In a recent Nature Geoscience study, scientists discovered that soil clings to water from the first rainfall of the summer and holds it so tightly it almost never mixes with other water. This discovery challenges the century-old assumption that rainwater, after it enters the soil through precipitation, displaces leftover water and pushes it deeper into the ground and eventually into neighboring streams.
Renée Brooks from the Environmental Protection Agency and colleagues determined soil water is actually separated into two “worlds”: mobile water that eventually runs to streams, and stationary water that is used up by plant roots.
These worlds are divided by pore size. Small pores in the soil around plant roots fill with water and serve as tiny reservoirs throughout the summer. Any water that enters the soil after the first summer rain moves through larger pores and almost never mixes. Once the plants use up the water in the small pores, the autumn rains replenish these reservoirs and the process repeats.
As the study’s co-author Jeff McDonnell explains in a press release from the University of Oregon:
Water in mountains such as the Cascade Range of Oregon and Washington basically exists in two separate worlds. We used to believe that when new precipitation entered the soil, it mixed well with other water and eventually moved to streams. We just found out that isn’t true. This could have enormous implications for our understanding of watershed function.
The discovery was made using a process called isotope analyses—that is, the researchers were able to identify specific water signatures to tell where it came from and where it moved.
The authors say the findings could affect the current understanding of how pollutants move through soils, how nutrients get transported from soils to streams, how streams function and how vegetation might respond to climate change.
Renée Brooks, J., Barnard, H., Coulombe, R., & McDonnell, J. (2009). Ecohydrologic separation of water between trees and streams in a Mediterranean climate Nature Geoscience DOI: 10.1038/NGEO722