High-resolution mapping and scaling behavior of passive L-band measurements and soil moisture retrieval in complex terrain

The Integrated Precipitation and Hydrology Experiment 2014 [1] was a ground-validation field campaign conducted in the Southern Appalachian Mountains (SAM) and adjacent Piedmont region May-June of 2014 in support of the Global Precipitation Measurement Mission (GPM). The Scanning L-band Active Passive (SLAP) instrument installed on NASA's King Air aircraft was flown over the eastern foothills of the SAM, complex terrain and heterogeneous land cover including forested areas, agricultural fields, and lakes, during IPHEx. [SLAP is an aircraft scale simulator of SMAP (Soil Moisture Active Passive) satellite mission.] A polarization mixing approach to wrap the georeferenced two-dimensional H-Pol and V -Pol SLAP brightness temperatures (TBs) on three-dimensional topography, hereafter referred to as topographic correction, was implemented. The algorithm requires the dynamic determination of global and local frames of reference for each EFOV (Extended Field of View) along the flight line, followed by the determination of correction factors calculated as the ratios of local and aircraft based antenna electric fields. The corrected TB measurements were subsequently used to estimate soil moisture with evaluation conducted against ground-based gravimetric soil moisture samples collected at various sites concurrently with the flight. The spatial organization of corrected TBs was assessed using the Topographic Wetness Index (TWI). The corrected TBs exhibit robust scaling behavior with TWI that is significantly different from that of the georeferenced SLAP observations without topographic correction. This finding suggests that TWI distributions can be used to downscaling SMAP products toward capturing the spatial organization of soil moisture conditioned by landform alone or in combination with methods that use land attributes such soil texture and vegetation [3].