How long does it take rain falling on a forest to reach a stream? What pathways does it take to get there? Humans have pondered such simple questions for millennia. Using a unique long-term data set on stream water chemistry from the Hubbard Brook Experimental Forest in New Hampshire, an international team of scientists has new answers.
Variation in the isotopic composition of water was used to determine the time that it took water to reach the stream, which varied from a median of 50 days during wet periods to 190 days during dry periods. At all times, there is a very small portion of the streamflow that is much older, as long as several years. These variations in water travel time matched changes in concentrations of substances dissolved in water, demonstrating how variations in water sources regulate water quality.
Distinct water sources detected in this study include shallow groundwater draining the soil and deeper groundwater draining geologic deposits. A mixing zone in riparian areas adjacent the stream determines how each of these sources deliver water to the stream. These results will help forest managers understand mechanistically how forests function to provide clean and reliable water supply and provide a benchmark for determining how climate change will affect this valuable resource in the future.