{"id":686,"date":"2015-11-30T13:32:00","date_gmt":"2015-11-30T18:32:00","guid":{"rendered":"https:\/\/water.phila.gov\/blog\/evapotranspiration-explained-invisible-flying-rivers\/"},"modified":"2023-01-19T14:18:36","modified_gmt":"2023-01-19T19:18:36","slug":"evapotranspiration-explained-invisible-flying-rivers","status":"publish","type":"post","link":"https:\/\/water.phila.gov\/blog\/evapotranspiration-explained-invisible-flying-rivers\/","title":{"rendered":"‘Invisible Flying Rivers’: How Trees Help Manage Stormwater"},"content":{"rendered":"

\"\"This diagram shows how trees move water from the soil to the air, creating what one NPR source describes as “massive, invisible flying rivers.” Image Credit: Philadelphia Water<\/span><\/em><\/p>\n

Evapotranspiration.<\/em> <\/strong><\/big>A real\u00a0mouthful, right?\u00a0But it\u2019s also a word we like to\u00a0use<\/a> when explaining how Green City, Clean Waters<\/a><\/em> and green stormwater infrastructure helps the city manage billions of gallons of excess water we get from storms.<\/big><\/p>\n

Essentially, we are talking about trees and plants moving water from the ground to the air, sucking it up through\u00a0the roots, and breathing it out in the air.<\/p>\n

Antonio Nobre<\/a>, one of Brazil\u2019s leading climate scientists, described the impact of evapotranspiration in a very cool way while talking to NPR<\/a>\u2019s Lourdes Garcia-Navarro in early\u00a0November:<\/p>\n

“What it creates, Nobre says, are these massive, invisible flying rivers.”<\/em><\/p>\n

Flying <\/em>rivers? Quite the image, eh?<\/p>\n

Nobre was talking about how the Amazon rainforest may be creating its own climate, and a big part of his \u201cflying rivers\u201d idea has to do with the massive, planetary\u00a0scale\u00a0of the South American rainforest, which covers more than 2.7 million square miles.<\/p>\n

At just 141\u00a0square miles and mostly not forest, Philly\u00a0doesn’t\u00a0have a fraction of the trees needed to create\u00a0the\u00a0effect Nobre refers to, so maybe instead of flying rivers<\/em>, we can think of our urban tree network as a system capable of releasing flying springs<\/em> of fresh water back into the air?\u00a0Still pretty cool, right?<\/p>\n

Here\u2019s a little more about why that process is important\u00a0for us in Philly:<\/p>\n

Like a lot of cities with older water infrastructure, we have a combined sewer system<\/a> in many neighborhoods that handles both sewage and<\/em> water from storms. When there\u2019s heavy rain, it can lead to overflowing sewers and a mixed mess of polluted stormwater and wastewater spilling into our rivers.<\/p>\n

Green infrastructure helps address that problem in a number of ways, but the basic goal of all our green tools<\/a> is to clean and filter water and keep too much of it from entering the sewers during and after storms. Tools like rain gardens<\/a> and tree trenches<\/a> slow down and filter stormwater, but they also allow plants to soak it up.<\/p>\n

This is where evapotranspiration<\/strong> comes into play.<\/p>\n

The first part of the word\u2014\u201cevapo\u201d\u2014refers to the evaporation that occurs when we use green infrastructure to slow down stormwater. Instead of rushing into the sewers and rivers, it\u2019s held closer to the surface with materials like soil and stone. There, it has a chance to slowly evaporate back into the atmosphere.<\/p>\n

Transpiration<\/strong>, the second part of the term, is a little more complex.<\/p>\n

We\u2019re used to thinking of plants and trees soaking water up through their roots, but lesser-known is the fact that this isn\u2019t a one-way street: trees and plants also put water back into the atmosphere through their leaves.<\/p>\n

And that\u2019s the transpiration Nobre refers to when he talks about those invisible flying rivers.<\/p>\n

Evapotranspiration is (you guessed it) the combined amount of water that enters the air from evaporation and transpiration, and it’s an important part of what green infrastructure does for us. Even if we can\u2019t begin to compare Philly to a rain forest, Green City, Clean Waters alone has added nearly 2,000 new trees to the city since 2011<\/a>.<\/p>\n

We\u2019ll need a lot more, but it\u2019s a good start, and every tree helps!<\/p>\n

For your listening pleasure, we\u2019ve embedded Antonio Nobre\u2019s interview with NPR below.<\/p>\n

They get into transpiration around the 2:35 mark, but the whole story is a great listen if you have time*:<\/p>\n