Brenda Toth

Canadian Experiment for Soil Moisture in 2010 (CanEx-SM10): Overview and Preliminary Results

The Canadian Experiment for Soil Moisture in 2010 (CanEx-SM10) was carried out in Saskatchewan, Canada, from 31 May to 16 June, 2010. Its main objective was to contribute to Soil Moisture and Ocean Salinity (SMOS) mission validation and the prelaunch assessment of the proposed Soil Moisture Active and Passive (SMAP) mission. During CanEx-SM10, SMOS data as well as other passive and active microwave measurements were collected by both airborne and satellite platforms. Ground-based measurements of soil (moisture, temperature, roughness, bulk density) and vegetation characteristics (leaf area index, biomass, vegetation height) were conducted close in time to the airborne and satellite acquisitions. Moreover, two ground-based in situ networks provided continuous measurements of meteorological conditions and soil moisture and soil temperature profiles. Two sites, each covering 33 km × 71 km (about two SMOS pixels) were selected in agricultural and boreal forested areas in order to provide contrasting soil and vegetation conditions. This paper describes the measurement strategy, provides an overview of the data sets, and presents preliminary results. Over the agricultural area, the airborne L-band brightness temperatures matched up well with the SMOS data (prototype 346). The radio frequency interference observed in both SMOS and the airborne L-band radiometer data exhibited spatial and temporal variability and polarization dependency. The temporal evolution of the SMOS soil moisture product (prototype 307) matched that observed with the ground data, but the absolute soil moisture estimates did not meet the accuracy requirements (0.04 m3/m3) of the SMOS mission. AMSR-E soil moisture estimates from the National Snow and Ice Data Center more closely reflected soil moisture measurements.

Validation of SMOS Data Over Agricultural and Boreal Forest Areas in Canada

This study was conducted as part of the Soil Moisture and Ocean Salinity (SMOS) calibration and validation activities over agricultural and boreal forest sites located in Saskatchewan, Canada. For each site covering 33 km × 71 km (i.e., about two SMOS pixels), we examined the SMOS brightness temperature (L1c) and soil moisture (L2) products from May 1 to September 30, 2010. The consistency of these data with respect to theory and to the temporal variation of surface characteristics was first discussed at both sites. Then, the SMOS L1c (prototype 346) and L2 (prototypes 305-309) products were evaluated using the Canadian Experiment for Soil Moisture in 2010 (CanEx-SM10) ground measurements and L-band passive microwave airborne measurements, in addition to AMSR-E soil moisture estimates and simulations from the zeroth order τ- ω radiative transfer model. For both study sites, the model underestimated SMOS brightness temperatures in V polarization, whereas an overestimation was observed in H polarization. The data sets showed that both the SMOS and AMSR-E soil moisture values were underestimated compared with ground measurements collected during CanEx-SM10 but less so for the AMSR-E estimates. The SMOS soil moisture product was underestimated with a RMSE varying from 0.15 to 0.18 m3/ m3. Furthermore, the overall results showed that errors in the soil moisture estimates increased with the absolute value of soil moisture.

Use of in situ soil moisture network for estimating regional-scale soil moisture during high soil moisture conditions

Improving remotely sensed soil moisture estimates requires calibration and validation from ground-based observations obtained from established monitoring networks. Network sites are often installed at the edges of fields (in grass strips), and it is unknown if the soil moisture conditions at the network sites are similar to those observed within the fields. Intensive field campaigns, that include extensive spatial sampling of soil moisture, can be used as a basis for comparison for network sites. This study utilized data from the Canadian Experiment for Soil Moisture in 2010 (CanEx-SM10). Regional mean soil moisture (at the scale required for passive microwave remote sensing) obtained from the network sites (32 in total) was compared to the mean soil moisture obtained from field locations (55–60 fields) within the same region for the 6 days of the field campaign. The mean difference between the regional mean network soil moisture and the regional mean field soil moisture was < 0.04 m3 m−3 for each day of the campaign. A bootstrapping technique, which randomly sampled the network data, determined that the regional field mean soil moisture fell within the 95% confidence interval for the network data for all days and resulted in a root mean square error (RMSE) between the network and the regional field soil moisture of < 0.03 m3 m−3. Thiessen polygons were used as an upscaling technique to determine the regional-scale soil moisture resulting from network and manual field measurements. The results indicated that the difference between the regional-scale soil moisture from the network versus the field measurements was < 0.041 m3 m−3 for all sampling days. A Monte Carlo analysis indicated that 25 of the network stations (within a region of approximately 1600 km2) would be required in order for the network mean to be within 0.04 m3 m−3 of the field mean soil moisture with 95% confidence.

Potentials of RADARSAT-2 data to monitor freezing/thawing cycles over agricultural lands in Canada

The target decompositions technique of Freeman-Durden is used to monitor freeze/thaw cycles, with fully polarimet-ric RADARSAT-2 images acquired over agricultural areas. In this paper is presented a preliminary analysis of the scattering mechanisms derived from three RADARSAT-2 acquisitions during the fall of 2008, over agricultural sites located in Saskatchewan, Ontario and Quebec. AMSR-E brightness temperatures data is also used in the analysis. Contrary to expectations, results indicate that surface scattering represents the main contribution to the signal over frozen soils.

Use of Radarsat-2 images to develop a scaling method of soil moisture over an agricultural area

Our objective is to describe and evaluate the spatial variability of the surface soil moisture at small scales using Radarsat-2 images acquired over an agricultural area. To reach this aim, a geostatistical analysis is applied to up scale the soil moisture from ground measurements to the resolution of radar images and also from retrieving soil moisture at different spatial resolution. To conduct this study, four RADARSAT-2 images acquired at different modes during the summer of 2008 over agriculture fields located in Saskatoon (Saskatchewan, Canada) were used. The available data are ground measurements of soil moisture, surface roughness and vegetation characteristics. The comparison between retrieval and ground soil moisture showed average absolute relative errors of about 35% and 55% for fine and ScanSAR mode images respectively. Indeed, a weak amelioration was obtained from the up scaling of the ground soil moisture data to the spatial scale of the different radar image.