Water keeps to itself

Scientists from Oregon State University and the Environmental Protection Agency used stable isotope analysis to ‘fingerprint’ water in the Cascade Range of Oregon and Washington, and keep track of exactly where it goes. They found that in dry areas, the first precipitation after a period of draught does not mix freely with older ground water. Instead, the new water is held by the soil surrounding plant roots.

Prior to this study, it was assumed that as new precipitation entered the soil, it became indistinguishably mixed with older water and trickled into nearby streams. Renée Brooks and her colleagues found that when the ground is dry, most of the precipitation that falls gets trapped in tiny pores surrounding plant roots. This water never mingles with ground water, but remains trapped in place until the plants draw it out via transpiration.

For example, after the first large rainstorm of the fall, 96% of the falling water was held around plant roots, and only 4% made its way into a stream. Later, after many rains had saturated the soil, 55% of falling water ended up in streams. Regardless of when the precipitation occurred, water that was originally held in soil around plant roots never ended up mixing and entering streams.

In other words, water can be divided into two separate ‘populations’ of molecules. This totally unexpected finding requires researchers to rethink everything from the movement of nutrients and pollutants through soil to how streams function.

Photo credit: Jina Lee, May 2007.