Large scale surface soil moisture by radar scatterometer

Global maps of the soil moisture content in the surface layer (0.5 - 2 cm) generated from the MetOp scatterometer (ASCAT) processed soon after each satellite orbit completion and presented in the natural projection of the image from sunsynchronous orbit. It is a coarse-resolution product (25 km), controlled by the instrument IFOV. It is run at the EUMETSAT Headquarters (backed by ZAMG) on the base of algorithms and software developed by TU-Wien prior to the start of H-SAF, and connected to H-SAF for further improvement.

  • Coverage: Strips of 1000 km swaths covering in succession the whole globe
  • Cycle: 36 hours for full coverage over Europe
  • Resolution: 25 km, constant through the field of view
  • Accuracy: 0.05 m³ m-³, degrading in the presence of forest, mountains, rock outcrops, water surfaces, urban areas
  • Timeliness: 130 min (potentially 30 min using the EARS service)
  • Dissemination: By EUMETCast
  • Formats: BUFR

Short description of the basic principles for product generation

The ERS-1/2 and MetOp scatterometers offer the opportunity to measure soil moisture in a relatively direct manner because of the high sensitivity of microwaves to the water content in the soil surface layer due to the pronounced increase in the soil dielectric constant with increasing water content. This is specifically the case in the low frequency region (1-10 GHz). However, scattering from land surfaces also depends on other factors (vegetation, surface roughness). The benefit of using the ERS and MetOp scatterometers for soil moisture retrieval is their unique sensor design, which enables direct accounting for the confounding effects of surface roughness, vegetation and dielectric properties. Outstanding sensor characteristics are:

  • the multi-incidence angle viewing capability which allows separating vegetation and soil moisture effects which both influence the measured backscatter coefficient;
  • the high temporal sampling rate which allows monitoring of changes in the highly variable soil wetness conditions through a change detection approach (while surface roughness can be assumed to be constant at this spatial scale);
  • the excellent radiometric accuracy which results in a low noise level and allows analysing multi annual time series.