COMBINING MULTI-SCALE OBSERVATIONS AND NEW TRANSPORT THEORY TO EXPLORE TIME VARIABILITY IN HYDROLOGIC TRANSPORT PROCESSES OPERATING IN NESTED PIEDMONT WATERSHEDS

The relationship between a watershed’s structure and the natural mechanisms controlling the transport of water and solutes through the watershed is not well understood. In this study, a multi-scale sampling approach is combined with a new time-variable transit time theory to elucidate the hydrologic processes controlling the transit time distribution (TTD) of water and solutes through a catchment. This new theory decomposes a catchment’s TTD into a part that depends on the time variability and partitioning of hydrologic inputs, and a part that depends on the way water of a given age is sampled by the streamflow. This age-sampling is defined by a probability density function (pdf) of the age-ranked storage, known as an rSAS function. The form of this pdf is theorized to be controlled by hydrologic processes operating at multiple scales and by the catchment’s structure which integrates these processes within its own spatial and temporal framework.

The rSAS function is estimated at a range of scales using stable water isotope samples and discharge data collected from six locations within a 383 hectare (ha) watershed of the Piedmont Physiographic Province (two springs, two forested watersheds, one suburban watershed, and the 383 ha watershed outlet). High frequency water parameters including temperature and specific conductance are also measured at these six locations. Repeat electrical resistivity surveys and the measured water chemistry parameters are used to contextualize the sources of variability in the rSAS function across scales and to help determine which hydrologic processes influence the form of the rSAS function.

Seasonal electrical resistivity surveys have been conducted since April 2014 within one of the forested watersheds, adjacent to the two springs. Weekly precipitation and stream isotope samples from the 383 ha and two forested watersheds have been collected since January 2014, while weekly sampling began at the other three sites in August 2014. Twelve-hourly isotope samples have also been collected at each sampling location along with water chemistry and discharge data. Initial results show clear differences in transport processes operating under low and high catchment storage conditions and between the forested catchments and the 383 ha watershed.