International audienceSince the launch of the ENVISAT satellite in 2002, the Radar Altimetry Mission provides systematic observations of the Earth topography. Among the different goals of the ENVISAT Mission, one directly concerns land hydrology : the monitoring of the water levels of lakes, wetlands and rivers. The ENVISAT Geophysical Data Records products contain, over different type of surfaces, altimeter ranges derived from four specialized algorithms or retrackers. However, none of the retrackers are intended to the processing of the radar echoes over continental waters. A validation study is necessary to assess the performances of the different ENVISAT-derived water levels to monitor inland waters. We have selected four test zones over the Amazon basin to achieve this validation study. We compare first the performances of these retracking algorithms to deliver reliable water levels for land hydrology. Comparisons with in-situ gauge stations, showed that Ice-1 algorithm, based on the Offset Centre Of Gravity technique, provides the more accurate water stages. Second, we examine the potentiality to combine water levels derived from different sensors (Topex/Poseidon, ERS-1&2, GFO)

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The floodplains of the Amazon basin influence the hydrology and fluxes of suspended solids and solutes on multiple scales. Our study focused on the floodplain of Lago Grande de Curuaí (Óbidos, Brazil), a 4000 km 2 segment of floodplain and local upland catchment representative of the lower Amazon. Based on in situ and satellite data acquired from 1997 to 2003, we calculated the exchanges of water between the floodplain and the river and determined the temporal dynamics of flooded area water derived from river flooding, rainfall, runoff, and exchange with groundwater annually for six years. The Amazon River dominated the inputs of water to the flooded area year-round, accounting about 77% of the annual total inputs; rainfall and runoff accounted for about 9% and 10%, respectively, while seepage from the groundwater system accounted for 4%. The hydrologic residence time of the lake was about three months, and the floodplain made a net contribution of water to the river. The exported volume (net balance between water input and 0022-1694/$ -see front matter ª a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j h y d r o l losses) varied between 4.2 and 7.3 km 3 depending on the year and represented about 0.75 times the maximal storage reached each year. ª

The monitoring of continental water stages is a requirement for meeting human needs and assessing ongoing climatic changes. However, regular gauging networks fail to provide the information needed for spatial coverage and timely delivery. Although the space missions discussed here were not primarily dedicated to hydrology, 18 years of satellite altimetry have furnished complementary data that can be used to create hydrological products, such as time series of stages, estimated discharges of rivers or volume change of lakes, river altitude profiles or leveling of in situ stations. Raw data still suffer uncertainties of one to several decimeters. These require specific reprocessing such as waveform retracking or geophysical correction editing; much work still remains to be done. Besides, measuring the flow velocity appears feasible owing to SAR interferometer techniques. Inundated surfaces, and the time variations of their extent, are currently almost routinely computed using satellite imagery. Thus, the compilation of the continuous efforts of the scientific community in these various investigative directions, such as recording from space the discharges of rivers or the change in water volume stored in lakes, can be foreseen in the near future.

The Amazon river basin has been recently affected by extreme climatic events, such as the exceptional drought of 2005, with significant impacts on human activities and ecosystems. In spite of the importance of monitoring freshwater stored and moving in such large river basins, only scarce measurements of river stages and discharges are available and the signatures of extreme drought conditions on surface freshwater dynamics at the basin scale are still poorly known. Here we use continuous multisatellite observations of inundation extent and water levels between 2003 and 2007 to monitor monthly variations of surface water storage at the basin scale. During the 2005 drought, the amount of water stored in the river and floodplains of the Amazon basin was ∼130 km 3 (∼70%) below its 2003-7 average. This represents almost a half of the anomaly of minimum terrestrial water stored in the basin as estimated using the Gravity Recovery and Climate Experiment (GRACE) data.

International audienceThe objective of this study is to derive the stage discharge relationship for 21 ?virtual gauge stations? located at the upper Negro River (Amazon Basin, Brazil). A virtual station can be defined as any crossing of water body surface (i.e., large rivers) by radar altimeter satellite tracks. Rating curve parameters are estimated by fitting with a power law the temporal series of water surface altitude derived from satellite measurements and the discharge. Discharges are calculated using ProGUM, a flow routing model based on the Muskingum Cunge (M C) approach considering a diffusion-cum-dynamic wave propagation [Leon, J.G., Bonnet, M.P., Cauhope, M., Calmant, S., Seyler, F., submitted for publication. Distributed water flow estimates of the upper Negro River using a Muskingum Cunge routing model based on altimetric spatial data. J. Hydrol.]. Among these parameters is the height of effective zero flow. Measured from the WGS84 ellipsoid used as reference, it is shown that the height of effective zero flow is a good proxy of the mean water depth from which bottom slope of the reaches can be computed and Manning roughness coefficients can be evaluated. Mean absolute difference lower than 1.1 m between estimated equivalent water depth and measured water depth indicates the good reliability of the method employed. We computed the free surface water slope from ENVISAT altimetry data for dry and rainy seasons. These profiles are in good agreement with the bottom profile derived from the aforementioned water depths. Also, the corresponding Manning coefficients are consistent with the admitted ranges for natural channels with important flows (superficial width >30.5 m [Chow, V.T., 1959. Open Channel Hydraulics. McGraw-Hill, New York]) and irregular section

. Floodplain water storage in the Negro River basin estimated from microwave remote sensing of inundation area and water levels. Remote Sensing of Environment, Elsevier, 2005, 99, pp.387-399 Abstract : The objective of this study is to determine spatio-temporal variations of water volume over inundated areas located in large river basins using combined observations from the Synthetic Aperture Radar (SAR) onboard the Japanese Earth Resources Satellite (JERS-1), the Topex/Poseidon (T/P) altimetry satellite, and in-situ hydrographic stations. Ultimately, the goal is to quantify the role of floodplains for partitioning water and sediment fluxes over the great fluvial basins of the world. SAR images are used to identify the type of surface (open water, inundated areas, forest) and, hence, the areas covered with water. Both radar altimetry data and in-situ hydrographic measurements yield water level time series. The basin of the Negro river, the tributary which carries the largest discharge to the Amazon river, was selected as a test site. By combining area estimates derived from radar images classification with changes in water level, variations of water volume (focusing on a seasonal cycle) have been obtained. The absence of relationship between water volume and inundated area, reflecting the diverse and widely dispersed floodplains of the basin, is one of the main result of this study.

In this study, rating curves (RCs) were determined by applying satellite altimetry to a poorly gauged basin. This study demonstrates the synergistic application of remote sensing and watershed modeling to capture the dynamics and quantity of flow in the Amazon River Basin, respectively. Three major advancements for estimating basin-scale patterns in river discharge are described. The first advancement is the preservation of the hydrological meanings of the parameters expressed by Manning's equation to obtain a data set containing the elevations of the river beds throughout the basin. The second advancement is the provision of parameter uncertainties and, therefore, the uncertainties in the rated discharge. The third advancement concerns estimating the discharge while considering backwater effects. We analyzed the Amazon Basin using nearly one thousand series that were obtained from ENVISAT and Jason-2 altimetry for more than 100 tributaries. Discharge values and related uncertainties were obtained from the rain-discharge MGB-IPH model. We used a global optimization algorithm based on the Monte Carlo Markov Chain and Bayesian framework to determine the rating curves. The data were randomly allocated into 80% calibration and 20% validation subsets. A comparison with the validation samples produced a Nash-Sutcliffe efficiency (E ns ) of 0.68. When the MGB discharge uncertainties were less than 5%, the E ns value increased to 0.81 (mean). A comparison with the in situ discharge resulted in an E ns value of 0.71 for the validation samples (and 0.77 for calibration). The E ns values at the mouths of the rivers that experienced backwater effects significantly improved when the mean monthly slope was included in the RC. Our RCs were not mission-dependent, and the E ns value was preserved when applying ENVISAT rating curves to Jason-2 altimetry at crossovers. The cease-to-flow parameter of our RCs provided a good proxy for determining river bed elevation. This proxy was validated against Acoustic Doppler current profiler (ADCP) cross sections with an accuracy of more than 90%. Altimetry measurements are routinely delivered within a few days, and this RC data set provides a simple and cost-effective tool for predicting discharge throughout the basin in nearly real time.