integration of the linear-kinematic wave equation. This method can be applied to any of the terrain-based flow-direction algorithms currently published. The method is robust and efficient.