Jean-François Crétaux

Annual vertical crustal motions predicted from surface mass redistribution and observed by space geodesy

Temporal variations of surface mass redistribution among atmosphere, oceans, and continental water reservoirs deform the Earths crust, in particular in the vertical direction. These displacements can now be measured by space geodesy and predicted from climatic loading data. In this study we first compute globally theoretical vertical displacements of the Earths crust caused by the main annual surface mass redistributions (atmosphere and ocean mass, soil moisture, and snow load). For that purpose we consider atmospheric pressure data from the National Centers for Environment Prediction (NCEP), soil moisture data from Huang et al. [1996] and from the Global Soil Wetness Project (GSWP), snow data from the International Satellite and Surface Climatology Project (ISLSCP) and GSWP, and ocean mass data from the Parallel Ocean Climate Model (POCM) and from TOPEX-Poseidon satellite altimetry after correcting for steric effects. Annual vertical displacements are computed for each load individually as well as for the total climatic contribution on global 2.5°×2.5° grids. We then present space geodesy-derived annual variations of vertical coordinates of 16 Doppler orbitography and radiopositioning integrated by satellite (DORIS) stations globally distributed around the globe. A comparison is then performed for each station between observed (by DORIS) and predicted (from climatology) results.

Evolution of Sea Level of the Big Aral Sea from Satellite Altimetry and Its Implications for Water Balance

Abstract The Aral Sea was one of the biggest lakes in the world before it started to shrink in the 1960s due to water withdrawal for land irrigation. Sea level decreases led to the separation of the Aral Sea into two basins—the Small Aral in the north and the Big Aral in the south. For several decades there were no continuous observations of Aral Sea level, and the few data that exist are fragmentary or unavailable. We present observations of the Big Aral Sea level estimated from the TOPEX/Poseidon (T/P) altimetry with high temporal resolution over the last decade (1993–2004). Since sea volume is one of the key parameters for the studies of water balance, we use the T/P-derived time series of sea level and a dedicated digital bathymetry model (DBM) to reconstruct temporal changes in the Aral Sea surface and volume. We introduce variations of the sea volume as the new constraint for the water budget of the Big Aral Sea. This is an important step toward estimating detailed seasonal and interannual changes of the water budget. We assess various existing components of the water budget of the Aral Sea and discuss the quality of the existing data and their applicability for establishing detailed water balance. In particular, large uncertainties in estimating the evaporation and underground water supply are addressed. Desiccation of the Aral Sea resulted in dramatic changes in the salinity regime and, consequently, affected its aquatic ecosystems. We also discuss changes in the aquatic fauna and their possible evolution under continuing desiccation of the Big Aral Sea. Combining satellite altimetry with other parameters of the water budget offers a promising potential for assessing temporal changes in the water budget of arid or semiarid regions, even those with a poor ground monitoring network.

Vertical crustal motions from the DORIS Space-Geodesy System

Five years (1993–1997) of DORIS data on the SPOT and Topex-Poseidon satellites have been analyzed to estimate vertical motions at 33 sites of the permanent DORIS network. Rates have been determined assuming constant velocity over the 5-year time span. Except for a few stations, vertical rates are lower than 5 mm/yr. Comparison with solutions provided by GPS, SLR or VLBI at colocated sites shows good agreement for most stations. Some space-geodesy vertical rates however disagree significantly. This may result from interannual to decadal fluctuations affecting the vertical motions, giving rise to different apparent linear trends if estimated over different time spans. Except for six high-latitude stations, post-glacial rebound produces negligible signal and cannot explain the observed motions. Other local or regional phenomena from geophysical or anthropogenic origin may be responsible for the estimated vertical motions.

Present‐day tectonic plate motions and crustal deformations from the DORIS space system

Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) data acquired between January 1993 and December 1996 from the SPOT-2, SPOT-3, and TOPEX/Poseidon satellites have been analyzed to determine velocities for 45 sites on eight major tectonic plates. For 28 sites far from deformation zones, the velocity estimates agree with plate model predictions. Least squares computation of poles of rotation, which model the plate motions, shows that for Eurasia, Africa, Pacific, and South America plates, the agreement is better with NUVEL-1, while for Australia, Antarctica, Nazca, and North America plates the DORIS Euler vectors are closer to NTJVEL-1A. In general, DORIS results do not differ significantly from other space geodetic techniques determinations but provide better estimates for plates poorly or inhomogeneously covered by Global Positioning System (GPS), Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) networks, such as Africa. The DORIS coverage of this plate allows discussion of intraplate deformations due to the motion of the eastern Africa part which constitues the Somalia plate. Sites located in deformation zones, such as western Eurasia boundaries, central Asia, southwestern America coast, South East Asia, show motion with respect to their own plates. Comparisons with other geodetic measurements for colocated stations, or with regional geodynamical models, show the interest of DORIS in active zones where global plate models are not valid.

Global surveys of reservoirs and lakes from satellites and regional application to the Syrdarya river basin

Large reservoirs along rivers regulate downstream flows to generate hydropower but may also store water for irrigation and urban sectors. Reservoir management therefore becomes critical, particularly for transboundary basins, where coordination between riparian countries is needed. Reservoir management is even more important in semiarid regions where downstream water users may be totally reliant on upstream reservoir releases. If the water resources are shared between upstream and downstream countries, potentially opposite interests arise as is the case in the Syrdarya river in Central Asia. In this case study, remote sensing data (radar altimetry and optical imagery) are used to highlight the potential of satellite data to monitor water resources: water height, areal extent and storage variations. New results from 20 years of monitoring using satellites over the Syrdarya basin are presented. The accuracy of satellite data is 0.6 km3 using a combination of MODIS data and satellite altimetry, and only 0.2 km3 with Landsat images representing 2–4% of average annual reservoir volume variations in the reservoirs in the Syrdarya basin. With future missions such as Sentinel-3A (S3A), Sentinel-3B (S3B) and surface water and ocean topography (SWOT), significant improvement is expected. The SWOT missions main payload (a radar interferometer in Ka band) will furthermore provide 2D maps of water height, reservoirs, lakes, rivers and floodplains, with a temporal resolution of 21 days. At the global scale, the SWOT mission will cover reservoirs with areal extents greater than 250 × 250 m with 20 cm accuracy.

Ice cover variability in the Caspian and Aral seas from active and passive microwave satellite data

The paper discusses time and space variations of ice extent in the Caspian and Aral seas during the last decade (1992–2002). It uses synergy of data from active (radar altimeter) and passive (radiometer) microwave nadirlooking instruments onboard the TOPEX/Poseidon satellite. The proposed approach is substantiated and validated using both in situ and satellite imagery data for the Caspian Sea. The results indicate significant spatial and temporal variability of ice conditions, with a significant decrease of both the duration of ice season and ice extent during the last four winters (1998–2002). The TOPEX/Poseidon-derived time series of sea ice extent are very valuable in view of the fragmentary and mostly unpublished data on ice conditions on the Caspian and Aral seas since the mid-1980s.

Water level estimation by remote sensing for the 2008 flooding of the Kosi River

Flood is a natural disaster which worsens when it is triggered by man-made constructions. This paper discusses one such flood event which occurred because of breach of a levee in the upper reach of the Kosi River in 2008, when floodwater spread over a large portion of the low-lying Ganga Plain of North Bihar, India. Here we have analysed a suite of space-based observations from radar altimetry, Moderate Resolution Imaging Spectroradiometer (MODIS) images, and Tropical Rainfall Measuring Mission (TRMM) precipitation data, together with in situ monthly precipitation data, with a main emphasis on the results from altimetry and MODIS data. A methodology to calculate water levels, using MODIS data and Envisat data together, is also discussed. Our analyses suggest a rise in water level of 1.0–1.4 m in the flooded region during the flood event and a maximum extent for the flooded area of around 2900 km2. Analyses of TRMM precipitation data do not indicate any influence of high precipitation in the upper catchment of the Kosi Basin on river water feeding into the plain area after breaching of dam. However, heavy and prolonged precipitation was found downstream of the dam over the flooded area during the flood period.

Remote Sensing-Derived Bathymetry of Lake Poopó

Abstract: Located within the Altiplano at 3,686 m above sea level, Lake Poopo is remarkably shallow and very sensitive to hydrologic recharge. Progressive drying has been observed in the entire Titicaca-Poopo-Desaguadero-Salar de Coipasa (TPDS) system during the last decade, causing dramatic changes to Lake Poopo’s surface and its regional water supplies. Our research aims to improve understanding of Lake Poopo water storage capacity. Thus, we propose a new method based on freely available remote sensing data to reproduce Lake Poopo bathymetry. Laser ranging altimeter ICESat (Ice, Cloud, and land Elevation Satellite) is used during the lake’s lowest stages to measure vertical heights with high precision over dry land. These heights are used to estimate elevations of water contours obtained with Landsat imagery. Contour points with assigned elevation are filtered and grouped in a points cloud. Mesh gridding and interpolation function are then applied to construct 3D bathymetry. Complementary analysis of Moderate Resolution Imaging Spectroradiometer (MODIS) surfaces from 2000 to 2012 combined with bathymetry gives water levels and storage evolution every 8 days.

Synergy of active and passive satellite microwave data for the study of first-year sea ice in the Caspian and Aral seas

The paper discusses application of active and passive microwave data for assessment of time and space variations of first-year ice cover. The Caspian and Aral seas are chosen as main study areas. The Caspian Sea evolution is primarily climate driven, while for the Aral Sea there is a mix of anthropic and climate factors. We analyze ice cover conditions using a novel method that combines active and passive satellite measurements for ice discrimination. This method uses the synergy of simultaneous data from active (radar altimeter) and passive (radiometer) microwave instruments onboard the TOPEX/Poseidon (T/P) satellite, launched in 1992. The benefits, drawbacks, and potential of ice cover studies using the proposed method are discussed. We analyze in detail how this method is influenced by the difference in footprints of the T/P sensors and by the radiometric properties of ice and snow at different stages of ice cover evolution. In order to link the T/P-derived results to historical observations that end in the mid-1980s, long time series of passive microwave data from SMMR and SSM/I sensors have also been analyzed. Satellite time series of ice cover extent and duration of ice period have been obtained for the Caspian and Aral seas since 1978. A good agreement is obtained between historical and satellite data, with significant spatial and temporal variability of ice conditions. There is a marked decrease of both duration of ice season and ice extent during the winters 1998/1999-2001/2002. These satellite-derived time series of sea ice parameters are very valuable in view of the heterogeneous and mostly unpublished data on ice conditions over the Caspian and Aral seas since the mid-1980s.

Inundations in the Inner Niger Delta: Monitoring and Analysis Using MODIS and Global Precipitation Datasets

A method of wetland mapping and flood survey based on satellite optical imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS) Terra instrument was used over the Inner Niger Delta (IND) from 2000–2013. It has allowed us to describe the phenomenon of inundations in the delta and to decompose the flooded areas in the IND into open water and mixture of water and dry land, and that aquatic vegetation is separated from bare soil and “dry” vegetation. An Empirical Orthogonal Function (EOF) analysis of the MODIS data and precipitation rates from a global gridded data set is carried out. Connections between flood sequence and precipitation patterns from the upstream part of the Niger and Bani river watersheds up to the IND are studied. We have shown that inter-annual variability of flood dominates over the IND and we have estimated that the surface extent of open water varies by a factor of four between dry and wet years. We finally observed an increase in vegetation over the 14 years of study and a slight decrease of open water.