The Relative Importance of Time-Variable Transport through Hillslope and Riparian Hydrogeomorphic Units on the Emergent TTD of a Small Forested Piedmont Watershed

Abstract:

The influence of watershed structure and the natural mechanisms controlling the transport of water and solutes through individual hydrogeomorphic units on the catchment transit time distribution (TTD) is still not well understood. In this study, a new formulation of time-variable TTD theory is combined with simple water balance models to elucidate the influence of individual hydrogeomorphic units and the hydrologic processes operating within them on the integrated catchment TTD. This new theory allows one to decompose a 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 discharge. This age-sampling is defined by a probability density function of the age-ranked storage, known as the relative StorAge Selection (rSAS) function. The form of the rSAS function for a given hydrogeomorphic unit is theorized to be controlled by hydrologic processes operating within that unit, while the TTD of the entire catchment represents an integration of the rSAS functions of these individual units.

Stable water isotope samples and discharge data are used to calculate the rSAS functions of hillslope and riparian hydrogeomorphic units contained within a 37 hectare watershed of the Piedmont Physiographic Province. The rSAS function and corresponding TTD of the 37 ha watershed are estimated. Water balances for the hillslope and riparian units are calculated along with the overall watershed water balance. Diurnal fluctuations in riparian water level are used to independently estimate evapotranspiration from the riparian area, rates of groundwater flux, and changes in riparian storage. Gaged springs are used to estimate changes in hillslope storage as well as discharge. The rSAS functions of the hillslope and riparian units are then weighted and combined and compared to the overall catchment TTD to elucidate the influence of each unit on the overall time-variable TTD.