Towards 1km Root Zone Soil Moisture from SMOS: The National Airborne Field Experiments

Jeffrey Walker, Rocco Panciera, Olivier Merlin and Jetse Kalma

Power Point Presentation

The National Airborne Field Experiments (NAFE) are a series of intensive experiments to be conducted in different parts of Australia. These experiments have been designed to answer questions on: i) estimation of surface soil temperature and vegetation water content from routine remote sensing data; ii) impact of dew, topography, surface roughness and surface rock on surface soil moisture retrieval; iii) transferability of current radiobrightness equations across scales ranging from 10’s meters to 10’s kilometers; iv) downscaling of low resolution passive microwave observations of surface soil moisture; and v) retrieval of root zone soil moisture estimates from surface soil moisture observations. Such questions can only be resolved through carefully planned and executed field experiments in well instrumented basins together with intensive ground and airborne measurements of the appropriate type and spatial/temporal resolution.

The NAFE’05 experiment was undertaken in the Goulburn River catchment during November 2005, with the objective to provide high resolution data for process level understanding of soil moisture retrieval, scaling and data assimilation. To meet this objective, the Polarimetric L-band Multibeam Radiometer (PLMR), a thermal imager and a tri-spectral scanner was flown onboard a small environmental research aircraft at four different altitudes from 625 ft to 10,000 ft AGL, resulting in L-band data at 62.5-m, 250-m, 500-m and 1-km resolution; thermal infrared data at approximately 1.25-m, 5-m, 10-m and 20-m resolution; and visible and near-infrared data at approximately 1-m resolution on one occasion. Flights were targeted across a 50km x 50km area containing 18 soil moisture profile monitoring sites. On one day per week the entire area was flown while on the remaining four days two sub-areas containing four extensively instrumented and ground monitored farms each were flown on alternate days. Coincident ground data at the farms was collected on embedded spatial scales of 6.25m to 1km. Additionally, data was collected on rock coverage and temperature, surface roughness, skin and soil temperature, dew amount, and vegetation water content.

The NAFE’06 experiment will be undertaken in the Murrumbidgee catchment during November 2006, with the objective to provide data for SMOS (Soil Moisture and Ocean Salinity) level soil moisture retrieval, downscaling and data assimilation. To meet this objective, PLMR (Polarimetric L-band Multibeam Radiometer) and supporting instruments (TIR and NDVI) will be flown at an altitude of 10,000 ft AGL to provide 1km resolution passive microwave data (and 20m TIR) across a 50km x 50km area every 2-3 days. This will both simulate a SMOS pixel and provide the 1km soil moisture data required for downscale verification, allowing downscaling and near-surface soil moisture assimilation techniques to be tested with remote sensing data which is consistent with that from current (MODIS) and planned (SMOS) satellite sensors. Additionally, two transects will be flown across the area to provide both 1km multi-angular passive microwave data for SMOS algorithm development, and on the same day, 50m resolution passive microwave data for algorithm verification. The study area contains a total of 13 soil moisture profile and rainfall monitoring sites for assimilation verification, and the transect fight lines are planned to go through 5 of these. Ground monitoring of surface soil moisture and vegetation for algorithm verification will be targeted at these 5 focus farms, with soil moisture measurements made at 250m spacing for 1km resolution flights and 50m spacing for 50m resolution flights.

This paper will present an overview of the experiment plans for both campaigns. Moreover, sample data from the NAFE’05 campaign will be presented together with preliminary analysis of the data to date.

While these experiments have a particular emphasis on the remote sensing of soil moisture, they are open for collaboration from interested scientists from all disciplines of environmental remote sensing and its application. See for more detailed information on these experiments.