Seasonal soil moisture patterns: Controlling transit time distributions in a forested headwater catchment

Abstract

AbstractThe Transit Time Distribution (TTD) of a catchment is frequently used for understanding flow paths, storage characteristics, and runoff sources. Despite previous studies, the connections between catchment characteristics and TTDs are still not fully understood. We present results from a 2 year stable isotope tracer investigation in the forested Wüstebach headwater catchment (38.5 ha), including precipitation, stream, and tributary locations. We used the gauged outlet to determine effective precipitation (peff), subdivided for wet and dry catchment state, and assumed it 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 respective subcatchment areas and the proportion of riparian zone within the subcatchments. Our approach gave insight into the spatial heterogeneity of TTDs along the Wüstebach River. We found that hydrological hillslope‐riparian zone disconnection was an important factor, as the catchment shifted between two distinct, time‐variant hydrological responses that were governed by seasonal changes of overall catchment wetness. 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. 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.