Space-based passive microwave soil moisture retrievals and the correction for a dynamic open water fraction

B. T. Gouweleeuw, A. I. J. M. van Dijk, J. P. Guerschman, P. Dyce, M. Owe
2012 Hydrology and Earth System Sciences  
<p><strong>Abstract.</strong> The large observation footprint of low-frequency satellite microwave emissions complicates the interpretation of near-surface soil moisture retrievals. While the effect of sub-footprint lateral heterogeneity is relatively limited under unsaturated conditions, open water bodies (if not accounted for) cause a strong positive bias in the satellite-derived soil moisture retrieval. This bias is generally assumed static and associated with large, continental lakes and
more » ... nental lakes and coastal areas. Temporal changes in the extent of smaller water bodies as small as a few percent of the sensor footprint size, however, can cause significant and dynamic biases. We analysed the influence of such small open water bodies on near-surface soil moisture products derived from actual (non-synthetic) data from the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) for three areas in Oklahoma, USA. Differences between on-ground observations, model estimates and AMSR-E retrievals were related to dynamic estimates of open water fraction, one retrieved from a global daily record based on higher frequency AMSR-E data, a second derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and a third through inversion of the radiative transfer model, used to retrieve soil moisture. The comparison demonstrates the presence of relatively small areas (&amp;lt;0.05) of open water in or near the sensor footprint, possibly in combination with increased, below-critical vegetation density conditions (optical density &amp;lt;0.8), which contribute to seasonally varying biases in excess of 0.2 (m<sup>3</sup> m<sup>−3</sup>) soil water content. These errors need to be addressed, either through elimination or accurate characterisation, if the soil moisture retrievals are to be used effectively in a data assimilation scheme.</p>
doi:10.5194/hess-16-1635-2012 fatcat:kehhe3xugrevtffyu3t6ziwid4