GRACE reveals depletion of water storage in northwestern South America between ENSO extremes

Dynamics of terrestrial water storage are determinant for many natural and social phenomena, with implications for water security and environmental sustainability. Here we use 2002-2017 data from the Gravity Recovery and Climate Experiment (GRACE) to study these dynamics in the Magdalena-Cauca river basin in northwestern South America. Through comparison with water balance-based estimates we assess the performance of multiple GRACE products in representing water storage dynamics in the basin, identifying the Mascon product from the Jet Propulsion Laboratory as the best suited for further analysis. We then investigate the existence of long term trends and show that terrestrial water storage in general and groundwater storage in particular have been gradually depleting in the basin since around the end of 2010. GRACE data reveal that this trend is not uniform across the basin but exhibits a clear-cut pattern in which the water depletion rate is more pronounced in the lower parts of the basin than it is in the upper basin. We explore the mechanisms behind the identified temporal trends and spatial patterns and show that water storage depletion largely coincides with a period between the La Nina and El Nino extreme phases of ENSO. Likewise, the pronounced contrast between depletion rates in the lower and higher parts of the basin largely coincides with marked biophysical differences between these regions, including the presence of major wetland systems in the lowlands, and the highlands of the Andean mountains.