The Rio de la Plata River is the estuary of the large South American rivers Paraná and Uruguay. The river basin extends across Brazil, Argentina, Uruguay, Paraguay and Bolivia and, at 2.6 million square kilometres, is the sixth-largest river system in the world.

The image shows the storage anomalies, i.e. the changes and deviations of the Terrestrial Water Storage (TWS) and Groundwater Storage (GWS) from the long-term average. A classification of how dry (red) or wet (blue) a month was in relation to the entire time series is highlighted in colour.

The TWS shows significantly greater annual variation than the GWS. This characteristic is frequently observed, as TWS also includes water storage compartments that react more quickly to changes than groundwater and can exhibit greater seasonality, such as soil moisture or water storage in rivers and lakes.

A prolonged drought has been observed in TWS since 2020. This drought is less pronounced in the groundwater storage. Therefore, we are only observing a groundwater drought to a limited extent. From this, we can conclude that the drought has occurred and continues to occur more in the near-surface reservoirs, such as surface water or soil moisture. Between 2015 and 2020, the data for both GWS and TWS show above-average moisture, with GWS remaining in the (too) wet range for significantly longer than TWS. This persistently wet period could have created a buffer in the groundwater for the current ongoing drought. Therefore, a lot of water from the near-surface reservoirs may have entered the groundwater during this period.

Conversely, the drought in the GWS in 2009 was more pronounced than in TWS. That could be related to the fact that the previous years (2003 to 2008) were already less humid in the groundwater than in the terrestrial water storage. It is, therefore, possible that more water remained in the near-surface reservoirs or was released from there into the atmosphere or the oceans instead of into the groundwater.

Various processes are involved in the accumulation of water in groundwater. In addition to infiltration rates of the soil or the connection of the aquifers to surface waters, the temporal and spatial distribution of precipitation or evaporation also plays a role. In the Rio de la Plata basin considered here, human intervention through the control of discharge from reservoirs is another important factor. 

Text & Graphic: Eva Boergens, GFZ