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Operational river monitoring from Sentinel-3 radar altimetry
A review of the potential to establish a global, operational river monitoring based on Sentinel-3 water surface elevation observations
Cécile M. M. Kittel, [email protected]
Peter Bauer-Gottwein
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DTU Environment, Technical University of Denmark 04 December 2017
Background and motivation
• In-situ observations declining
• Need for observations for operational and long-term management purposes
• Solution:
– Alternative observation technologies combined with reliable simulations
– Remote sensing observations:
• Observations of important components of the land-surface water balance
• Global coverage and gridded data
• New missions are publically accessible and offer new possibilities
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1900 1920 1940 1960 1980 20000
2000
4000
6000
8000
Nu
mb
er
of s
tatio
ns
Sentinel-3 satellite
River discharge, GRDC, 2014
SITAEL, 2016
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DTU Environment, Technical University of Denmark 04 December 2017
Measuring discharge from space?
• Discharge not directly observable
• Two evolving approaches:
– Water level from satellite altimetry
• Discharge estimated from rating curves or using hydrodynamic models
– Water surface extent from multi-mission remote sensing data
• E.g. Pekel et al., Nature (2016)
– Combining the two: volume/height and extent holds great potential for hydrological applications
• Existing networks with altimetry observations for global water bodies:
– Hydroweb (Crétaux et al. ASR (2011))
– DAHITI (Schwatke et al. HESS (2015))
New missions, e.g. Sentinel-3, will ensure continuity
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DTU Environment, Technical University of Denmark 04 December 2017
Altimetry observations
• Altimeter sends a pulse and records the return echo
• Waveform is then ”retracked” to extract nadir height
• Water height in inland water bodies.
• Uncertainties in range of 0.5 m
• Main issue for inland water bodies: surrounding land cover and topography
• Position must be precisely known
http://altimetry.info, Credits: CNES/D. Ducros
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DTU Environment, Technical University of Denmark 04 December 2017
Example: The Ogooué, Africa
• Extracted using Sentinel-1 water mask – four altimetry missions combined (Kittel et al., 2017, HESSD)
• Water surface elevation (WSE) above a reference (geoid) for the entire river
• Interpolate to annual water height amplitude
• Drifting groundtrack = longitudinal WSE profile
• Fixed groundtrack = Time series at “Virtual Station”
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ICESat Water Elevation [m]
0-47
47-95
95-142
142-189
427-474
CryoSat-2 Water Elevation [m]
0 - 47 47 - 95 95 - 142 142 - 189 189 - 237 237 - 284 284 - 332 332 - 379 379 - 427 427 - 474
Legend
0 50 100 150 200 km
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DTU Environment, Technical University of Denmark 04 December 2017
Sentinel-3
• European Space Agency (ESA) mission
– Sentinel-3A launched in 2016
– Sentinel-3B scheduled for launch in 2018
• Operates in Synthetic Aperture Radar (SAR) mode
• Ground track separation of 104 km (52 km with Sentinel-3B)
• Revisit time of 27 days
• For 98 of the worlds largest rivers:
– 2940-5898 virtual stations
– One VS every 44 km of river
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Sentinel-3 virtual stations, Amazon river
Virtual stations concept
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DTU Environment, Technical University of Denmark 04 December 2017
Sentinel-3
What can we expect?
• Unmonitored basin, the Ogooué
– 39 VS
– 1540 km river
– Observations every ~40 km
– Currently no monitoring
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DTU Environment, Technical University of Denmark 04 December 2017
Discharge from water surface elevation (WSE)
Different approaches:
Method 1: Ground surveys at virtual stations to establish rating curve
– Michailovsky et al. HESS (2012)
Method 2: Regression results from large global datasets
– Bjerklie et al., J Hydrol (2003)
– Gleason and Smith, PNAS (2014)
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Correlation between hydraulic geometry coefficients (Gleason and Smith, 2014)
Rating curves (Michailovsky et al., 2012)
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DTU Environment, Technical University of Denmark 04 December 2017
Discharge from water surface elevation (WSE)
Method 3: Informing hydraulic models with WSE
– E.g. Kittel et al., HESSD (2017), Schneider et al., HESS (2017)
Method 4: UAV surveys at VS locations to establish rating curve
– Hydraulic variables observable from UAV platform
• E.g. Bandini et al., J Hydrol (2017)
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Qy
w
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DTU Environment, Technical University of Denmark 04 December 2017
Conclusion and Outlook
Great potential in operational monitoring using satellite altimetry
• Sentinel-3 will be a significant inland water monitoring asset
• Value will increase significantly if rating curves can be estimated accurately and efficiently
• We need a new, innovative technology to properly setup rating curves
• UAVs attractive for estimating rating curves at Sentinel-3 VS locations
– Bandini et al., Session 8, 17:40
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Wang, 2016