The La Plata Basin, covering 3.5 million square kilometers, feeds five countries and contains the Pampas, one of the world's richest pasturelands. Ground-based observations are sparse here, and satellite vegetation imagery only sees the surface. This work stands apart: neighboring studies address the pulse of civilization, rhythms of deserts, and wet interludes in Tibet, but none directly weigh water in a major river basin. GRACE gravimetry has for the first time measured the total water stored in soil, snow, and groundwater without a single sensor on the ground.
The GRACE satellites flew in tandem 220 kilometers apart, continuously measuring microscopic changes in that distance. When they passed over an area with excess or deficit of water, gravity shifted slightly, and the satellites moved closer together or farther apart. From 86 monthly gravity field maps spanning April 2002 to August 2009, the scientists subtracted atmospheric and oceanic effects, filtered out noise, and derived changes in water storage with an accuracy of 1.5 centimeters. For comparison, they used the GLDAS model, which accounts for precipitation and evaporation but not groundwater, as well as data from 27 wells in the Argentine Pampas.
GRACE showed that in the lower La Plata, water was disappearing at a rate of up to 3.5 centimeters per year, while the GLDAS model gave only 2.2 centimeters. The drought began in the spring of 2008, by the fall of 2009 it had engulfed the entire basin, and the water deficit reached 12 centimeters of equivalent height, equivalent to 248 gigatons. That is nearly half of the annual water consumption in the United States. Rainfall data from GPCP and TRMM confirmed that precipitation had been below average since 2003, and 2009 was the driest year in 12 years. Dry years coincided with La Niña, wet years with El Niño.
Unlike neighboring studies that rely on models or indirect indicators, this paper provides a direct measurement of total water storage from space. It showed that models like GLDAS systematically underestimate losses because they ignore groundwater. Adding well data to the model markedly improved agreement with GRACE. This is the first quantitative assessment of such a severe drought in La Plata and proof that satellite gravimetry can serve as an independent arbiter for climate models.
The GRACE mission has been extended at least through 2013, and a new data release (release 5) will improve accuracy thanks to better background models. This will allow tracking the recovery of water stores after the drought and checking how quickly the basin returns to normal. In the long run, such measurements will underpin early warning systems for droughts on a continental scale.
The work affects hydrology of large river basins, climate modeling, and water resource management in South America. It also sets a benchmark for validating land surface models.
The next step is to incorporate GRACE data into operational drought monitoring systems and compare them with more detailed ground measurements in other basins. It is also important to separate the contributions of natural variability and human groundwater extraction.
For hydrology, the unresolved question remains how to separate the effects of climate and groundwater pumping on storage depletion. This study shows that satellite gravimetry can provide the overall picture, but without a dense well network, separating the causes is impossible.
The deficit of 248 gigatons of water is equivalent to a layer 12 centimeters thick spread over an area comparable to the combined territory of Argentina and Uruguay.
The idea of weighing water from space echoes Arthur C. Clarke's novel 'The Fountains of Paradise,' where orbital engineers use gravitational anomalies to build a space elevator.