Estimating long term surface soil moisture from satellite microwave observations in Illinois, USA

A database of long-term soil moisture was compared to satellite microwave observations over a test site in the midwestern United States. Night-time microwave brightness temperatures were observed at a frequency of 6.6 GHz by the Scanning Multichannel Microwave Radiometer (SMMR). At 6.6 GHz, the instrument provides a spatial resolution of approximately 150 km, and a temporal frequency over the test area of about two to three night-time orbits per week. Vegetation radiative transfer characteristics, such as the canopy transmissivity, were estimated from vegetation indices such as the NDVI and the MPDI. Because the time of satellite coverage does not always coincide with the ground measurements of soil moisture, the existing ground data were used to calibrate a water balance for the top 10 cm surface soil layer in order to interpolate daily surface moisture values. Passive microwave remote sensing presents the greatest potential for providing regular spatially representative estimates of surface soil moisture at global scales.