Spatially enhanced passive microwave derived soil moisture: Capabilities and opportunities

Low frequency passive microwave remote sensing is a proven technology for providing soil moisture estimates, but the coarse resolution of its data restricts the range of applications. Downscaling, otherwise known as disaggregation, has been proposed as the solution to spatially enhance these coarse resolution soil moisture observations, through association with complementary observations, or ancillary information about land surface features at higher spatial resolution. Such information includes solar reflectance, thermal emission, passive microwave emissions at a higher frequency, radar backscatter, soil or surface attributes such as topography and soil properties, and land surface modelling. Each of these ancillary data sources has its own strengths and limitations in terms of, for example, sensitivity to surface soil moisture dynamics and availability. This paper provides an extensive review of the capabilities and opportunities of current soil moisture downscaling approaches which provide a deterministic pattern of soil moisture, together with their strengths and limitations.