Chloride circulation in a lowland catchment and the formulation of transport by travel time distributions

Abstract

[1] Travel times are fundamental catchment descriptors that blend key information about storage, geochemistry, flow pathways and sources of water into a coherent mathematical framework. Here we analyze travel time distributions (TTDs) (and related attributes) estimated on the basis of the extensive hydrochemical information available for the Hupsel Brook lowland catchment in the Netherlands. The relevance of the work is perceived to lie in the general importance of characterizing nonstationary TTDs to capture catchment transport properties, here chloride flux concentrations at the basin outlet. The relative roles of evapotranspiration, water storage dynamics, hydrologic pathways and mass sources/sinks are discussed. Different hydrochemical models are tested and ranked, providing compelling examples of the improved process understanding achieved through coupled calibration of flow and transport processes. The ability of the model to reproduce measured flux concentrations is shown to lie mostly in the description of nonstationarities of TTDs at multiple time scales, including short-term fluctuations induced by soil moisture dynamics in the root zone and long-term seasonal dynamics. Our results prove reliable and suggest, for instance, that drastically reducing fertilization loads for one or more years would not result in significant permanent decreases in average solute concentrations in the Hupsel runoff because of the long memory shown by the system. Through comparison of field and theoretical evidence, our results highlight, unambiguously, the basic transport mechanisms operating in the catchment at hand, with a view to general applications.

Time series of tritium, stable isotopes and chloride reveal short-term variations in groundwater contribution to a stream

“…Hrachowitz et al, 2013;. Results from a dynamic model of chloride transport revealed that water in transit was generally younger than storage water (Benettin et al, 2015). Differences between RTs and TTs also indicate that the assumption of complete mixing was not met for the Teviot Brook catchment.…”

“…Hrachowitz et al, 2013;Birkel et al, 2015) and/or higher sampling frequencies (e.g. Birkel et al, 2012;Benettin et al, 2013Benettin et al, , 2015. Nonetheless, Seeger and Weiler (2014) recently noted that in the current state of research, the calculation of timeinvariant TT distributions from lumped models still represents a useful alternative to more complex, computerintensive modelling methods.…”

“…Accurate quantification of TT is of prime importance for water resource management issues, in particular for the assessment of catchment sensitivity to anthropogenic inputs such as fertilizers or herbicides (e.g. van der Velde et al, 2010;Benettin et al, 2013), and for the provision of additional constraints on catchment-scale hydrological models (e.g. Gusyev et al, 2013).…”

“…One of the reasons is that this non-stationarity is not accounted for in the models commonly used in catchment TT research. In the last 5 years, an ever-growing number of studies has transferred its focus to assessing dynamic TT distributions (Hrachowitz et al, 2010(Hrachowitz et al, , 2013Roa-García and Weiler, 2010;Rinaldo et al, 2011;Cvetkovic et al, 2012;Heidbüchel et al, 2012Heidbüchel et al, , 2013McMillan et al, 2012;Tetzlaff et al, 2014;Birkel et al, 2015;Benettin et al, 2015;Harman, 2015;Klaus et al, 2015a;Kirchner, 2015). Most of these studies agreed on the importance of considering storage dynamics, because the RT distribution of storage water and the TT distribution of water transiting at the outlet of the catchment are likely to be very different.…”

Aggregation in environmental systems – Part 2: Catchment mean transit times and young water fractions under hydrologic nonstationarity

“…The model used here shares a similar overall structure with many other conceptual models (e.g., Benettin et al, 2013), with several simplifications. However, although the model used here is typical in many respects, I will use it in an unusual way.…”