Muriel Bergé-Nguyen

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.

Absolute calibration of Jason radar altimeters from GPS kinematic campaigns over Lake Issykkul

This study presents results of calibration/validation (C/V) of Jason-1 and Jason-2 satellite altimeters over Lake Issykkul located in Kyrgyzstan, which was chosen as a dedicated radar altimetry C/V site in 2004. The objectives here are to estimate absolute altimeter biases and to quantify the altimetry instrument error budgets for lakes studies, including errors associated with the atmosphere media delay corrections of the radar signal, and with the different retracking mode used over “nonocean” surface. The C/V is conducted using various equipments, for example, GPS local network, moving GPS along the satellites tracks over the Lake Issykkul, in situ level gauges and weather stations. The absolute bias obtained for Jason-1 and Jason-2 from field campaigns conducted in 2008, 2009 and 2010 are 96 ± 40 mm and 162 ± 42 mm, respectively. The bias calculated for Jason-1 is in close agreement with estimates done at other dedicated C/V sites in the ocean, but the Jason-2 bias still differs by 1–2 cm with ocean C/V estimates. The relative bias (Jason-2 minus Jason-1) deduced from measurements during the tandem mission is estimated at 81 ± 10 mm. The accuracy of Jason-1 and Jason-2 for the determination of the lake level variations as compared to the historical in situ gauges of the Lake Issykkul is 33 and 31 mm RMS, respectively. The bias between the two modes of retracking available on Jason-2 Geophysical Data Records (GDRs) has been estimated over the two first years of Jason-2 data over the Lake Issykkul. We found an average value of 234 ± 40 mm, which is in good agreement with results obtained in a previous study on the Envisat satellite (Crétaux et al. 2009). Lake level studies using Jason-2 or Envisat must therefore take this relative bias into account if both retracked altimetry heights are used.

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.

Lakes Studies from Satellite Altimetry

Accurate and continuous monitoring of lakes and inland seas has been possible since 1993 thanks to the success of satellite altimetry missions: TOPEX/POSEIDON (T/P), GFO, JASON-1, and ENVISAT. Global processing of the data of these satellites can provide time series of lake surface heights over the entire Earth at different temporal and spatial scales with a subdecimeter precision. Large lakes affect climate on a regional scale through albedo and evaporation. In some regions, highly ephemeral lakes provide information on extreme events such as severe droughts or floods. On the other hand, endorheic basin lakes are sensitive to changes in regional water balance. In a given region covered by a group of lakes, if the records of their level variations are long enough, they could reveal the recurrence of trends in a very reliable and accurate manner. Lakes are thought to have enough inertia to be considered as an excellent proxy for climate change. Moreover, during the last century, thousands of dams have been constructed along the big rivers worldwide, leading to the appearance of large reservoirs. This has several impacts on the basins affected by those constructions, as well as effects on global sea level rise. The response of water levels to regional hydrology is particularly marked for lakes and inland seas of semiarid regions. Altimetry data can provide a valuable source of information in hydrology sciences, but in-situ data (river runoff, water level, temperature, or precipitation) are still strongly needed to study the evolution of the water mass balance of each lake.

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.

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.

The Benefits of the Ka-Band as Evidenced from the SARAL/AltiKa Altimetric Mission: Quality Assessment and Unique Characteristics of AltiKa Data

The India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedicated to oceanography. The mission objectives are primarily the observation of the oceanic mesoscales but also include coastal oceanography, global and regional sea level monitoring, data assimilation, and operational oceanography. The mission ended its nominal phase after 3 years in orbit and began a new phase (drifting orbit) in July 2016. The objective of this paper is to provide a state of the art of the achievements of the SARAL/AltiKa mission in terms of quality assessment and unique characteristics of AltiKa data. It shows that the AltiKa data have similar accuracy at the centimeter level in term of absolute water level whatever the method (from local to global) and the type of water surfaces (ocean and lakes). It shows also that beyond the fact that AltiKa data quality meets the expectations and initial mission requirements, the unique characteristics of the altimeter and the Ka-band offer unique contributions in fields that were previously not fully foreseen.

Aral Sea Hydrology from Satellite Remote Sensing

Space technologies have been widely used over the last 10 years for water surface monitoring worldwide and they have shown their capability to monitor components of the water cycle and water balance at regional scales and on time scales ranging from months to decades. We present here the applications of space data from radar altimetry and satellite imagery (Terra/MODIS) over the Aral Sea Basin (ASB). Radar altimetry, which has been designed to study the ocean, has opened a new era in monitoring lakes, rivers and reservoirs. The recent missions of satellite altimetry (Topex-Poseidon, Jason-1/2, Envisat, ERS-1 and ERS-2) have made it possible to measure with great precision inland sea level variations that can be used to determine water mass balances. Radar altimetry, coupled with complementary in situ data, has allowed quantifying precisely the water balance of the Aral Sea since 1992 as well as balances for large reservoir systems along the Syr Darya, in particular Chardarya and Toktogul, and for Lake Aydarkul. This approach has also made it possible to ascertain the water balances of lakes and wetlands in the deltas of the Syr Darya and Amu Darya.

Absolute Calibration or Validation of the Altimeters on the Sentinel-3A and the Jason-3 over Lake Issykkul (Kyrgyzstan)

Calibration/Validation (C/V) studies using sites in the oceans have a long history and protocols are well established. Over lakes, C/V allows addressing problems such as the performance of the various retracking algorithms and evaluating the accuracy of the geophysical corrections for continental waters. This is achievable when measurements of specific and numerous field campaigns and a ground permanent network of level gauges and weather stations are processed. C/V consists of installation of permanent sites (weather stations, limnigraphs, and GPS reference points) and the organization of regular field campaigns. The lake Issykkul serves as permanent site of C/V, for a multi-mission purpose. The objective of this paper is to calculate the altimeter biases of Jason-3 and Sentinel-3A, both belonging to an operational satellite system which is used for the long-term monitoring of lake level variations. We have also determined the accuracy of the altimeters of these two satellites, through a comparison analysis with in situ data. In 2016 and 2017, three campaigns have been organized over this lake in order to estimate the absolute bias of the nadir altimeter onboard the Jason-3 and Sentinel-3A. The fieldwork consisted of measuring water height using a GPS system, carried on a boat, along the track of the altimeter satellite across the lake. It was performed at the time of the pass of the altimeter. Absolute altimeter biases were calculated by averaging the water height differences along the pass of the satellite (GPS from the boat system versus altimetry). Jason-3 operates in a Low Resolution Mode (LRM), while the Sentinel-3A operates in Synthetic Aperture Radar (SAR) mode. In this study we found that the absolute biases measured for Jason-3 were −28 ± 40 mm with the Ocean retracker and 206 ± 30 mm with the Ice-1 retracker. The biases for Sentinel-3A were −14 ± 20 mm with the Samosa (Ocean like) retracker and 285 ± 20 mm with the OCOG (Ice-1-like) retracker. We have also evaluated the accuracy of these two altimeters over Lake Issykkul which reached to 3 cm, for both the instruments, using the Ocean retracker. Remote Sens. 2018, 10, 1679; doi:10.3390/rs10111679 www.mdpi.com/journal/remotesensing Remote Sens. 2018, 10, 1679 2 of 17