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S. Hasenauer, Z. Bartalis, V. Naeimi, C. Pathe, D. Sabel, M. Doubkova, W. Wagner, A. Bartsch:
"Operational drought monitoring with METOP-ASCAT and ENVISAT-ASAR";
Talk: United Nations/Austria/European Space Agency Symposium on Space Applications for Sustainable Development to Support the Plan of Implementation of the World Summit on Sustainable Development "SPACE TOOLS AND SOLUTIONS FOR MONITORING THE ATMOSPHER, Austrian Academy of Sciences, Graz (invited); 2008-09-09 - 2008-09-12.

Soil moisture is an important parameter for understanding the water cycle and for applications concerning vegetation and plant growth. It is a widely used parameter in hydrological modelling, as well as numerical weather prediction, flood forecasting or drought monitoring. The Vienna University of Technology (TU Wien) has experience in long-term monitoring of soil moisture datasets from several microwave satellite sensors.

The scatterometers onboard ERS-1/2 (European Remote Sensing satellite, AMI instrument) and METOP (Meteorological Operational satellite, ASCAT instrument) offer the opportunity to measure soil moisture in a relatively direct manner due to the high sensitivity of microwaves to the water content in the soil surface layer because of the pronounced increase in the soil dielectric constant with increasing water content. The soil moisture retrieval method is based on a change-detection approach that accounts for the effects of surface roughness, vegetation, and heterogeneous land cover. The influence of vegetation is determined by exploiting the multi-incidence angle viewing capabilities of the scatterometer sensor and allows the retrieval of surface soil moisture information for the topmost 2 cm with a spatial resolution of 50 and 25km, respectively. Given the heritage of ERS and the series of three METOP scatterometers, a global coverage can be achieved for a time scale of 1991 to about 2021. Moreover, this service has the advantage of operational availability and standardised product format via EUMETSAT facilities.

Furthermore, TU Wien is developing a scheme for spatial downscaling by transferring the change detection algorithms from the scatterometer to ENVISAT (Environmental Satellite, ASAR instrument), in order to derive a 1 km surface soil moisture product for selected regions.

This work gives an overview of the soil moisture retrieval methods, highlights the importance of soil moisture for monitoring droughts and introduces the products at different spatial scales.