How can we anticipate and measure the planet's water resources in order to adapt to the effects of global changes? This is one of the objectives of the future SWOT satellite, developed as part of a Franco-American collaboration launched nearly 10 years ago, which will revolutionize practices at an altitude of 891 km with an altimeter capable of monitoring the rivers and lakes of our planet. Takeoff planned for April 2021.
An 11 November 2016, in Marrakech, seven French water stakeholders, research organizations, financiers and managers (AFD, CNES, IOWater, Compagnie Nationale du Rhône, IRD, Irstea, BRLi) officially signed a working group agreement in order to use the future SWOT satellite to monitor the planet's water resources.
Anticipating its commissioning in 2021, scientists in Irstea's laboratories are working to develop algorithms to convert satellite data into operational data for monitoring water resources worldwide. With more than satisfactory results...
A NASA report, the Gravity Recovery and Climate Experiment (GRACE) project, carried out between 2003 and 2013, states that fresh water is becoming scarce and that we are increasingly short of it. In fact, out of 37 rivers studied, 21 are in the process of being depleted. Eight of them have little or no water resources at present. They could therefore potentially run dry in the future, which would be a disaster for all the life forms they support and for us humans. It is therefore essential to monitor them ... closely.
Satellites have revolutionized oceanography, they will revolutionize hydrology tomorrow. The Franco-American Swot (Surface Water and Ocean Topography) mission is expected to be the cornerstone of this revolution by carrying into space an instrument of technological breakthrough, a wide swath interferometric radar called KaRIn.
In 2021, the SWOT satellite, built by Thales Alenia Space, will fly over the earth equipped with this breakthrough technology. , which will take surface water height measurements over two 60 km swaths on either side of the satellite's vertical (compared to a few kilometres at present!). All this will be done with an altimetric accuracy (sea level observation technique) of around 10 cm, when the high-resolution measurements are averaged over areas of 1 km². Among the assets of the satellite approach: the possibility of covering the rivers of the poorly accessible and less well-informed areas of the planet, the transparency of information between countries, especially on transboundary basins, a global coverage of the globe, as well as homogeneity of the measurement, its processing and archiving. Thus, for example, only two hydrometric stations were still recently operational on the Congo River, which is 4700 km long and crosses four countries. The $1 billion SWOT project is funded two-thirds by NASA and one-third by CNES.
High-tech for river flow measurement
Several French and American scientific teams have been working for about seven years on converting data of the type to be acquired by satellite into river flows. Raw data received from space are translated into water heights relative to the geoid. . Specialists in hydraulics and data assimilation from the Irstea Montpellier centre joined the project three years ago and applied a new flow calculation method that proved to be particularly effective. "Our method is based both on a complete 1D hydrodynamic model, the SIC software developed for more than 25 years by Irstea and adapted to many contexts, and the 4D-VAR method developed by the University of Grenoble and used by Météo France and the European Centre for Medium-Range Weather Forecasts  for weather forecasts in particular in France and Europe. », explains Pierre-Olivier Malaterre in charge of the Montpellier team Hydraulic Management, Optimisation and Supervision of Water Transfers. "Our technique, more sophisticated than those used by our colleagues, gave the best results in terms of flow estimation".
To support this statement, the scientists refer to a comparative work of the different methods, carried out for the time being on two of the 19 "test rivers" chosen within the framework of the SWOT project. "On the Garonne, we worked on data generated by our models and reproducing a real hydrological scenario that took place in 2010, with a wide range of flow values (from 50 to nearly 2000 m3/s)". Other tests are in progress, within the framework of a thesis led by Irstea and co-funded by Irstea and CLS  in collaboration with the University of Ohio. They concern flow scenarios on the rivers Po in Italy and Sacramento in California, based on a priori very realistically simulated data identical to those to be provided by the SWOT satellite.
Thus the data simulator integrates various disturbances (satellite roll, thermal expansion of the satellite masts carrying the radar instruments, effects of the wet troposphere, radar wave reflections on the relief, etc.) which in space will disturb the signal. "Here again, the first comparative trials give the advantage to Irstea's calculation method. This will reinforce our strong expertise (already recognized internationally!) in the field of hydraulic modeling and data assimilation. »
A new technology to complement the existing hydrometric network
However, despite the good results posted for the measurement of hydrological flows using SWOT data, satellite technology is not intended to replace field hydrological measurement networks. Indeed, the current resolution of the satellite will only allow monitoring of large rivers over 100 metres wide (or 50 m if possible). In France, only our five main rivers will be able to benefit from it.
On the other hand, given the satellite's trajectory around the earth and its speed, the frequency of measurements at a given location will be only two to five measurements per 21-day cycle. "Hence the interest in perpetuating and/or developing the current network of measurements in the field. In France, this represents more than 3,000 stations that automatically measure river levels at high frequencies (a few minutes to a few hours). These data are combined with height-discharge relationships regularly calibrated by the field teams who carry out punctual flow measurements, called "gauges", explains Michel Lang, a hydrologist at the Irstea centre in Lyon. "Some experimental basins, such as those managed by Irstea for more than 50 years (Réal Collobrier and Orgeval), are equipped with multiple sensors that allow us to finely monitor the various components of the water cycle and improve our hydrological modelling tools".
The data acquired on hydrometric sites and those provided by satellites are complementary. The former have a very fine temporal resolution that allows monitoring of rapid hydrological phenomena such as flash floods in rivers, and are available over several decades, making it possible to monitor the effects of global change on water resources. The latter can, on large rivers, provide rapid flow estimates at multiple points, some of which are difficult to access from the ground.
Space hydrology federates in France
In view of the mass of data that the satellite will provide from 2021 (seven terabits  per day!), it is a matter of preparing to receive, process and analyse them beforehand. CNES is thus preparing to strengthen its data analysis capacity, helped in this by the contributions of the Future Investment Program (PIA).
This open-access data will enable the emergence of new tools to address major environmental issues in the field of water: protection, flood warning and monitoring, resource management, navigation, hydropower production, agriculture, etc.
(Source: IRSTEA - 8/12/2016)
 A wide swath interferometric radar. The swath refers to the band analysed by 2 radar antennas located at the ends of 2 arms of 10 metres each.
2] The geoid is the reference hydrological ellipse established from the mean sea level by suppressing the action of wind, currents and tides.
3] ECMWF Reading, UK
4] Satellite Location Collection
5] One terabit represents 1012 (= 1,000,000,000,000) bits.