Seasonal Soil Moisture Patterns Control Transit Time Distributions in a Forested Headwater Catchment

Abstract:

The Transit Time Distribution (TTD) is an integrated measure of catchment-wide water transport, frequently used to analyze water flow paths, catchment storage and runoff sources. Despite previous studies, the connections between catchment characteristics and TTDs are still not fully understood. We present results from a two-year stable isotope tracer investigation of the forested Wuestebach headwater catchment (38.5 ha), including precipitation, stream and tributary locations. Using the gauged outlet, we determined effective precipitation (p_eff), subdivided for wet and dry catchment states, and assumed p_eff to be spatially uniform. We then calculated TTDs of 14 ungauged stream and tributary locations where stable isotope tracer information was available and compared them to their respective subcatchment areas and the proportion of riparian zone within the subcatchments. Our approach gave insight into the spatial heterogeneity of TTDs along the Wuestebach River. We found that hydrological hillslope-riparian zone disconnection was an important factor, as the catchment shifted between a wet and a dry state, resulting in two distinct, time-variant hydrological responses. TTD results showed a negative correlation between riparian zone proportion and Mean Transit Time (MTT), corroborated by the dense network of soil water content measurements. No correlation between subcatchment size and MTT was found. Thus, we conclude that the functional landscape entity of riparian zone proportion controls the catchment water transport function. The difference in hydrological behavior of the riparian zone and hillslopes could explain the often encountered old water phenomenon, where considerable amounts of old water quickly appear as runoff.