Spatial and Temporal Variability of Soil Surface Roughness during SMAPVEX16-IA.

The SMAPVEX16-IA field campaign was held May 25 to August 16, 2016, in the watershed of the South Fork Iowa River in Central Iowa. The purpose of the experiment was to determine why NASA's SMAP satellite mission is not performing as well as expected in agricultural regions like Iowa and the rest of the Corn Belt. Iowa was chosen as the location of the experiment because of the existence of the 20-node permanent in situ South Fork soil moisture network. One possible reason for SMAP's unexpected performance is an incorrect estimation of soil surface roughness, the mm-scale topography of the soil surface. Soil surface roughness, like soil moisture, alters the emissivity of the soil surface, and most be accounted for in order to retrieve soil moisture from SMAP observations. We characterized soil surface roughness at more than 20 sites within the South Fork using three different methods. First, 3-m transects using a 1-cm grid board parallel and perpendicular to row direction. Second, 3-m transects using a pin board parallel and perpendicular to row direction. Third, a mobile lidar system that measured a variety of transects parallel and perpendicular to row direction. We will compare and contrast the three methods, and quantify the temporal and spatial variability in soil surface roughness that was observed across the South Fork. Finally, we will estimate the impact of soil surface roughness on soil moisture retrieval.