In ungauged basins, space-based information is essential for the monitoring of the hydrological w... more In ungauged basins, space-based information is essential for the monitoring of the hydrological water cycle, in particular in regions undergoing large flood events where satellite data may be used as input to hydrodynamic models. A method for near 3D flood monitoring has been developed which uses synergies between radar altimetry and high temporal resolution multi-spectral satellites. Surface reflectances from the Moderate Resolution Imaging Spectroradiometer (MODIS) Terra instrument are used to map areas of open water as well as aquatic vegetation on a weekly basis, while water level variations in the inundated areas are provided by the radar altimetry from the Topex/Poseidon (T/P) and Envisat satellites. We applied this synergistic approach to several regions across the world (Diamantina Floodplain in Australia, Inner Niger Delta and Lake Chad in Africa, Andean Altiplano in South America, and Ganga River Delta in Asia). Based mainly on optical and Near Infra Red (NIR) imagery for ...
<strong>1.Summary</strong> This document describes the database that accompanies the ... more <strong>1.Summary</strong> This document describes the database that accompanies the article written by the authors of this dataset and accepted by Geophysical Research Letters (doi: 10.1029/2019GL082027).The database is distributed as a set of shapefiles, containing polylines that define the geometry of river centerlines located between 60°N and 56°S, with attributes described below. The shapefiles are organized according to continent and further broken into major basins to allow for manageable file sizes. A more complete dataset is available in the netCDF format upon request (please email Renato Frasson at frasson.1@osu.edu). This database was partially funded by the Algorithm Definition Team contract to the Ohio State University, University of North Carolina at Chapel Hill, and Remote Sensing Solutions, Inc. <strong>2.Polyline geometry</strong> The centerline geometry is defined by sets of points located approximately every 30 m based on the Global River Widths from Landsat (GRLW) database (Allen &amp; Pavelsky, 2015; 2018). Each line describes a meander and features the following attributes. <strong>3.Attribute description</strong> <strong>SegmentID:</strong> identification number of the river segment (segments are parts of a river delimited by confluences). <strong>lakeFlag:</strong> 0 – river, 1 – lake, 2 – river under the influence of tide, 3 – canal, 4 – unable to connect GRWL with HydroSHEDs, 5 – dam, -9999 – no data. <strong>Width:</strong> average width in the meander, disregarding small river widths assigned to locations undetected by Landsat but known to be inundated. Locations where no width could be produced are marked as -9999. <strong>Elevation:</strong> mean elevation from SRTM (90m) per river meander in meters. SRTM pixels are assigned to equally spaced points (every ~30m) over the river centerlines using the nearest neighbor approach. The average elevation of all valid points per meander is reported here. Locations where no elevation could be produced are marked as -9999. <st [...]
ABSTRACT The Surface Water and Ocean Topography (SWOT) mission is a swath mapping radar interfero... more ABSTRACT The Surface Water and Ocean Topography (SWOT) mission is a swath mapping radar interferometer that will provide global measurements of water surface elevation (WSE). The revisit time depends upon latitude and varies from two (low latitudes) to ten (high latitudes) per 22-day orbit repeat period. The high resolution and the global coverage of the SWOT data open the way for new hydrology studies. Here, the aim is to investigate the use of virtually generated SWOT data to improve discharge simulation using data assimilation techniques. In the framework of the SWOT virtual mission (VM), this study presents the first results of the automatic calibration of a global flow routing (GFR) scheme using SWOT VM measurements for the Amazon basin. The Hydrological Modeling and Analysis Platform (HyMAP) is used along with the MOCOM-UA multi-criteria global optimization algorithm. HyMAP has a 0.25-degree spatial resolution and runs at the daily time step to simulate discharge, water levels and floodplains. The surface runoff and baseflow drainage derived from the Interactions Sol-Biosphère-Atmosphère (ISBA) model are used as inputs for HyMAP. Previous works showed that the use of ENVISAT data enables the reduction of the uncertainty on some of the hydrological model parameters, such as river width and depth, Manning roughness coefficient and groundwater time delay. In the framework of the SWOT preparation work, the automatic calibration procedure was applied using SWOT VM measurements. For this Observing System Experiment (OSE), the synthetical data were obtained applying an instrument simulator (representing realistic SWOT errors) for one hydrological year to HYMAP simulated WSE using a &amp;quot;true&amp;quot; set of parameters. Only pixels representing rivers larger than 100 meters within the Amazon basin are considered to produce SWOT VM measurements. The automatic calibration procedure leads to the estimation of optimal parametersminimizing objective functions that formulate the difference between SWOT observations and modeled WSE using a perturbed set of parameters. Different formulations of the objective function were used, especially to account for SWOT observation errors, as well as various sets of calibration parameters.
The principal instrument of the SWOT mission is KaRIn, a Ka-band interferometric SAR system opera... more The principal instrument of the SWOT mission is KaRIn, a Ka-band interferometric SAR system operating on two near nadir swaths on opposite sides of the satellite track. This article describes the specificities of images from such a SAR system as compared to images acquired by conventional spaceborne SAR systems. Both radiometric and geometric aspects are covered.
In ungauged basin, space-based information is essential for the monitoring of hydrological water ... more In ungauged basin, space-based information is essential for the monitoring of hydrological water cycle, in particular in regions undergoing large flood events where satellite data may be used as input to hydrodynamic models. A method for near 3D flood monitoring has been developed which uses synergies between radar altimetry and high temporal resolution multi-spectral satellite. Surface Reflectance from the MODIS Terra instrument are used to map areas of open water as well as aquatic vegetation on a weekly basis, while water level variations in the inundated areas are provided by the radar altimetry from the Topex / Poseidon (T/P) and Envisat satellites. We present this synergistic approach to three different regions: Niger Inner delta and Lake Tchad in Africa, and Ganga river delta in Asia. Based mainly on visible and Near Infra Red (NIR) imagery is suitable to the observation of inundation extent. This method is well adapted for arid and semi arid regions, but less for equatorial ...
ABSTRACT Satellite measurements are used for hydrological investigations, especially in regions w... more ABSTRACT Satellite measurements are used for hydrological investigations, especially in regions where in situ measurements are not readily available. The future Surface Water and Ocean Topography (SWOT) satellite mission will deliver maps of water surface elevation (WSE) with an unprecedented resolution and provide observation of rivers wider than 100 m and water surface areas above 250 x 250 m over continental surfaces between 78°S and 78°N. The purpose of the study presented here is to use SWOT virtual data for the optimization of the parameters of a large scale river routing model, typically employed for global scale applications. The method consists in applying a data assimilation approach, the Best Linear Unbiased Estimator (BLUE) algorithm, to correct uncertain input parameters of the ISBA-TRIP Continental Hydrologic System. In Land Surface Models (LSMs), parameters used to describe hydrological basin characteristics are generally derived from geomorphologic relationships, which might not always be realistic. The study focuses on the Niger basin, a trans-boundary river, which is the main source of fresh water for all the riparian countries and where geopolitical issues restrict the exchange of hydrological data. As a preparation for this study, the model was first evaluated against in-situ and satellite derived datasets within the framework of the AMMA project. Since the SWOT observations are not available yet and also to assess the skills of the assimilation method, the study is carried out in the framework of an Observing System Simulation Experiment (OSSE). Here, we assume that modeling errors are only due to uncertainties in Manning coefficient field. The true Manning coefficient is then supposed to be known and is used to generate synthetic SWOT observations over the period 2002-2003. The satellite measurement errors are estimated using a simple instrument simulator. The impact of the assimilation system on the Niger basin hydrological cycle is then quantified. The optimization of the Manning coefficient using the BLUE algorithm from June 2002 to December 2003 leads to a significant improvement of the water levels over the river, and also at the 8 locations with gages. Indeed, the relative bias of the water level is globally improved (a 30 % reduction) and the amplitude of the water level is closer to the truth with assimilation than without assimilation. The relative bias of the Manning coefficient is also reduced (40% reduction) and the Manning coefficient globally converges towards an optimal value despite potential problems related to equifinality. The discharge is also improved by the assimilation, but to a lesser extent than for the water levels (7%). Moreover, the method allows a better prediction of the occurence and intensity of flood events in the inner delta and showed skill in simulating the maxima and minima of water storage anomalies in several continental reservoirs, especially the groundwater and the aquifer reservoirs, for which its evolution is difficult to observe. Finally, the study shows that the method is useful for hydrological forecasting over longer time periods than those of the calibration.
Abstract. This work was supported by the National Center for Space Studies in the framework of th... more Abstract. This work was supported by the National Center for Space Studies in the framework of the future mission surface water and ocean topography (SWOT). The estuarine zones with their complex hydrodynamics represent a great challenge for the future SWOT mission. The Seine estuary was selected by the SWOT Science Definition Team for the calibration/validation of the mission. Thus the aim of this study is to investigate the temporal hydrodynamic variability in the Seine estuary and the SWOT ability to reproduce this variability and hydrodynamic phenomena. For this, we used several statistical techniques and frequency analyses applied on the tide-gauge measurements of the Seine estuary and on SWOT simulated data. SWOT samples (SWOT simulated data) have been extracted synthetically from observations using the number of overpasses per repeat cycle [21 days (d)] in order to study the ability of SWOT to reproduce the time variability of the hydrodynamics. Results have demonstrated similar frequencies of multiyear, annual, and intraannual along the estuary from the Seine discharge to the Cherbourg sea level. The 11- to 22-d frequency has been only observed for estuarine and coastal time series suggesting the tide effect in these contexts. The distribution of levels has shown a high influence of tides and discharges at the downstream and the upstream, respectively. Low frequencies of water level are explained by the climate patterns of North Atlantic Oscillation with a mean range of 65%, whereas the high frequencies are associated to the periods of flooding and stormy events. Finally, simulated SWOT samples of water level show a better restitution of observations for high overpasses number. The results have shown a better restitution of hydrodynamic variability by SWOT in the river and upstream of estuary than in the downstream of estuary and coastal zone: in the estuary downstream and the coast, the annual mode is underexpressed while the 3-month (m) mode is overexpressed, which means the annual mode is underestimated and the 3-m mode is amplified by SWOT.
In ungauged basins, space-based information is essential for the monitoring of the hydrological w... more In ungauged basins, space-based information is essential for the monitoring of the hydrological water cycle, in particular in regions undergoing large flood events where satellite data may be used as input to hydrodynamic models. A method for near 3D flood monitoring has been developed which uses synergies between radar altimetry and high temporal resolution multi-spectral satellites. Surface reflectances from the Moderate Resolution Imaging Spectroradiometer (MODIS) Terra instrument are used to map areas of open water as well as aquatic vegetation on a weekly basis, while water level variations in the inundated areas are provided by the radar altimetry from the Topex/Poseidon (T/P) and Envisat satellites. We applied this synergistic approach to several regions across the world (Diamantina Floodplain in Australia, Inner Niger Delta and Lake Chad in Africa, Andean Altiplano in South America, and Ganga River Delta in Asia). Based mainly on optical and Near Infra Red (NIR) imagery for ...
<strong>1.Summary</strong> This document describes the database that accompanies the ... more <strong>1.Summary</strong> This document describes the database that accompanies the article written by the authors of this dataset and accepted by Geophysical Research Letters (doi: 10.1029/2019GL082027).The database is distributed as a set of shapefiles, containing polylines that define the geometry of river centerlines located between 60°N and 56°S, with attributes described below. The shapefiles are organized according to continent and further broken into major basins to allow for manageable file sizes. A more complete dataset is available in the netCDF format upon request (please email Renato Frasson at frasson.1@osu.edu). This database was partially funded by the Algorithm Definition Team contract to the Ohio State University, University of North Carolina at Chapel Hill, and Remote Sensing Solutions, Inc. <strong>2.Polyline geometry</strong> The centerline geometry is defined by sets of points located approximately every 30 m based on the Global River Widths from Landsat (GRLW) database (Allen &amp; Pavelsky, 2015; 2018). Each line describes a meander and features the following attributes. <strong>3.Attribute description</strong> <strong>SegmentID:</strong> identification number of the river segment (segments are parts of a river delimited by confluences). <strong>lakeFlag:</strong> 0 – river, 1 – lake, 2 – river under the influence of tide, 3 – canal, 4 – unable to connect GRWL with HydroSHEDs, 5 – dam, -9999 – no data. <strong>Width:</strong> average width in the meander, disregarding small river widths assigned to locations undetected by Landsat but known to be inundated. Locations where no width could be produced are marked as -9999. <strong>Elevation:</strong> mean elevation from SRTM (90m) per river meander in meters. SRTM pixels are assigned to equally spaced points (every ~30m) over the river centerlines using the nearest neighbor approach. The average elevation of all valid points per meander is reported here. Locations where no elevation could be produced are marked as -9999. <st [...]
ABSTRACT The Surface Water and Ocean Topography (SWOT) mission is a swath mapping radar interfero... more ABSTRACT The Surface Water and Ocean Topography (SWOT) mission is a swath mapping radar interferometer that will provide global measurements of water surface elevation (WSE). The revisit time depends upon latitude and varies from two (low latitudes) to ten (high latitudes) per 22-day orbit repeat period. The high resolution and the global coverage of the SWOT data open the way for new hydrology studies. Here, the aim is to investigate the use of virtually generated SWOT data to improve discharge simulation using data assimilation techniques. In the framework of the SWOT virtual mission (VM), this study presents the first results of the automatic calibration of a global flow routing (GFR) scheme using SWOT VM measurements for the Amazon basin. The Hydrological Modeling and Analysis Platform (HyMAP) is used along with the MOCOM-UA multi-criteria global optimization algorithm. HyMAP has a 0.25-degree spatial resolution and runs at the daily time step to simulate discharge, water levels and floodplains. The surface runoff and baseflow drainage derived from the Interactions Sol-Biosphère-Atmosphère (ISBA) model are used as inputs for HyMAP. Previous works showed that the use of ENVISAT data enables the reduction of the uncertainty on some of the hydrological model parameters, such as river width and depth, Manning roughness coefficient and groundwater time delay. In the framework of the SWOT preparation work, the automatic calibration procedure was applied using SWOT VM measurements. For this Observing System Experiment (OSE), the synthetical data were obtained applying an instrument simulator (representing realistic SWOT errors) for one hydrological year to HYMAP simulated WSE using a &amp;quot;true&amp;quot; set of parameters. Only pixels representing rivers larger than 100 meters within the Amazon basin are considered to produce SWOT VM measurements. The automatic calibration procedure leads to the estimation of optimal parametersminimizing objective functions that formulate the difference between SWOT observations and modeled WSE using a perturbed set of parameters. Different formulations of the objective function were used, especially to account for SWOT observation errors, as well as various sets of calibration parameters.
The principal instrument of the SWOT mission is KaRIn, a Ka-band interferometric SAR system opera... more The principal instrument of the SWOT mission is KaRIn, a Ka-band interferometric SAR system operating on two near nadir swaths on opposite sides of the satellite track. This article describes the specificities of images from such a SAR system as compared to images acquired by conventional spaceborne SAR systems. Both radiometric and geometric aspects are covered.
In ungauged basin, space-based information is essential for the monitoring of hydrological water ... more In ungauged basin, space-based information is essential for the monitoring of hydrological water cycle, in particular in regions undergoing large flood events where satellite data may be used as input to hydrodynamic models. A method for near 3D flood monitoring has been developed which uses synergies between radar altimetry and high temporal resolution multi-spectral satellite. Surface Reflectance from the MODIS Terra instrument are used to map areas of open water as well as aquatic vegetation on a weekly basis, while water level variations in the inundated areas are provided by the radar altimetry from the Topex / Poseidon (T/P) and Envisat satellites. We present this synergistic approach to three different regions: Niger Inner delta and Lake Tchad in Africa, and Ganga river delta in Asia. Based mainly on visible and Near Infra Red (NIR) imagery is suitable to the observation of inundation extent. This method is well adapted for arid and semi arid regions, but less for equatorial ...
ABSTRACT Satellite measurements are used for hydrological investigations, especially in regions w... more ABSTRACT Satellite measurements are used for hydrological investigations, especially in regions where in situ measurements are not readily available. The future Surface Water and Ocean Topography (SWOT) satellite mission will deliver maps of water surface elevation (WSE) with an unprecedented resolution and provide observation of rivers wider than 100 m and water surface areas above 250 x 250 m over continental surfaces between 78°S and 78°N. The purpose of the study presented here is to use SWOT virtual data for the optimization of the parameters of a large scale river routing model, typically employed for global scale applications. The method consists in applying a data assimilation approach, the Best Linear Unbiased Estimator (BLUE) algorithm, to correct uncertain input parameters of the ISBA-TRIP Continental Hydrologic System. In Land Surface Models (LSMs), parameters used to describe hydrological basin characteristics are generally derived from geomorphologic relationships, which might not always be realistic. The study focuses on the Niger basin, a trans-boundary river, which is the main source of fresh water for all the riparian countries and where geopolitical issues restrict the exchange of hydrological data. As a preparation for this study, the model was first evaluated against in-situ and satellite derived datasets within the framework of the AMMA project. Since the SWOT observations are not available yet and also to assess the skills of the assimilation method, the study is carried out in the framework of an Observing System Simulation Experiment (OSSE). Here, we assume that modeling errors are only due to uncertainties in Manning coefficient field. The true Manning coefficient is then supposed to be known and is used to generate synthetic SWOT observations over the period 2002-2003. The satellite measurement errors are estimated using a simple instrument simulator. The impact of the assimilation system on the Niger basin hydrological cycle is then quantified. The optimization of the Manning coefficient using the BLUE algorithm from June 2002 to December 2003 leads to a significant improvement of the water levels over the river, and also at the 8 locations with gages. Indeed, the relative bias of the water level is globally improved (a 30 % reduction) and the amplitude of the water level is closer to the truth with assimilation than without assimilation. The relative bias of the Manning coefficient is also reduced (40% reduction) and the Manning coefficient globally converges towards an optimal value despite potential problems related to equifinality. The discharge is also improved by the assimilation, but to a lesser extent than for the water levels (7%). Moreover, the method allows a better prediction of the occurence and intensity of flood events in the inner delta and showed skill in simulating the maxima and minima of water storage anomalies in several continental reservoirs, especially the groundwater and the aquifer reservoirs, for which its evolution is difficult to observe. Finally, the study shows that the method is useful for hydrological forecasting over longer time periods than those of the calibration.
Abstract. This work was supported by the National Center for Space Studies in the framework of th... more Abstract. This work was supported by the National Center for Space Studies in the framework of the future mission surface water and ocean topography (SWOT). The estuarine zones with their complex hydrodynamics represent a great challenge for the future SWOT mission. The Seine estuary was selected by the SWOT Science Definition Team for the calibration/validation of the mission. Thus the aim of this study is to investigate the temporal hydrodynamic variability in the Seine estuary and the SWOT ability to reproduce this variability and hydrodynamic phenomena. For this, we used several statistical techniques and frequency analyses applied on the tide-gauge measurements of the Seine estuary and on SWOT simulated data. SWOT samples (SWOT simulated data) have been extracted synthetically from observations using the number of overpasses per repeat cycle [21 days (d)] in order to study the ability of SWOT to reproduce the time variability of the hydrodynamics. Results have demonstrated similar frequencies of multiyear, annual, and intraannual along the estuary from the Seine discharge to the Cherbourg sea level. The 11- to 22-d frequency has been only observed for estuarine and coastal time series suggesting the tide effect in these contexts. The distribution of levels has shown a high influence of tides and discharges at the downstream and the upstream, respectively. Low frequencies of water level are explained by the climate patterns of North Atlantic Oscillation with a mean range of 65%, whereas the high frequencies are associated to the periods of flooding and stormy events. Finally, simulated SWOT samples of water level show a better restitution of observations for high overpasses number. The results have shown a better restitution of hydrodynamic variability by SWOT in the river and upstream of estuary than in the downstream of estuary and coastal zone: in the estuary downstream and the coast, the annual mode is underexpressed while the 3-month (m) mode is overexpressed, which means the annual mode is underestimated and the 3-m mode is amplified by SWOT.
Uploads
Papers by Christine Lion