Towards SMOS validation in south-eastern Australia

The upcoming Soil Moisture and Ocean Salinity (SMOS) mission of the European Space Agency will be the first satellite dedicated to the measurement of soil moisture. It will measure soil moisture for the top 5 cm at a spatial resolution of 35 to 50 km. Comparison of the soil moisture data from SMOS with insitu monitoring network measurements across areas the size of a SMOS footprint will play a vital role in the validation of this new data stream. Possible validation sites in south-eastern Australia include the monitoring networks located in the Goulburn Catchment of the upper Hunter and Yanco area of the Murrumbidgee Catchment. This study has assessed the suitability of these two networks for the validation of SMOS soil moisture measurements using L-band passive microwave aircraft observations obtained from the National Airborne Field Experiments (NAFE) of 2005 and 2006. Here the soil moisture content for the entire SMOS sized area of each site has been estimated from validated aircraft observations across the region, providing a measurement that is not readily achieved by traditional ground-based sampling. This spatial average is then compared with averages from ground-based point measurements of the soil moisture taken at approximately 2 km spacing, as well as the existing in-situ networks on a number of dates and moisture conditions. From this assessment, both the number of point measurements required to accurately obtain the average soil moisture for the area and the suitability of the existing monitoring networks in each of the sites has been assessed. Using three days of extensive regional soil moisture data from the Goulburn catchment with coincident airborne soil moisture data covering a range of soil moisture conditions from wet to dry, it was found that 5 to 15 point measurements were needed to obtain the spatially averaged surface soil moisture for the area within a root mean square error of 0.04 m/m; the greater number of points were required in wetter conditions. However, the current monitoring station networks in the Goulburn River catchment and the Yanco region were found to not yield accurate estimates of average soil moisture of their respective areas when using all available station data (13 stations in the Goulburn and 9 stations at Yanco). Moreover, the error in average soil moisture for the areas did not asymptotically decrease as more measurements were included. This demonstrated that several of the monitoring stations installed are not representative of the broader area. However, through comparison of monitoring station combinations that produced the smallest error in average soil moisture, representative monitoring stations were identified for each area. Specifically, it was found that stations K3, M5, M6 and S1 produced a spatially averaged soil moisture estimate with a RMSE of less than 0.07 m/m for the Goulburn catchment, and that stations Y5, Y7, Y10 and Y12 produced a spatially averaged soil moisture estimate with a RMSE of less than 0.04 m/m for the Yanco area. An explicit assumption in the RMSE estimates is that both the aircraft and point measurements of soil moisture are error free.