How often do you think about where stormwater goes after it hits the pavement and runs into the storm drain? It can come as a surprise when you realize how much pollution—sediment, excess nutrients, viruses, bacteria and more—all of the runoff carries to our lakes, reservoirs, streams, and rivers. This pollution comes at a cost, not only in how much money is spent to make water drinkable but also in the health risks it poses to water recreation.

Trees offer a simple and beautiful way to intercept some of that runoff, and they provide a lot of other benefits, too. So, how do trees affect stormwater?

Most of the rainfall that trees capture is consumed through the processes of transpiration and evapotranspiration. Transpiration occurs when a tree pulls water from the soil and releases it as a vapor during photosynthesis. Evapotranspiration occurs when water intercepted by a tree’s limbs and leaves is evaporated into the air as a vapor. Trees also reduce throughfall, which means rainfall volume and velocity are reduced. Less runoff moving more slowly translates into less soil erosion. Finally, tree roots and root decomposition increases the capacity for soil to soak up rain water. This reduces how much stormwater runs off. The image below illustrates how water cycles through a tree.