In order to make progress, it is necessary to have long time series, and have an overall view. In the absence of terrestrial systems that fulfi l these two conditions, space remote sensing, although still in its infancy if one considers the time scales sought after, is promising. For example, satellite altimetry can be used to monitor variations in the levels of major rivers, lakes and wetlands. The same is true of gravimetry* missions. Optical and infrared sensors can detect the presence of water at different spatial resolutions (from a metre to a kilometre), providing essential information.
© ESA.
By supplementing in situ measurement networks with observations from space and incorporating this information into hydrological models, we can begin to get a better understanding of the processes that take place on the scale of large river basins, although it will still be very diffi cult to separate natural effects from effects due to humans, a challenge that must be met in the future.
To do this, we need a global and integrated vision of continental hydrological systems. This can only be achieved through close international cooperation, via major structuring programmes, as has been the case for instance for operational oceanography or meteorology. From this perspective, cooperation between hydrologists, climatologists, river basin agencies, designers of space systems from both the private and public sector, and political decision makers is essential.
Jean-François Crétaux
LEGOS
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