Remote Sensing

Groundwater seen from space

A new satellite system will soon resume collecting aquifer data from around the globe

Imagem: Edson Silva /Folhapress One of the recharge areas of the Guarani aquifer, Saibra Lagoon, in Ribeirão Preto, was partially dried up in 2014Imagem: Edson Silva /Folhapress

Falcon 9—a rocket almost 70 meters tall and 4 meters in diameter—is set to conduct another flight this March from a California air force base in the US to deliver the German-built twin Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) satellites into orbit. GRACE-FO will replace the original mission, GRACE, which was decommissioned in 2017 after 15 years in orbit 500 kilometers above the surface (see Pesquisa FAPESP, issue no. 181). Because the force of gravity is determined by mass, which in turn varies with the amount of water accumulated in rivers, snow, in the ground, and in aquifers, GRACE uses measurements of gravitational fields to provide information about surface freshwater reserves, and is the only satellite system capable of measuring groundwater reserves thanks to its unique mode of operation.

GRACE data, with a spatial resolution of 300 to 400 km2, has supported comprehensive analytics supplementing conventional methods of assessing the amount of water stored in aquifers. The Mineral Resources Research Company (CPRM) monitors 27 aquifers in Brazil using water-level meters installed in 380 artesian wells. “The data collection network is too small to monitor the entire country,” says geologist Maria Antonieta Mourão, a senior hydrologist at the company. “With the new GRACE mission, we could use the artesian well meters only to calibrate measurements of aquifer recharge.”

CPRM found that groundwater levels in São Paulo, Triângulo Mineiro and Mato Grosso had receded by two to four meters in 2014 compared with 2011, and has warned about the intense exploitation of aquifers potentially leading to depletion of water reserves. The Guarani aquifer, one of the world’s largest underground water reserves, spanning 1.2 million km2, is being tapped by some 4,000 artesian wells in the states of São Paulo, Paraná, Santa Catarina, Rio Grande do Sul, and Mato Grosso do Sul. The Alter do Chão aquifer, in the North, extends underground for more than 500,000 km2 and is also suffering from overexploitation—a total of 15,000 wells are estimated to be in operation in Manaus—and from contamination by waste dumps, service stations, cemeteries, and sewers, especially in major cities.

GRACE allowed Brazilian researchers to measure the dramatic depletion of aquifer reserves in Brazil during the drought of 2014, which began in October the previous year in the state of São Paulo and extended into 2015 throughout the Southeast. Between 2011—a year with high rainfall—and 2015, the drought depleted around 150 cubic kilometers (km3) of surface water and groundwater in the Southeast of Brazil. This volume is equivalent to half of the maximum capacity of hydro power reservoirs in the Paraná basin, a river system draining an area of approximately 900,000 km2 and providing water supply to around 60 million people, including almost 20 million people living in Greater São Paulo.

The drought in 2014 affected the middle and northeastern areas of the Paraná basin, with water volumes declining most in reservoirs closest to the sources of the rivers, says civil engineer Davi Melo, a researcher at the São Carlos School of Engineering at the University of São Paulo (EESC-USP). “The larger reservoirs, although depleted by more than 50% between 2011 and 2015, were still able to compensate for losses in smaller reservoirs and prevent the effects of the drought from spreading,” he says. He described in detail the impacts from the 2014 drought on the Paraná river basin in a paper published in November 2016 in Hydrology and Earth System Sciences and in his doctoral research, which he completed in 2017 under civil engineer Edson Wendland, a professor of hydrology at EESC-USP.

The northeast and India 
GRACE satellite data has also shown that groundwater volumes declined at a rate of 49 km3 per year in Brazil’s Northeast and at a slightly higher rate—56 km3 per year—in the Southeast between February 2012 and January 2015, according to a study published in the Journal of Hydrometeorology in February 2016. The author, Rio de Janeiro–born civil engineer Augusto Getirana, has worked at NASA since 2011 and has extensively used GRACE data to evaluate drought-affected areas in the United States. The resulting maps are published by institutions collaborating with NASA, such as the US Department of Agriculture and the National Drought Mitigation Center at the University of Nebraska. “We can use GRACE data alone to provide a high-level picture and to quantify aquifer volumes in a given region, or combine it with data from other sources, such as hydrological models to model the flow of water through the ground or in the atmosphere, and field observations where greater detail is needed,” he says.

In 2007, a team from the University of Texas at Austin found 82% agreement between field measurements and satellite data for a 450,000 km2 aquifer in the Midwest of the US. In 2009, a NASA team led by Matthew Rodell observed that aquifers were being depleted much faster than they were being replenished in northwestern India, a region that is home to 114 million people. The withdrawal of 109 km3 of water for rice irrigation from August 2002 to October 2008 was 20% more than the government’s estimates and twice the capacity of India’s largest surface reservoir.

The possibilities offered by GRACE data will be discussed in the Latin-American Symposium on Groundwater Monitoring due to be held in Belo Horizonte in April 2018, when the new satellite will already be in operation.

Project
Water availability during extreme climate events: Droughts in the Paraná River basin (no. 16/23546-7); Grant Mechanism Postdoctoral Grant; Principal Investigator Edson Cezar Wendland (USP); Scholarship Beneficiary Davi de Carvalho Diniz Melo; Investment R$196,780.00.

Scientific articles
MELO, D. C. D. et al. Reservoir storage and hydrologic responses to droughts in the Paraná River basin, south-eastern Brazil. Hydrology and Earth System Sciences. Vol. 20, no. 11, pp. 4673–88. 2016.
GETIRANA, A. Extreme water deficit in Brazil detected from space. Journal of Hydrometeorology. Vol. 17, pp. 5919–99. 2016.