As the Amazon basin is difficult to access, the condition of its water table remained virtually unknown. The mystery was solved thanks to an assessment made by an international team linked to several IRD units and led by LEGOS researchers. This result is all the more interesting in that access to this fresh water is a major issues for populations
The groundwater reserves (or water table) of the Amazon Basin are insufficiently evaluated as the operation involves continuous on-site measurements in virtually inaccessible areas. Using a multi-satellite approach, a French-Brazilian team has managed to overcome this pitfall (1).
The researchers started from the assumption that all inland water reserves consist of groundwater plus surface water — lakes, flood plains, canals, etc. — and soil moisture. “However, the recently analysed satellite measurements of the GRACE mission [Gravity Recovery and Climate Experiment] ?Space missions of NASA and the German space agencyhave given us all land reserves. In the Amazon Basin, surface water is also measured by satellite. Lastly, digital models are used to estimate the moisture contained in soil, explains Fabrice Papa, expert in water cycle and climate study through space-based observations at LEGOS. With a simple subtraction, we were able to calculate the proportion of groundwater and its monthly variations from January 2003 to September 2010”.
This indirect evaluation led researchers to make a number of observations. Their approach shows a decrease in water tables in 2005 which may be due to the drought of that year. Furthermore, this deficit remains visible until the end of their observations, in 2010, in the Alter do Chão aquifer?Geological formation containing temporarily or permanently water, located in the centre of the Amazon Basin. “In fact, we do not know how long it takes for the reserves to replenish”, adds Frédéric Frappart, also from LEGOS. However, this observation allows for a better understanding of the not so obvious links between surface water and groundwater: little rain means less water on the surface, which affects the amount of water deep down. In other words, it seems that the water on the surface refills the water table.
Another finding, valid for the entire Amazon Basin, is that groundwater contributes 20% to 35% on average to the variation in total inland water reserves. This proportion varies however from one area to the next. It rises up to 70% in the Solimões, Xingu and Tapajos basins, i.e. drainage basins subject to infrequent flooding, while it is lower in other areas. Yet, the total stock of inland water for a given area depends on the rainfall this area receives, as well as on the water it loses to evaporation and runoff. Therefore, in areas subject to infrequent flooding, i.e. where surface water varies little, such as the aforementioned basins, it appears that water tables are the main reason for the variability of all inland water reserves.
The final observation is that the variation in surface water reserves is in sync with that of total reserves, but not with that of groundwater reserves, which is sometimes off by several months. Researchers have not yet determined the reasons why and suggest pursuing this matter further, to shed light on exchanges between groundwater and surface water, such as seepage or infiltration which would explain this time lag.
This indirect evaluation of water tables is a world first, the significance of which extends beyond the Amazon. “Most of the fresh water used for human activities comes from these underground reserves, and access to this water is a global issue, points out Fabrice Papa. Our method is applicable on a global scale. This evaluation should now be carried out worldwide, particularly in India and the Sahel, where water tables are regularly affected by droughts and greatly influenced by anthropic pressure”.
1. F. Frappart, F. Papa, A. Güntner, J. Tomasella, J. Pfeffer, G. Ramillien, T. Emilio, J. Schietti, L. Seoane, J. da Silva Carvalho, D. Medeiros Moreira, M.-P. Bonnet, F. Seyler, The spatio-temporal variability of groundwater storage in the Amazon River Basin , Advances in Water Resources , 14 décembre 2018