Power‐law residence time distribution in the hyporheic zone of a 2nd‐order mountain stream

We measured the hyporheic residence time distribution in a 2nd‐order mountain stream at the H. J. Andrews Experimental Forest, Oregon, and found it to be a power‐law over at least 1.5 orders of magnitude in time (1.5 hr to 3.5 d). The residence time distribution has a very long tail which scales as t −1.28 , and is poorly characterized by an exponential model. Because of the small power‐law exponent, efforts to characterize the mean hyporheic residence time ( t s ) in this system result in estimates that are scale invariant, increasing with the characteristic advection time within the stream channel ( t ad ). The distribution implies the hyporheic zone has a very large range of exchange timescales, with significant quantities of water and solutes stored over time‐scales very much longer than t ad . The hyporheic zone in such streams may contribute to short‐time fractal scaling in time series of solute concentrations observed in small‐watershed studies.