Global Drought Monitoring Service through the GEOSS Architecture ...

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Architectural Implementation Pilot, Phase 3 Version: 2.0 Global Drought Monitoring and European Drought Observatory-Water SBA Engineering Report Date: 11/Feb/2011 • Near real-time reporting on disasters (droughts, floods) • Medium and long-term forecasting (also with respect to climate change) • Promotion of new satellite-capabilities (e.g. Sentinel 1) • Matching new satellite-capabilities to specific user requirements GLOWASIS will be made interoperable with the Water Information System for Europe (WISE-RTD), linking water demand and supply with existing tools, such as the European Drought Observatory (EDO) and PCR-GLOBWB, a global hydrological model (the same model used in DEWFORA), combining complex water cycle variables in a standardized format with respect to water scarcity information. Sources of information and data are: • Already existing GMES (Global Monitoring for Environment and Security) data, such as the LMCS (Land Monitoring Core Service) of GEOLAND2, • in-situ data from GEWEX (Global Energy and Water Cycle Experiment) and Global Terrestrial Network on Hydrology (GTN-H) initiatives, such as the International Soil Moisture Network, • statistical databases (e.g. AQUASTAT and SEEAW) Results of GLOWASIS can be used in research, for practical implementation and management purposes. Therefore, end-users encompass river basin management organizations (Rhine, Danube, Elbe, Oder), the European Environment Agency, UN-Water, the Australian Bureau of Meteorology, etc. GLOWASIS is coordinated by DELTARES, the Netherlands. The Institute of Photogrammetry and remote Sensing (IPF) leads one work package (user requirements) and is involved in all others. As one of IPF’s most successful projects on soil moisture, SHARE will also contribute to GLOWASIS. The following sub-chapters give an overview about soil moisture products from ASAR (advanced synthetic aperture radar) and scatterometer sensors. SHARE is a DUE Tiger Project of the European Space Agency, which offers an operational soil moisture monitoring service. The synergistic use of ENVISAT's ASAR sensor and scatterometers (on METOP and ERS) allows for frequent, high resolution monitoring of regional soil moisture dynamics. An algorithm was developed at IPF to detect surface soil moisture from active microwave systems. Active sensors are sensitive to soil moisture mainly due to distinct dielectric properties of water stored in soil. Microwaves of the Advanced Synthetic Aperture Radar (ASAR) and the advanced scatterometer (ASCAT) cannot penetrate soil deeper than a few centimetres. In case of ASCAT an algorithm was developed, which models the soil water content in deeper layers (the soil water index, SWI). It is obtained by filtering surface moisture time series with an Page 22
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 Page 23
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