The accuracy of snow depth estimation is affected significantly by the regional surface type. We have developed a theoretical model of vector radiative transfer (VRT) for snowpack/vegetation canopies at SSM/I channels. The vegetation canopy is modelled by a layer of nonspherical particles, and the snowpack is modelled as a layer of dense spherical particles. By numerically solving two coupled VRT equations for multi-layer models of different surface types such as tree/snow, grass/snow and snowpack only, two scattering indices SI1 = T B19v - T B37v and SI2 = T B22v - T B85v are obtained for a variety of snow depths (SD) and ice-grain sizes. These results are combined as a mesh graph in the figure of SI1 versus SI2 . When the SSM/I TB data is observed, its location in the mesh graph can indicate the estimation of SD. Our results compare well with the SSM/I data of the U.S.A. east coast January blizzard, 1996.