Suchergebnisse

© 2021 by the authors. ; We proposed a direct approach to validate hectometric and kilometric resolution leaf area index (LAI) products that involved the scaling up of field-measured LAI via the validation and recalibration of the decametric Sentinel-2 LAI product. We applied it over a test study area of maize crops in northern China using continuous field measurements of LAINet along the year 2019. Sentinel-2 LAI showed an overall accuracy of 0.67 in terms of Root Mean Square Error (RMSE) and it was used, after recalibration, as a benchmark to validate six coarse resolution LAI products: MODIS, Copernicus Global Land Service 1 km Version 2 (called GEOV2) and 300 m (GEOV3), Satellite Application Facility EUMETSAT Polar System (SAF EPS) 1.1 km, Global LAnd Surface Satellite (GLASS) 500 m and Copernicus Climate Change Service (C3S) 1 km V2. GEOV2, GEOV3 and MODIS showed a good agreement with reference LAI in terms of magnitude (RMSE ≤ 0.29) and phenology. SAF EPS (RMSE = 0.68) and C3S V2 (RMSE = 0.41), on the opposite, systematically underestimated high LAI values and showed systematic differences for phenological metrics: a delay of 6 days (d), 20 d and 24 d for the start, peak and the end of growing season, respectively, for SAF EPS and an advance of −4 d, −6 d and −6 d for C3S. ; This research was funded by the National Natural Science Foundation of China under Grant (41971282; 42001303), the Sichuan Science and Technology Program (2021JDJQ0007; 2020JDTD0003), the Copernicus Global Land Service (CGLOPS-1, 199494-JRC), and the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant 835541.

Abstract. Detachments of large parts of low-angle mountain glaciers in
recent years have raised great attention due to their threats to lives and
properties downstream. While current studies have mainly focused on
post-event analysis, a few opportunities have presented themselves to assess
the potential hazards of a glacier prone to detachment. Here we present a
comprehensive analysis of the dynamics and runout hazard of a low-angle
(∼20∘) valley glacier, close to the Qinghai–Tibet
railway and highway, in the East Kunlun Mountains on the Qinghai–Tibet
Plateau. The changes in morphology, terminus position, and surface elevation
of the glacier between 1975 and 2021 were characterized with a stereo-image
pair from the historical KH-9 spy satellite, six digital elevation models
(DEMs), and 11 high-resolution images from Planet Labs. The surface flow
velocities of the glacier tongue between 2009 and 2020 were also tracked
based on cross-correlation of Planet images. Our observations show that the
glacier snout has been progressively advancing in the past 4 decades,
with a stepwise increase in advance velocity from 4.55±0.46ma-1 between 1975 and 2009 to 30.88±2.36ma-1 between 2015 and 2020. DEM differencing confirms the
glacial advance, with surface thinning in the source region and thickening
in the tongue. The net volume loss over the glacier tongue was about
11.21±2.66×105 m3 during 1975–2018. Image
cross-correlation reveals that the surface flow velocity of the glacier
tongue has been increasing in recent years, with the mean velocity below
4800 m more than tripling from 6.3±1.8ma-1 during
2009–2010 to 22.3±3.2ma-1 during 2019–2020. With
a combined analysis of the geomorphic, climatic, and hydrologic conditions
of the glacier, we suggest that the flow of the glacier tongue is mainly
controlled by the glacier geometry, while the presence of an ice-dammed lake
and a supraglacial pond implies a hydrological influence as well. Taking the
whole glacier and glacier tongue as two endmember avalanche sources, we
assessed the potential runout distances of these two scenarios using the
angle of reach and the Voellmy–Salm avalanche model. The assessments show
that the avalanche of the whole glacier would easily travel a distance that would threaten the safety of the railway. In contrast, the detachment of the
glacier tongue would threaten the railway only with a small angle of reach
or when employing a low-friction parameter in the Voellmy–Salm modeling.

The Daguangbao mega-landslide (China), induced by the 2008 Wenchuan earthquake (Mw = 7.9), with an area of approximately 8 km2, is one of the largest landslides in the world. Experts predicted that the potential risk and instability of the landslide might remain for many decades, or even longer. Monitoring the activity of such a large landslide is hence critical. Terrain Observation by Progressive Scans (TOPS) mode from the Sentinel-1 satellite provides us with up-to-date high-quality Synthetic Aperture Radar (SAR) images over a wide ground coverage (250 × 250 km), enabling full exploitation of various InSAR applications. However, the TOPS mode introduces azimuth-dependent Doppler variations to radar signals, which requires an additional processing step especially for SAR interferometry. Sentinel-1 TOPS data have been widely applied to earthquakes, but the performance of TOPS data-based time series analysis requires further exploitation. In this study, Sentinel-1 TOPS data were employed to investigate landslide post-seismic activities for the first time. To deal with the azimuth-dependent Doppler variations, a processing chain of TOPS time series interferometry approach was developed. Since the Daguangbao landslide is as a result of the collapse of a whole mountain caused by the 2008 Mw 7.9 Wenchuan earthquake, the existing Digital Elevation Models (DEMs, e.g. SRTM and ASTER) exhibit height differences of up to approximately 500 m. Tandem-X images acquired after the earthquake were used to generate a high resolution post-seismic DEM. The high gradient topographic errors of the SRTM DEM (i.e. the differences between the pre-seismic SRTM and the actual post-seismic elevation), together with low coherence in mountainous areas make it difficult to derive a precise DEM using the traditional InSAR processing procedure. A re-flattening iterative method was hence developed to generate a precise TanDEM-X DEM in this study. The volume of the coseismic Daguangbao landslide was estimated to be of 1.189 ± 0.110 × 109 m3 by comparing the postseismic Tandem-X DEM with the preseismic SRTM DEM, which is consistent with the engineering geological survey result. The time-series results from Sentinel-1 show that some sectors of the Daguangbao landslide are still active (and displaying four sliding zones) and exhibiting a maximum displacement rate of 8 cm/year, even eight years after the Wenchuan earthquake. The good performance of TOPS in this time series analysis indicates that up-to-date high-quality TOPS data with spatiotemporal baselines offer significant potential in terms of future InSAR applications. ; This work was supported by the National Natural Science Foundation of China under Grant No. 41474003. The research stay of Dr. Tomás at Newcastle University was funded by the Ministry of Education, Culture and Sport within the framework of Project PRX14/00100. Additional funding was obtained from the Spanish Government under projects TIN2014-55413-C2-2-P and ESP2013-47780-C2-2-R. Part of this work is also supported by the UK Natural Environmental Research Council (NERC) through the Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET, ref.: come30001) and the LICS and CEDRRiC projects (ref. NE/K010794/1 and NE/N012151/1, respectively), the ESA-MOST DRAGON-3 projects (ref. 10607 and 10665), the ESA-MOST DRAGON-4 project (ref. 32244) and the Open Fund from the Key Laboratory of Earth Fissures Geological Disaster, Ministry of Land and Resources (ref.: gla2013001).