A research team has refined a highly original method for studying ground water based on altitude measurements taken by satellite. This technique was originally only intended for the study of oceans, and has been used for only a few years to observe continental surface water bodies. After years of work to calibrate and validate the data in the Amazon basin, the researchers measured the altitude and variations in levels of more than 500 rivers, lakes, and flood zones.
Through this observation network, the densest every deployed on this scale, the researchers were able to create the first maps of Amazon groundwater. In the dry season, surface water reservoirs are at the same level as the aquifer that feeds them: altitude measurements of the surface water then made possible direct observations of the height of the groundwater. The researchers then mapped the groundwater ceiling at low water periods, which is to say at its lowest level of the year, from 2003 to 2008. The maps obtained were found to be consistent with direct measurements of water depth carried out in wells.
The "memory" of the groundwater and its impact
These first maps offer a way to monitor changes in the groundwater over these five years. Following the drought in 2005, the researchers observed the abrupt drop in its low water level in most of the zone under study. Then, this level gradually rose from the north to the south, to only return to its average value between 2007 and 2008. This result suggests an important "memory effect" in the groundwater. This could in turn have a strong impact on the climate. Because of this, if an abnormally low water level persists, this could contribute to reducing evapotranspiration, limit the moisture level in the atmosphere, and reduce rainfall in time.
The maps obtained are a source of essential and unprecedented information on the spatial and temporal structure of the Amazon groundwater and a major advance for hydrology. They help better understand the large-scale underground hydrological processes involved in the water cycle, carbon cycle, and maintenance of biodiversity in the Amazon. In fact, until now, groundwater was a major unknown in these reports.
- Julia Pfeffer, Frédérique Seyler, Marie-Paule Bonnet, Stéphane Calmant, Frédéric Frappart, Fabrice Papa, Rodrigo C.D. Paiva, Frédéric Satgé, Joecila Santos Da Silva. Low-water maps of the groundwater table in the central Amazon by satellite altimetry. Geophysical Research Letters, 2014; 41 (6): 1981 DOI: 10.1002/2013GL059134
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