Trees help rain seep into soil because living and decaying roots make soil porous by creating a network of well-connected, minuscule channels in the soil. Rainwater seeps into soil with such channels several hundred times faster than it seeps through soil without channels.
Additionally, when plant debris falls on the soil and starts to organically degrade, it helps soil maintain integrity and form small aggregated clumps. These clumps also ensure that soil is porous.
Once water is absorbed by soil, just as rainwater percolated downward into soil, water can percolate horizontally in soil as well. This kind of underground “water flow” can feed water into streams and rivers wherever the water table intersects the streambed. Ground-water velocities can be as low as 1 foot per year or 1 foot per decade, but not higher than 1 foot per day. This underground component is known as base flow.
Studies have found that water more easily infiltrates soil in forested areas than in crops, degraded forests and grazed pasture.
In the UK, studies showed that soil infiltration rates were 67 times greater in plots planted with trees compared to grazed pasture. Grazed pasture is especially bad at absorbing water because animal hoofs compact the soil, which makes it lose its porosity.
The greater absorption of water in forests also translates into ensuring rivers stay perennial when forests are intact.
