Challenges & Research issues

The study of continental surface water is doubly important, as it represents the majority of water resources available to society in general (drinking water, agriculture, industry and leisure activities) and because its study reveals many of the processes at work within catchment areas (e.g. changing rainfall patterns, soil erosion, changes in land use and diffuse or specific pollution). Water resources thus act as sentinels for global and environmental change. In fact, changes in the water cycle (exceptional flooding, droughts and pollution) are often the most visible signs alerting our society to ongoing changes.

Monitoring hydrological processes requires repeated observations in space and time so as to detect the sources and modes of transport of the flows of water and material in catchment areas. As hydrological processes are highly heterogeneous by nature, the higher the frequency of observations, the more reliable they are and the more information they provide. Carbonneau and Piégay (2012) observe that the need to study and measure catchment areas holistically naturally leads to remote sensing as a fundamental tool, because only satellite data provide continuous measurements at the scale of the whole catchment area.

There are currently many techniques for studying the water cycle by remote sensing, especially over land, but this is a recent development and the discipline is therefore among the most dynamic of remote sensing applications. As a result, the techniques employed are extremely varied, and include active radar techniques, in which the phase (interferometry) and/or the power of the electromagnetic wave transmitted (SAR, altimetry) are taken into account; passive radar (microwave emissivity of surfaces); laser techniques; or spectral analysis at short wavelengths (multi-, super- or hyperspectral imaging in the visible and infrared). However, while there are an increasing number of satellite missions and applications, they must be geared to the issues facing hydrologists and managers of water resources. The people developing remote sensing tools are very often not those using them, and despite the spectacularly « demonstrative » nature of many remote sensing studies, their actual usefulness for those working in the field may be questionable. There is therefore a need for a concerted effort to develop robust products that address the problems faced by research, management and society at large.

The first satellite studies in hydrology initially focused on the observation of processes related to the transfer of water and material in catchment areas, but without directly observing the water itself. There have been many studies since the 1970s on the use of remote sensing for studying the geomorphology of watercourses (Millington and Townshend 1986), surface deformations (Wegmüller et al. 1998), changes in land use (Matthews 1983) or the use of microwave emissivity to estimate soil moisture content (Schmugge and Jackson 1994). Technical progress and the availability of new data sources have given birth to numerous applications. It is now possible to monitor hydrometeorological flows, including snow cover (Papa et al. 2007), the distribution of flooded plains (Hess et al. 2003; Martinez and Le Toan 2007), variations in water masses (Frappart et al. 2005; Papa et al. 2008), and the water level of rivers (Santos da Silva et al. 2010). It is now possible to “look into » watercourses and lakes. Very-high-resolution sensing has had a revolutionary effect by making it possible to map bathymetry phenomena in small watercourses (Fonstad and Marcus 2005), micro-habitats (Marcus et al. 2003) or even river beds (Carbonneau et al. 2005) with the use of passive optical or lidar imaging. The present is therefore an exceptional moment in terms of technical developments, and will hopefully lead to great progress in water cycle research themes. In particular, it is certain that the synergy between space observation, in situ observation and modelling tools will enable us to address fundamental questions related to the transfer of water and matter from the physics of small-scale processes right up to climate forecasting.

Theia Products on Water issues

Monitoring Water Volume in Watersheds and Floodsplains Soil moisture at very high spatial resolution and at plot level for Arles aera (France). Theia Drought Index provide worldwide data with a 25-km resolution from SMOS satellite since 2010. Snow Cover Duration for the Pyrenees during a hydrological year (1 September 2016 to 31 August 2017), produced by synthesis of Theia snow-covered surface products (Level 3). Theia Water Quality product documents in particular Suspended Matter Level. Here for the Rhone River in Arles (France). Map for Evapotranspiration level at plot scale in Morroco, 2017. Theia product presenting the irrigated crops in 2015 in the South-West of France. Produced from 13 LANDSAT8 level 2A images. Localisation of all Hydroweb virtual stations in November 2020

Concerned Theia SECs

Prototyping SEC

Colours of the continental waters SEC

Variables & products

Digital Soil Mapping

Incubating SEC

Drought SEC

Variables & products

Evapotranspiration

Incubating SEC

Irrigation SEC

Producing SEC

Land cover SEC

Incubating SEC

Mapping and monitoring of water bodies SEC

Producing SEC

Risks associated with infectious diseases SEC

Variables & products

Snow-covered surface

Producing SEC

Soil moisture with very high spatial resolution SEC

Variables & products

Surface Reflectance

Producing SEC

Surface Soil Moisture SEC

Prototyping SEC

Surface water volumes of floodplains SEC

Producing SEC

Water levels of lakes and rivers SEC

Theia News on Water

PhD & PostDoc

[PhD] Spatio-temporal dynamics of suspended particulate matter in the estuary/coastal sea continuum

Thesis project aiming to integrate and interact high-frequency in situ data (COAST-HF, SYNAPSES) with water color satellite data (Sentinel 2/3 – Landsat – MODIS) to enhance our understanding of the land-sea interface.

News

SWOT data that exceeds expectations

The SWOT space mission was launched at the end of 2022 by CNES and NASA to document the water masses of the entire planet – oceans and continental waters. Data […]

Products

OSO 2023 Data : a little more patience ahead

THEIA Land Cover product is produced each year by CESBIO and CNES. It provides a 24-class typology of land cover, downloadable by department for mainland France. Production begins in January […]

Products

THEIA Sentinel-2 Surfwater, now an active production on hydroweb.next

Discover Theia Surfwater Product now produced continuously for Sentinel-2 and available on hydroweb.next

Applications

The study of water resources is a largely interdisciplinary field. Consequently, the products developed in the framework of the Theia cluster are designed to be used in delivery structures observing catchment areas in all their complexity and over the long term. A significant example of an application follows.

• The HYBAM National Observation Service investigates the hydrology and geochemistry of the Amazon Basin (www.ore-hybam.org). This SNO is tasked with observing the long-term evolution of the major Amazonian watercourses and understanding the hydrological variability and transfer of materials in relation to climatology and anthropogenic activities. It has been using satellite observations such as altimetry and colour for several years to supplement the observations of conventional networks. HYBAM is supported by an international consortium representing eight countries and consisting of partners in both research and operational hydrology (a hydro-weather service).