Soil surface roughness affects both the radiometric and radar observations. Radar observations can, in certain conditions, be more sensitive to surface roughness than soil moisture itself due to the increased scattering of the incoming radiation. Moreover, surface roughness affects the radiometric observations by effectively increasing the surface area of electromagnetic wave emission. Its effect on observations at L-band frequency has been difficult to quantify, and therefore it is crucial to characterise the spatial variation of this parameter across the different land cover types.

During SMAPEx-3 surface roughness was characterized at 3 locations within each major land cover type in the six focus radar pixel areas and at the 50 sites in the forest sampling area. A dedicated team was in charge of the surface roughness measurements. At each of the locations two, 3m-long surface profiles were recorded, one oriented parallel to the look direction of the PLIS radar (East-West) and one perpendicular (North-South). Surface roughness was also measured at selected forest sites once during the campaign. Two perpendicular measurements per site were taken. The location was to be selected by team leader and could be inside of one of the subplots or in the buffer areas, but must be located under the forest canopy following the established protocols. For sites with sparse forest cover the roughness were taken for the most representative vegetation type (e.g. grass, shrubs, etc.). For each roughness profile photos of the surrounding vegetation were taken. Complete details on the surface roughness sampling strategy and protocols can be found in the SMAPEx-3 workplan.

Surface roughness sampling schedule