Scientists from Oregon State University and the US Environmental Protection Agency employed updated techniques in stable isotope analysis in order to ‘fingerprint’ water and determine where it came from and where it moved to.

The prevailing belief was that when new precipitation entered the soil, it mixed well with other water and eventually moved to streams. However, the new study, published in Nature Geoscience, showed that small pores around the roots of plants cling to the first rainfall following a dry summer and hold it there in order to hydrate the soil. Yet this moisture hardly ever mixes with the water from subsequent precipitation, which passes through larger pores in the soil.

In one test, only four per cent of the first large rainfall after summer ended up in a stream – the majority was absorbed by the soil around plants to rehydrate it. But when the soil’s moisture was fully recharged a month later, 55 per cent of precipitation went directly into streams, and during the winter rains, almost all of the water that was initially held in the soil remained in place without mixing with precipitation.

The findings may mean that scientists have to reconsider how nutrients or pollutants move through the soil. ‘This could have enormous implications for our understanding of watershed function,’ said Jeff McDonnell, a hydrology professor at the university. ‘It challenges about 100 years of conventional thinking.’

April 2010