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Topography can be important for future passive microwave remote sensing of soil moisture from space. One of the problems in assessing the importance of topography is that a digital elevation model (DEM) for the surface does not provide an intuitive estimation of when topography will be important. This is especially true given the large footprint of L-band radiometers on future space missions such as the Soil Moisture and Ocean Salinity (SMOS), Aquarius and the Soil Moisture Active-Passive (SMAP) missions. To address this issue, the DEM has been replaced with a probability density function (pdf) for slopes. It is shown that the slope pdf can be separated into “smooth” and “rough” distributions that provide insight into when topography will be important. The model is applied to the site of the 2004 Soil Moisture Experiment (SMEX04) in Arizona, and to the site of the Australian Airborne Cal/Val Experiment for SMOS (AACES) and shown to produce results comparable to the direct application of the DEM.