WP 2: Archive remote sensing data
Archive remote sensing data and historic photos taken in the landscape shall be analyzed to reconstruct hydrological changes taking place during the last decades. This WT aims at consolidating and completing hydrological information reconstructed from geo-archives for the last decades. Furthermore, this WP will support the interpretation of SAR (Synthetic Aperture Radar) signatures in terms of soil moisture retrieval on the basis of signatures provided by past, recent, and future remote sensors. Hence, this approach will improve the connectivity between different data sources, and hence promote our understanding of hydrological changes taking place at around the present time. The paramount goals of this WP are to identify annual changes of spatio-temporal patterns of soil surface moisture and identifying trends in the subsurface hydrology over the last decades.
Time series of soil surface moisture maps will be derived from archive SAR data. For the investigation area C-band SAR data are available from the European Space Agency’s (ESA) missions ERS-1 (1991 - 2000), ERS-2 (1995 - today), and ENVISAT (2002 - today) providing typically one data frame per month with spatial resolution of better than 30 m. Furthermore, archives of optical- and multispectral remote sensing data will be considered to determine the SAR pixels whose backscatter provides reliable moisture information. Processing of SAR raw data to calibrated stacks of geo-referenced and co-registered Single Look Complex images (SLC) will be done routinely by the GFZ section 1.4. Time-series of soil moisture will then be generated from the backscatter (the real part of the SLC´s), which typically varies by about 6 dB (factor 4) for soil moisture ranging from completely dry to saturated.
Ground-truth soil moisture data measured in-situ is compulsory for developing, calibrating and validating interrelations between soil moisture and SAR backscatter. Such ground-truth information is available for the German Aerospace Center (DLR) test site “Durable Environmental Multidisciplinary Monitoring Information Network” (DEMMIN) that is part of the investigation area. Once the retrieval approach is calibrated for the DEMMIN test site, it will be applied to a larger area encompassing the entire investigation area including amongst others the Ucker catchment. Than time-series of soil moisture will be generated for the SAR pixels identified to carry reliable moisture information on the basis of the optical- and multispectral remote sensing data.
As already mentioned, the study area comprises a large number of lakes whose levels dropped significantly over the last decades. Due to the generally shallow ground water table in this region, many of the lakes are strongly coupled with both, the sub-surface hydrology, and also with the moisture within the upper soil layers (root-zone). These instances will be utilized for deriving perennial subsurface hydrological parameters from time series of lake levels that can be derived from archives of satellite images and aerial photos. In addition to these remote sensing data we will call the resident population to provide us with photos taken at specific lakefronts to reconstruct lake level variations.