Streamwater transit time distributions at the catchment scale: constraining uncertainties through identification of spatio-temporal controls

Abstract

Precipitation water traveling through a catchment takes faster and slower flow paths to reach the outlet. The knowledge about the distribution of relevant flow paths in a catchment and their respective transit times of water is important when considering that water is the main transportation agent for pollutants and that anthropogenic impacts to natural systems can alter the hydrology dramatically, thus endangering water resources. However, the exact processes governing water transport through a catchment are unknown, as no measurement technology exists to capture them in situ. Tracers such as the stable isotopes of water (δ18O and δ2H) are used to model these transport processes. The Transit Time Distribution (TTD) is a model estimate that integrates different flow paths of precipitation water through a catchment to the outlet. Due to different sources of uncertainties, e.g., the model structure, the estimates of TTDs are inherently uncertain. The conclusions of present day studies that want to elucidate the hydrological behavior of catchments, compare catchments or predict the hydrology of ungauged catchments from TTDs inherently suffer from these uncertainties.<br /> The aim of this study was to investigate spatiotemporal influences on the uncertainty of TTDs with the overall goal to ensure better estimates of TTDs. A simple, conceptual model was applied to two humid, small to medium scale catchments to investigate three hypothesis: that (1) heterogeneities of TTDs of a small catchment stem from different soil types, (2) canopy-induced changes in the tracer signal of stable isotopes of water due to interception will influence TTD estimates, and (3) a higher temporal resolution of tracer data will lead to differences in TTDs.<br /> The obtained results indicate that the soil types can indeed explain the spatial patterns of TTDs in a small scale catchment and could be used to limit uncertainty in e.g., ungauged catchments. When calculating TTD for forested catchments, interception must be considered, as it decreases the uncertainty of TTD estimates. Furthermore, a higher temporal resolution of tracer data led to drastically different estimates of TTDs, indicating that the usually applied weekly data is not enough to understand faster flow paths through a catchment.<br /> Thus, this study is a step forward in decreasing uncertainties in TTD estimates by considering canopy interception and arguing for higher resolution tracer data. Future work will have to concentrate on automatization of high-resolution measurements of tracer data to establish the data basis needed for less uncertain TTD estimates.