Erupting volcanoes inject particles -sulfate aerosols- high up into the the atmosphere which spreads out and reflect sunlight back into space. ThVolcano eruption satilitee eruption of Pinatubo for example, discharged some 17 million tonnes of sulphur dioxide 40 kilometers into the lower atmosphere, where it lingered for several years.

volcanic geo engineeringAdvocates of “radiation management’ have proposed injecting massive quantities of similar particles by means of cannons , planes or giant tubes to simulate the effects of a volcano. The reason for this is that these particles that reflect the sunlight effectively “cools down” the planet. And it has been suggested as a means of counter acting global warming.

But a new study found that this could reduce river flows by up to 10%.

In the first study of its kind, University of Edinburgh scientists Carley Iles and Gabriele Hegerl compared annual water flow in 50 rivers around the world with the timing of major volcanic eruptions, notably Agug in 1963, El Chichon in 1982 and Pinatubo in 1991.
 
For some rivers, records went back into the 19th century, making it possible to take into account earlier eruptions too.
 
They discovered that a year or two after these volcanoes belched massive amounts of debris into the upper atmosphere — creating a partial sunscreen — the flows of tropical rivers decreased.
 
“It was known that volcanic eruptions affect global rainfall,” Iles explained by email.
 
Less sunlight means less evaporation, while cooler surface temperatures diminish the capacity of the atmosphere to retain water.
 
“But it was previously unclear to what extent this translated into changes in river flow,” she added.
 
The study does not specify by how much water volume was reduced, but a rough calculation for the Nile and the Amazon — the world’s largest river by discharge — showed a drop of about 10 percent, she told AFP.
That may be enough to have an impact on agriculture and the wellbeing of humans that depend on those rivers, especially if highly populated regions, she suggested.
Globally, the net impact of big eruptions has been a drop in water flows. But in some regions — southern South America and southwestern United States — the amount of water coursing through rivers actually increased.
The differing results, Iles explained, stem from variations in regional weather patterns.
“Our findings suggest that these kinds of geo-engineering schemes are likely to have side effects on river flow, so caution is advised,” Iles said.