In regional and global numerical weather prediction and climate models, sub-grid scale effects are often accounted for as part of the surface and boundary-layer parametrizations. There are several proposals (Wood and Mason 1993; Belcher et al. 1993; Taylor et al. 1989) regarding the parametrization of drag over small-scale topography. We review these proposals and make comparisons between them, utilizing results from an efficient numerical model with second-order turbulence closure (Non-linear mixed spectral finite difference model—Xu et al. 1994). Model results cover an extensive range of parameters, such as hill slope, roughness length, horizontal scale and hill shape. We initially confine ourselves to the neutrally stratified surface boundary-layer but extensions to the planetary boundary-layer and for stable stratification are in progress.