VASHYB / Validation of Altimetric Satellites for HYdrology in Brazil

Daniel Medeiros Moreira, Joecila Santos da Silva, Stéphane Calmant

1. Proposal

1.1. Scientific Objectives

The objective of this project is to propose an ambitious program of deployment of in-situ stations on some major Brazilian Rivers for the validation of the future altimetric satellites, namely ESA's SENTINEL-3 missions and the NASA/CNES mission SWOT. These missions will operate altimeters in modes that have not yet been fully validated over rivers, the SAR and the SAR Inteferometric modes. Assessment of the performance of these new altimetric measurements is a prerequisite before a monitoring of the Brazilian rivers can be foreseen by means of satellite altimetry.

Given the importance of the Brazilian rivers in 1- the global balance of the water cycle on Earth, 2- the water resources for the Brazilian Industry, power production, and human consumption, the Brazilian institutions bearing the present project and involved in the monitoring of the water resources at the National level intend to actively participate to the assessment of these performances.

1.2. State of the Art of the Scientific Purpose

Water resources on the continental surface are limited, and the spatial and temporal distribution of this resource does not always meet the most crucial needs. Water resources readily available for human consumption and for ecosystems are found in lakes and rivers, corresponding to only 0.27% of the fresh water and about 0.007% of the total amount of water in the world ( The quantity and quality of the fresh water supplies will be a major problem in the coming decades. The world consumption of fresh water already reached 54% of the total stock in 1995. It is likely to equal the total fresh water resource available in North Africa and South Asia by 2025, when Asia will be using ten times more water than the rest of the world (World Resources Institute,

One billion people lack sufficient water for domestic consumption today, and it is estimated that, in 30 years, 5.5 billion people will be living in areas with moderate to serious water shortages (Population Reference Bureau, 1997). In the 1970s, the emergence of environmentalism, followed by the adoption of the sustainability principle, in combination with the threat of a worldwide water shortage in the years to come, has led a number of countries to completely revise their strategies and governance tools applied to the integrated management of water resources. The monitoring of continental surface water stored in rivers, lakes, and wetlands has turned into the scientific hydrologist community's primary objective and a major concern for a majority of state organizations.

Unfortunately, our knowledge of the global dynamics of terrestrial surface waters is still very poor (SWOT, Satellite Water Ocean Topography, Yet, studies of the integrated, global nature of the hydrological cycle are essential to our understanding of natural climate variability and to predict a climatic response to anthropogenic forcing (Koster et al., 1999). Most international water management groups underline the need for core hydrological data. Nowadays, monitoring of the continental water resource (temporal variability of river stages and river discharges) is provided via hydrologic networks. These networks are organized on a national basis. The challenges common to most regions include inadequate monitoring networks, gaps in records, a general decline in the number of stations, chronic under-funding, differences in processing and quality control, and differences in data policies (WMO, 2003). Consequently, the global monitoring of hydrological cycles worldwide is continually decreasing (Vörösmarty et al., 2010) and some basins, e.g., the Congo basin, are now almost devoid of stage and flow measurements Major issues in the poorest regions of the world indeed include poor status or outright lack of monitoring networks, support infrastructure and data quality. For example, the WMO first identified those National Hydrological Networks in Africa that still maintain a hydrological data archive on paper. Replies to a questionnaire sent to Hydrological Advisors in 39 countries showed that 82 per cent use paper for archiving their data (WMO, 2004), which means that they are not available to the scientific community or the decision makers. Roads et al. (2003) compared climate model outputs with ground-truth data over the continental United States. Using predictions from various climate models, they found that the runoff predictions are often in error by 50%, and even mismatches with observations as high as 100% were not uncommon. Coe (2000) found similar results for many of the world's large river basins. Thus, these models are now becoming limited as a result of the decline in observations of water discharge and water storage (Alsdorf et al., 2003). Nowadays, untouched rivers and catchments can only be found amongst small tributaries since the catchment of most major contributors undergo human stress, i.e., a place subject to either water removal, deforestation or flow regulation with dams. These small tributaries are nowadays rarely monitored properly. As rivers, lakes and wetlands form the main fresh water resource, there is a strong demand for a global, homogeneous, continuing (over many years) monitoring system which delivers fast-access data on continental water stages and, wherever possible, on water volumes and river discharges too.

Such a monitoring system can be provided by satellite altimetry in complement of the existing in-situ networks and there is a strong need for an application specific to continental waters of the existing and forthcoming satellite missions.

Progress made over two decades of research to propose water level estimates were mostly based on studies of the Amazon basin. Because the Amazon basin is the largest basin in the world and it includes the widest river reaches in the world, it has been selected for many applications of satellite altimetry over rivers (see the reviews by Calmant and Seyler, 2006 Hall et al., 2011, Calmant et al., in press). This includes the very first demonstrations of satellite altimetry capability to recover water levels over rivers and most of the validation studies (Guzkowska et al., 1990; Koblinsky, 1993; Leon et al., 2006; Frappart et al., 2005; Silva et al., 2010; Calmant et al., 2012; Seyler et al., 2012).

Seasat (NASA Sea Satellite) was launched in 1978 and provided the first dataset of altimetric measurements used to assess water levels in the Amazon River (Guzkowska et al., 1990; Cudlip et al., 1990; Mertes et al., 1996; and Dunne et al., 1998). These works include the task of reprocessing the radar echoes received onboard Seasat, (called retracking procedure). Birkett (1998) demonstrated the first use of Topex/Poséidon (T/P, NASA / CNES Topography Experiment / Poseidon satellite) data over the Amazon basin. Birkett et al. (2002) merged the results from these studies to produce the first altimetric profile of the Amazon main stem over 4000 km. Koblinsky (1993) was the first to attempt retrieval of a water level time series over the Amazon main stem by retracking the GEOSAT echoes. Comparison with local gauges resulted in standard deviations between 19 cm and 1.09 m. This result demonstrated the interest of the GEOSAT data for water level monitoring, at least in the Amazon basin and the strong need for a validation of the satellite-derived value using well maintained in-situ gauges. Launched by ESA on July 17 1991, ERS-1 was placed in different orbits over its mission lifetime, including the 35-day repeat orbit that was further used by ERS-2 (1995-2003), Envisat (2002-2012) and now SARAL (since 2013). As with the previous missions, ranges tracked with the standard algorithm are of no value since retracked ranges are not provided in the official geophysical data records (GDRs). Investigators interested in working with this data had to perform their own retracking.

Envisat was the first mission to propose retracked ranges in the GDRs and it was a major step for altimetry over rivers. Frappart et al. (2006) demonstrated that when such a retracking is performed by the Space Agencies, it was no more mandatory to reprocess the radar waveform to get reliable water levels over the Amazon basin. They found that best results were achieved with the ranges retracked by ICE-1 (Bambers, 1994). An example of such a series over the Negro River is presented in Fig. 1. Silva et al. (2010) conducted a more extensive study of validation over 70 series. This study shows that standard deviation of around 30 cm can be obtained using these ranges when according time series are compared to gauge series. The main point highlighted by these studies is that a fine selection of the data is as important as the range retracking itself. Frappart et al. (2006), followed by Silva et al. (2010), showed that in around 30% of cases over the Amazon basin the onboard tracking can be locked over an energy spike, generating estimates of slant measurements. Geometrical details can be found in Silva et al. (2010). This effect of off-nadir measurements is particularly common over narrow reaches where a height parabola (the range being elongated according to distance between the satellite and the target) affects the along track height profile over several kilometers (Fig. 4). Consequently, the water level can be better estimated by fitting these parabolas rather than if only one measurement over the river was available. Significantly, this beneficial effect also occurred on other past missions, e.g., ERS1 and ERS2, but also on current flying missions such as Jason-2 and SARAL.

Currently, only two missions are flying, namely SARAL and Jason-2. From 2015, there should be additional missions; Copernicus /ESA's Sentinel-3A and CNES/NASA/NOAA/EUMETSAT's Jason-3. Jason-3 will simply be the continuation of Jason-2 whereas the constellation of Sentinel-3 missions will be of major importance for the monitoring of rivers. Sentinel-3A and Sentinel-3B will embark altimeters working in SAR mode over the continents. The preliminary results presented by Bercher et al. (2013) using the Cryosat-2 data in SAR mode are highly promising. Sentinel-3A is scheduled for a launch in late 2015 and Sentinel-3B in 2017. They will be placed on two interleaved orbit with a 27-day repeat cycle.

The SWOT (Surface Water Ocean Topography) mission will constitute a major step for the monitoring of water levels over rivers (Alsdorf et al., 2007). SWOT is a NASA/CNES mission planned for launch in 2020. It will be the first mission primarily dedicated to the measurement of river stages. It will use KaRIN (Ka-band Radar Interferometer), a Ka band interferometer to provide continuous mapping of the river levels over ~2 60 km wide swaths. The repeat cycle has been established at 21 days so that full coverage is performed by the according number of swaths, except for narrow nadir bands. Because of full coverage, sampling will be performed twice every cycle of 21 days. The spatial resolution is expected to be at least 100 m 100 m with a centimeter level accuracy at the 1 km² level of resolution. A LRM nadir altimeter working in the Ku band should sample the nadir band. Taking into account the contrast in back-scattering between ground and water, it is expected that SWOT will also provide wet surface mapping. This mapping should be very helpful for automatic determination of the reaches' width and its possible temporal variation. Thus, SWOT will provide river surface level, slope and width all together. There is obviously no need for height or discharge everywhere along a river. The advantage of SWOT can be understood as a capability to provide information at any place it is desired, at control sections or river mouth for instance. Right after the launch, SWOT wil be placed ona temporary orbit for ~2 months. This orbit will be a fast sampling orbit, with a 1 day repeat period. This phase will be the one when we will focus the calval work.

1.3. Detailed Description of the Proposal

Maintaining a hydrological network in remote basins is a costly task. To give the example of the Brazilian part of the Amazon basin network, managed by ANA (Agencia Nacional de Aguas) and other federal or state entities, there is on average one limnimetric station for each 7200km2, About 270 limnimetric stations only are managed by ANA in the Amazon basin, from which only 210 are regularly surveyed, and only 61% are still operating today. The cost of this network operation is about 3 million dollars per year, a huge expense for the country. Brazil is therefore a country that could greatly benefit of the complement to this network that altimetry could provide. ANA and CPRM are institution involved in the monitoring of the Brazilian rivers at the national level. Their willing to incorporate the altimetry data into the hydrological information that they distribute throughout the country via the hidroweb web service bears the present proposal.

The network of validation stations presented hereafter intends to bring the necessary ground information for the following calibration/validation questions:

  • What is the accuracy on the water levels in different contexts, in particular for a large range to river widths.
  • How accurate are the slope and widths estimates.
  • Is the long term orbit error significant.
  • Is there significant roll errors and how large is the impact of the errors at the swath edges.

Also, in close collaborations with other proposal for the SWOT ST, hydro-morphological informations such as bathymetry of the cross section and flow velocity will be collected in order that proposals dealing with the qualification of discharge algorithms can test the latters. After 20 years of highly accurate altimetry over the oceans, it is well known that no altimetry mission is devoid of systematic bias. Biases can have two different origins. They may have an instrumental origin or come from some mismodelling of the echoes. This is true even though the retracking algorithm is tuned to best-fit the radar echoes, as is the case for the oceanic domain (Rodriguez, 1988). Thus, the estimate of altimetric, instrumental, bias over the oceans has been the subject of extensive literature for all successive missions since T/P (Bonnefond et al., 2013, and herein references). Clearly, it is mandatory to have independent estimates for continental waters where the fit of the retracking algorithms to the echo waveforms is much more empirical than it is over the oceans. Birkinshaw et al. (2014) recall that the biases are retracker-dependent and mission-dependent. However, very few studies have been conducted with this objective. Calmant et al. (2012), Seyler et al. (2012) and more recently Silva et al. (2015) estimated the bias for ranges retrieved using the ICE-1 algorithm, respectively for ENVISAT, Jason-2 and SARAL. The fact that for the same retracking algorithm very different value are found for each mission highlight the need for a systematic determination of these biases for any mission and any technique used to get the range for the radar echoes. Because we have no knowledge of the possible biases that will affect the SAR and SARIN measurements of the forthcoming missions, it is absolutely necessary that studies address this question for these missions if one wants to link the temporal variations of the water level observed by these new missions with those observed by the past missions since the 90's. Since it is a major concern, this question of the biases is also driving the present proposal.

Installation of New Stations

We propose to install 60 new stations in the Amazon and Paraguay basins. A bird eye view of the location of the stations is given in Figure 1.

The location of the stations has been selected according to the following criteria (in order or priority):

  1. Be located within the swaths of the SWOT fast sampling orbit
  2. Inside the swath, be either at the center of the swath (minimum error expected) or at the rims of the swath (maximum error expected),
  3. At a crossing with a Sentinel-3 track, in order to enable the inter-calibration of the Sentinel-3 SAR data in Ku band with the SWOT data in interferometric mode in Ka band.

Each station will consist in:

  1. A level gauge, made either of a traditional rule with twice-a-day readings, an automatic station or an immerged pressure sensor where it is not possible to have an operator
  2. A met sensor including at least rainfall rates and atmospheric pressure

    In either case, a leveling will be performed in order that the local water stages can be converted into absolute heights (referenced to GRS80). The location of the stations is also intended to spread as long as possible along the tracks in order that the long wavelength errors can be estimated. In collaboration with colleagues of the South Am Early Adopters of SWOT from French Guiana, Colombia and Bolivia we will seek for a way to also establish similar validation sites along the Maroni River for the East track and along the Negro and Orinoco and river (west track) in order to increase as much as possible the extend of the wavelength that can be analyzed.

  3. An ADCP profile through the cross section of the river. This ADCP profile will provide an estimate of the river bathymetry, referenced to GRS80, an estimate of the river width, and an estimate of the flow velocity and its variations throughout the cross section.
  4. Whenever possible (but not mandatory everywhere), an estimate of the local slope of the free surface and that of the river bed will be collected using a GNSS station embarked on a boat cruising over ~1 km on both sides of the station.

Levelling of Existing Gauges

Through the PhD work conducted by D. Medeiros on the use of GNSS systems for the leveling of gauges either in static or cinematic PPP modes, the team has acquired the expertise to conduct a state-of-the art leveling of the Brazilian network of gauges. The specific methodology that we have developed consists in using the boats cruising the rivers to collect the GNSS data that can be used in post-processing (using CNES' GINS-PC software) to get a rapid, cost-effective leveling of the gauge with an accuracy at the 2 cm level, including taking into account the crustal deformation due to the hydrological loading. Note that this technique also provides an estimate of the local slope of the free surface. Until the launch of SWOT, priority will be given to the stations included in the swaths of the fast sampling phase. Leveling the rest of the stations will be performed after the launch.

Computation of Altimetric Series

The expertise of the team in processing altimetric data for hydrology has been recognized at the international level through many publications (). Today, the team maintains a database of > 1000 series in the Amazon and Paraguay basins.

In the frame of the present project, the team will:

  1. Continuously update the series of existing missions working in LRM. This part mostly includes the SARAL and Jason-3 missions. Priority will be given to the Virtual Stations in the SWOT fast sampling orbit.
  2. Start the processing of the new missions operating in SAR mode, i. e. the Sentinel-3 missions, but also the Jason-CS mission if data are available before the launch of SWOT. The project will start with the VS located in the swaths of the Fast Sampling orbit of SWOT and will extend the database to all the VS in the two basins, possibly to other basins in Brazil according to funding.

Creation of a Web Service Distributing the Information Derived of Satellite Altimetry in Brazil

We intend to take the opportunity of this proposal to create a web service distributing the altimetry information collected by the proponents either in the frame of this proposal or in the frame of research projects. In a first stage, this web service will distribute the ~1000 series already computed in the Amazon and Paraguay basins. These series will be regularly updated. Series built at additional virtual stations created at the crossing with new orbits (Sentinel-3 A & B, Jason-2 interleaved orbit) will be incorporated continuously.

In a second step, we intend to include the distribution of other products such as rated discharges, gauge leveling, leveled bathymetry sections, and softwares.

1.4. Schedule


  • Installation of the first 20 stations. Since we plan to start with the Negro river, we present in Figure 2 the network of validation sites that we propose to install on this river
  • Update of existing LRM series located in the swaths of the Fast Sampling orbit
  • Start processing the Sentinel-3A data to initiate the series. Priority will be given to the series located into the swaths of the Fast sampling orbit.
  • Start the coding of a web service
  • Organize a short course about LRM altimetry
  • Participate to the SWOT ST meeting


  • Installation of 20 more stations
  • Update of the LRM and Sentinel-3A series located in the swaths of the Fast Sampling orbit
  • Collect the data of the 20 first stations
  • Opening of the web service
  • Organize a meeting of all the proponents to follow the advances of the project
  • Organize an advanced course focusing on the SAR and SARIn techniques
  • Participate to the SWOT ST meeting


  • Installation of the last 20 stations
  • Update of the LRM and Sentinel-3A series located in the swaths of the Fast Sampling orbit
  • Start processing the Sentinel-3B data to initiate the series. Priority will be given to the series located into the swaths of the Fast sampling orbit.
  • Collect the data of the 40 stations installed in 2016 and 2017
  • Organize a meeting of all the proponents to follow the advances of the project
  • Participate to the SWOT ST meeting


  • Update of existing LRM series located in the swaths of the Fast Sampling orbit
  • Pursue the Sentinel-3A and 3B series.
  • Collect the data of the 60 stations
  • Upload all the available data into the web service
  • Organize a meeting of all the proponents to follow the advances of the project
  • Participate to the SWOT ST meeting


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