Every year, 285 billion metric tons, or 285 cubic kilometres of water by volume, rises and falls in the Amazon floodplain, accounting for just five per cent of the total water flow into the ocean.  Until now, the sheer size of the area and the difficulties of access meant that researchers had to rely on data taken from sporadic field visits, leading them to believe that the water flow was as much as 30 per cent.

The findings will be of critical importance to scientific efforts to understand climate change, and will impact significantly on future predictions and analyses of flood levels.

Four satellites were used to study the floodplain, looking at water level changes during the wet and dry seasons between 2003 and 2006. The measurements were taken using a satellite called GRACE which measures changes in the planet's gravity field and records the weight (and hence the volume) of water resting on the Earth's surface. 

The study was conducted by a team of experts from the Universities of Ohio, Baltimore, Bristol and California, and is reported online in the journal Remote Sensing of Environment.

Prof Paul Bates, from Bristol’s School of Geographical Sciences, said: “This is the first time we've been able to actually measure the amount of water that flows every year from the Amazon River channel to its floodplain and back again.”

Prof Bates’ team, reputed for its flood assessment work using computer model simulations, and with an expertise in Amazon hydraulics, was able to critically review the data produced using the satellites.

“You can't do this work from the ground because of the vast area and difficulty of getting around, so satellites and models really are the only options,” added Prof Bates. “The work is important as we currently don't know how much water flows from the Amazon into the ocean, or the route that water takes to get there.  These new measurements tell us more about both of these things, and provide a baseline from which we can look for future changes in the Amazon flood wave.”

The combined data from the satellites reveals how the Amazon landscape has changed as a result of highland rains creating an overflow into the lowland jungle.

At the height of the rainy season, water flowed into various locations on the Amazon floodplain at a rate of 5,500 cubic meters (5,500 metric tons) per second, and during the dry season, it drained away into the Amazon River - and, ultimately, into the Atlantic Ocean - at a rate of 7,500 cubic meters (7,500 metric tons) per second.

Paper:

Alsdorf et al. Seasonal water storage on the Amazon floodplain measured from satellites. Remote Sensing of Environment, 2010; DOI: 10.1016/j.rse.2010.05.020.