At Barcelona Expert Center (BEC) we are able to provide a Level 4 (L4) Surface Soil Moisture (SSM) product with 1 km spatial resolution that meets the requirements of land hydrology applications. To do so, we use a downscaling method that combines highly-accurate, but low-resolution, SMOS radiometric information with high resolution, but low sensitivity, visible-to-infrared imagery to SSM across spatial scales. A sample L4 SSM map from September 1, 2014 (6 AM) is shown in Figure 1.
This downscaling approach was first presented in [1] along with results of its application to a set of SMOS images acquired during the commissioning phase over the Oznet network, South-East Australia. Using reprocessed SMOS data obtained with the latest L1 and L2 processors, we have further developed and validated this technique; we now use SMOS polarimetric and multi-angular information in the downscaling method, which results in improved fine-scale soil moisture estimates [2].
The temporal and spatial variability of two years of SMOS-BEC L4 fine-scale (1km) SSM estimates over the Iberian Peninsula has been evaluated through comparison with ground-based measurements acquired at the in situ soil moisture measurement network (REMEDHUS) located in the central part of the Duero basin, Spain [2]. Results show that the downscaling method improves the spatial representation of SMOS coarse soil moisture estimates (SMOS L2) while maintaining temporal correlation and root mean squared differences with ground-based measurements. Figure 1 shows the temporal evolution of SSM time-series (i.e. SMOS L2, SMOS-BEC L4, in-situ) over REMEDHUS. It can be seen that area-averaged downscaled estimates match well with in situ data (circles are enclosed within the network’s soil moisture variability in shaded green). Scatter plots of Fig. 2 display the agreement between remotely sensed and REMEDHUS in situ SSM time-series, with segments illustrating the linear fit of seasonal data. Results are shown for a representative station of rainfed cereals, the most common land-use in the area, for SMOS L2 (left plot) and for SMOS-BEC L4 (right column). It can be seen that the slope of the linear correlation is significantly improved in the L4 maps (it is closer to the 1:1 line) and the dynamic range of in situ soil moisture measurements is reproduced in the high resolution maps, including stations with different mean soil wetness conditions (see further results in [2]). This evaluation study supports the use of this downscaling approach to enhance the spatial resolution of SMOS observations over semi-arid regions such as the Iberian Peninsula.
Fine-scale soil moisture maps over the Iberian Peninsula from years 2010 to present can be freely accessed through the SMOS-BEC data distribution and visualization service (cp34-bec.cmima.csic.es). Global SMOS data as well as MODIS data over the Iberian Peninsula are received in NRT at SMOS-BEC facilities and, since June 2012 the downscaling algorithm is triggered twice a day, corresponding to SMOS ascending and descending passes to serve high-resolution soil moisture maps in Near Real-Time NRT (delay of < 6h). As a prime NRT application, these maps are being used by local fire prevention services in their early warning system to detect extremely dry soil and vegetation conditions posing a risk of fire. BEC has recently been chosen as an SMAP Early Adopter to foster the use of remotely sensed soil moisture data in forest fire risk prevention services.
[1] Piles, M., A. Camps, M. Vall-llossera, I. Corbella, R. Panciera, C. Ruediger, Y. Kerr, J. Walker (2011) “Downscaling SMOS-derived soil moisture using MODIS Visible/Infrared data”, IEEE Transactions on Geoscience and Remote Sensing, vol. 49, pp. 3156-3166. [2] Piles, M., N. Sánchez, M. Vall-llossera, A. Camps, J. Martínez-Fernández, J. Martínez, V. González-Gambau (2014) “A Downscaling Approach for SMOS Land Observations: Evaluation of High-Resolution Soil Moisture Maps Over the Iberian Peninsula”, IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol.7, no.9, pp.3845-3857.