Global Drought Monitoring Service through the GEOSS Architecture ...

Architectural Implementation Pilot, Phase 3 Version: 2.0
Global Drought Monitoring and European Drought
Observatory-Water SBA Engineering Report
Date: 11/Feb/2011
exponential function (WAGNER et al., 1999). Being able to model the profile soil moisture up to
one metre facilitates estimations of infiltration capacities and plant available water (defined as
the difference between field capacity and permanent wilting point). This is the approach used in
the agricultural monitoring and forecasting models cited in section 1.3 above. Flooded soils are
more prone to cause flooding, as noted in section 2.1.2 above.
The two systems to obtain soil moisture data:
• Medium resolution soil moisture from an imaging Advanced Synthetic Aperture Radar
(ASAR) onboard ENVISAT can be operated in global monitoring or wide swath mode. It
was the first system to deliver global backscatter measurements in C-Band (5.3 GHz) at a
spatial resolution of one kilometre. Spatial resolutions of 150 meters can be achieved by
SCAN SAR wide-swath mode. In the SHARE project, regions on three continents have
been monitored once or twice a week. Soil roughness and vegetation effects of each pixel
are “corrected” by change detection method – the subtraction of a reference image from a
SAR image. This way the inhomogeneous distribution of soil water in the topmost
centimetres of the unsaturated zone, where evapotranspiration takes place, can be
considered. The most recent version of the ASAR data viewer is online at:
http://www.ipf.tuwien.ac.at/radar/dv/ipfdv/index.php?dataviewer=asar2
• Scatterometers onboard METOP (ASCAT), ERS-1 and ERS-2 (SCAT) are non-imaging
sensors and characterised by higher temporal (1-2 days), but lower spatial resolution.
Change detection works similar to the SAR system. ASCAT is a collaboration of
EUMETSAT and IPF. It was declared operational in December 2008 and is now
produced in near real-time by EUMETSAT, using the WARP-NRT software. This
software had been prototyped by EUMETSAT and developed by IPF. ASCAT soil
moisture is a Level 2 product delivered in orbit geometry at two different grid spacings:
25 km and 12.5 km. The two products are derived directly and on the same grid as the
equivalent ASCAT Level 1b products (normalized backscatter).Consequently, the
resolution of the soil moisture values is approximately 50km and 35 km.
Thorough validation of ERS scatterometer and ASAR demonstrated a good
correspondence of satellite and in-situ data (DORIGO, 2010). The correlation of ASAR results to
in-situ measurements is slightly weaker than the ones of scatterometers on board ERS, mainly
due to its lower radiometric resolution. However, the correlation of ASAR and in-situ data
improves significantly when averaged over larger areas (PATHE et al., 20009, MLADENOVA
et al., 2010). ASCAT products are spatially variable with high quality over grassland and
agricultural areas and lower quality in more densely vegetated areas and deserts. The
investigation of soil moisture at medium scale is a critical assess for IPF’s efforts for
downscaling of active and passive sensors. Field studies showed that, despite high spatiotemporal
variability of soil moisture, its correlation to the mean soil moisture over a larger area is
significant in the temporal domain.
Recent flood events (January 2011) in Eastern Australia affected more than 200 000
people and an area as big as the size of France and Germany combined. ASAR observations can
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