Scaling Properties of L-band Passive Microwave Soil Moisture: From SMOS to Paddock Scale

Remote sensing of soil moisture is a powerful and inexpensive way to map surface soil moisture for a range of applications, including weather prediction, water resources management and irrigation practices. Passive microwave sensors operating at L-band wavelength have shown the most potential for this task. However, this technology faces some major challenges, mostly related to correct interpretation of the spatially averaged measurements provided by such sensors and the mismatch between typical observation scales at L-band (50km) and the resolution at which data are needed for many environmental applications (1km). Due to the imminent launch (2007) of the first dedicated soil moisture mission, the European passive microwave Soil Moisture and Ocean Salinity (SMOS) satellite, it is important to address these issues by investigating the effect of sub-pixel variability in soil moisture and land surface features (vegetation cover, soil type, soil temperature) on radiobrightness observations, so that suitable downscaling schemes may be developed. The National Airborne Field Experiment 2005 (NAFE’05) conducted in the Goulburn catchment was designed with this primary objective. The NAFE’05 dataset is unique worldwide due to the wide range of spatial scales at which passive microwave observations were made, ranging from 1km to 62.5m resolution. Supporting ground measurements were also made at a range of scales from 1km to 6.25m resolution. In this study, airborne observations from the NAFE’05 campaign are used to examine the scaling properties of L-band passive microwave signatures across scales from 1km to 62.5m. The spatial properties of passive microwave signatures are compared across multiple observation scales. High resolution observations are also spatially aggregated and compared with the microwave signature as observed at subsequently lower resolutions. Results indicate a limited impact of land surface variability on the scaling behaviour of brightness temperatures, with the area-averaged brightness temperature measured at different resolutions varying of less then 6K.