A method is developed to estimate the scalar roughness length for water vapor z0v using the complementary evaporation concept. On the basis of the advection-aridity approach, estimates of z0v and of kB-1 (kB-1 = ln(z0/z0v), where z0 is the momentum roughness length) are available that do not require any estimate of surface temperature or humidity or of stomatal resistance. When applied to grassland data from Cooperative Atmosphere-Surface Exchange Study-1997, the resulting values of kB-1 corresponded well with those found from surface temperatures measured with a radiometer having an oblique view of the surface. They also displayed variability with z0+ (z0+ = u*z0/ν, where u* is the friction velocity and ν is the kinematic viscosity), which corresponded well with theoretical and wind tunnel test results from the literature. These results imply that useful estimates of kB-1 are available with this method, and they seem to support the complementary evaporation concept. Strict screening of the data was required to select only those data yielding the smallest uncertainties in the resulting values of kB-1.