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The Forested Watershed: the Giant Sponge and the Big Straw

Posted: May 19, 2015

Despite it feeling like a wet and gray winter, it hasn’t translated into a very wet spring. We are blessed to have the giant sponges that are forested watersheds to help us out. Penn’s Woods isn’t just about the trees.

Where has all the water gone? Two weeks ago on a Sunday hike at Penn State’s Rock Springs agricultural research farm west of State College, there were flowing drainages, puddled water full of amphibian eggs, and plenty of places for a thirsty dog to get a drink. This past weekend those cooling spot were gone in the agricultural areas; but in the woods there were still good spots with flowing water to take a welcome break.

Water is in the news more than ever. On the national perspective, we’re watching the droughts continue in the western United States. Debates are held and feasibility studies undertaken on moving water from the Great Lakes westward, who has the right to the water in the Colorado River, and when do agricultural water needs trump community needs. Here in Pennsylvania, twenty-seven of our sixty-seven counties remain under a drought watch. By comparison to many places in the world, we are very fortunate. But despite it feeling like a wet and gray winter, it hasn’t translated into a very wet spring. We are blessed to have the giant sponges that are forested watersheds to help us out. Penn’s Woods isn’t just about the trees.

Forests and water are intricately interwoven. Forest soils are one of the most absorptive types of soils that exist. Hydrologists use a device called an infiltrometer to determine permeability, an instrument that measures the time it takes for a soil or surface to absorb water. For example, asphalt does not absorb water; water sits there until it evaporates in a hot sun. Lawn has the ability to absorb about one inch per hour and then water starts flowing over the top, making its way to the storm sewers or puddling until the hot sun dries it out. Grass roots don’t go very deep and the soil is often compacted by lawn mowers. Forest soils have been shown to absorb up to 15 inches or more per hour. We rarely experience rain events that exceed that rate!

Forest soils are giant sponges. The top layer, the duff layer, is covered in leaves and vegetation that intercept falling water and slow it down – preventing raindrops from hitting the soil and dislocating soil particles to move away, as sediment, in flowing water. Forest soils have lots of macropores (big holes made by worms, insects, dead tree roots, live tree roots) that allow water to move quickly into the ground. And they have micropores (small holes) that hold onto the water and keep it available for plants and animals to use it. Water infiltrates until it saturates the soil or fills the holes completely. It then begins to move through the soil, until it hits bedrock or clay or another surface that it can’t get through. Water then flows sideways downhill until it comes out in streams or gets held in groundwater reservoirs. It takes a long time for water to move through that system. The soil organisms also help filter the water, removing pollutants and excess nutrients that could harm stream life. Many, many urban centers (Boston and New York City, for example) use forested watersheds to provide their clean drinking water with minimal treatment. The forest soils absorb, hold, and filter water, releasing it steadily over time.

At the same time, forests can be thought of as big straws. Trees move a lot of water through their trunks and out their leaves as they photosynthesize and breathe. In the last two weeks, most all of our trees have leafed out (the black walnuts are still breaking bud in our neck of the woods). The giant straw is coming online. Without additional rain, the trees pull that water from the sponge of the soil, which means that stream flow drops. Groundwater will continue to flow into those streams keeping it at base flow levels, but perhaps not as much moisture is passing through the soil. Trees move the water out of the soil and into the atmosphere, contributing to moisture in the air, and hopefully cloud formation that will bring more precipitation and keep us green and vibrant.

Both the giant sponge and big straw are vital to the water cycle and its continuity. As more forests are paved over, as more agricultural lands converted to development, we lose the ability for those soils to work in our favor, keeping streams clean and flowing, providing drinking water, slowing down and absorbing storm water, and ensuring a continuous water cycle from which we benefit, including hot dogs.

Contact Information

Allyson Brownlee Muth, Ed.D.
  • Forest Stewardship Program Associate
Phone: 814-865-3208