Soil moisture and precipitation intensity control the transit time distribution of quick flow in a flashy headwater catchment

Abstract. The rainfall-runoff transformation in catchments usually follows a variety of slower and faster flow paths which leads to a mixture of “younger” and “older” water in streamflow. Previous studies have investigated the time-variable distribution of water ages in streamflow (Transit Time Distribution, TTD) by stable isotopes of water (δ¹⁸O, δ²H) together with transport models based on StorAge Selection (SAS) functions. This function traditionally formulated based on soil moisture to mimic the preferential release of younger water as the system becomes wetter. However, besides soil moisture, it is plausible to assume that precipitation intensity may also play a critical role in how quickly water flows through a catchment. In this study, we tested whether fast flow and its transit times are controlled by soil moisture only or also by precipitation intensity in a heterogeneous catchments with a significant fast runoff response component. We analyse high-resolution δ18O data (weekly and event streamflow δ18O samples) in a 66 ha agricultural catchment. We estimate TTDs by a tracer transport model based on SAS functions. We test two scenarios of the SAS function parameter for the quick release of young water into streamflow, one as a function of soil moisture only, and one as a function of both soil moisture and precipitation intensity. The results that accounting for both soil moisture and precipitation intensity to define the shape of SAS functions for quick flow, improved the tracer simulation in streamflow (increase in Nash-Sutcliffe Efficiency from 0.31 to 0.51). Even though the estimation of the TTs younger than 90 days were similar for both SAS approaches, the shorter travel times (TTs younger than 7 days) were not represented well when only accounting for soil moisture in the SAS function parameterization, in particular, in the summer and autumn months. This is due to flow processes that promote the direct contribution of precipitation to the stream (e.g tile drain) and infiltration excess overland flow processes. It appears that a significant portion of event water bypasses the soil matrix through fast flow paths (overland flow, tile drains, and/or preferential flow paths) also in dry soil condition for both low and high-intensity precipitation. Thus, in catchments where preferential flows and overland flow are important flow processes, soil-wetness-dependent and precipitant-intensity-conditional SAS functions may be required to describe and identify the mechanisms behind the quick streamflow generation and their time scale.