Exciting possibilities  for coupling isotope tracers and satellite/remote sensing data for Water Resources  Assessment

Ms Janine Halder, Division of Physical and Chemical Science, International Atomic Energy Agency

The advent of new satellite observation technologies has opened up exciting opportunities for assessing surface water and aquifers, and is increasingly incorporated into large scale hydrological models. Moreover, the increasing precision of remote sensing techniques and improved spatial cover of satellite missions enable scientists to focus on smaller watersheds and to couple on-the-ground use of geochemical and isotope tracers to satellite/remote and sensing outputs.

Isotope tracers are frequently used in case study sites (e.g. local river watershed, lakes, and groundwater aquifers) to determine hydrological processes and impacts (as evaporation and recycling of water), the origin of water (e.g. precipitation, surface, ground and ocean water), and water ages. To assess variations in the isotopic composition of water sources, quantitative as well as spatial hydro(geo)logical data are crucial but not always obtained on a regular basis for logistical, safety or remote access reasons. Here satellite and remote sensing data can contribute valuable information at any timeframe, and help focus the spatial extent of field programmes. Examples of possible joint projects proposed during an IAEA Technical Meeting on the coupled use of isotope tracers and Satellite/Remote Sensing data in Water Resources Assessment and Management in November 2016 include:

·         Thermal infrared imaging (TIR)

TIR imaging provides satellite or airborne derived surface water temperatures. Temperature anomaly maps can be used to identify groundwater inputs to lakes, rivers, and coastal waters. Informed field assays of radioisotopes (³H, ²²²Rn) and water stable isotopes (¹⁸O/²H), can thereafter verify observed thermal signals as groundwater discharge and detect localized seepage points or groundwater “hotspots”. 

·         Remote Sensing/Airborne images

Remote sensing covers large areas which are not easily accessible (e.g. peatlands, wetlands, glaciers) and can provide information about snow cover, flooding, and vegetation. Satellite and isotope methods complement each other in understanding snow melt processes, streamflow components, and in establishing catchment water budgets.

·         Altimetry

Radar altimeter measurements track lake and river levels over time and can therefore estimate river discharges or determine whether an upstream lake/wetland is gaining or losing water. This is especially useful in ungauged basins or difficult terrains.  Radioisotopes and water stable isotopes as well as water chemistry can be employed to quantify water sources contributing to discharge.

·         Gravity Recovery And Climate Experiment (GRACE)

GRACE observations are based on time variable gravity and can be converted to monthly water storage changes for large areas (>150,000 km²). Isotope age dating may be coupled with GRACE secular trends (net storage change rates) to estimate absolute storage and replenishment rates. Moreover, isotopes could also be used to partition GRACE total water storage into snow, ice, surface water, soil moisture and groundwater, and fluxes on boundaries.

In summary, the coupling of isotope tracers and satellite/remote sensing data opens up new interdisciplinary opportunities for assessing surface water and aquifers. Especially with the recent development of laser spectroscopy technologies for stable isotope analysis, the approaches are likely to be increasingly integrated within water quantity and quality studies. This will, however, require cooperation of isotope hydrologists, remote sensing experts and hydrological modellers.