Combining New Theory and Multi-Scale Observations to Explore Hydrologic Transport Processes and Their Relationship to Catchment Structure in a Small Piedmont Watershed

Abstract:

The influence of catchment structure on hydrologic processes controlling the transit time distribution (TTD) of water and solutes through watersheds is still not well understood. However, recent advances in time-variable transit time theory offer a new framework that decomposes controls on a catchment’s TTD into a part that is dependent on the time variability of flow and water balance partitioning, and a part that depends on the way water of different ages is sampled by the streamflow. Age-sampling is characterized using a probability density function over the storage ranked by age, herein referred to as an Ω function. The form of the Ω function at the catchment scale is hypothesized to be controlled by hydrological processes operating at that scale and by the integration of processes from smaller-scale units as dictated by the catchment’s structure.

Here stable water isotope samples and water parameters including conductivity and fDOM are measured at six sampling locations within a 383 ha watershed of the Piedmont Physiographic Province (the outlet, two forested watersheds, one suburban watershed, and two springs). Analysis of the hydrology and the Ω function estimated at each spatial scale is used to understand the controls of variability in hydrologic inputs and outputs, the partitioning of outputs, and the heterogeneity of flow pathways. These three sources of variability are then examined within the context of measured water chemistry parameters to further elucidate hydrologic processes across scales.

Isotope data has been collected from each site at a range of sampling intervals from minutes during a storm event to weekly over a period of months to parameterize Ω functions over a range of time scales. Precipitation and stream samples from the 383 ha and two forested watersheds have been collected weekly since January 2014, while weekly sampling began at the other three sites in August 2014. Twelve-hourly isotope samples have been collected at each sampling location for several one-week periods along with hourly and event data. Initial results reveal clear differences in the processes controlling the forested and suburban watersheds and suggest different transport dynamics operate under high and low catchment wetness conditions.