What can flux tracking teach us about water age distribution patterns and their temporal dynamics?

Abstract

Abstract. The complex interactions of runoff generation processes underlying the hydrological response of streams remain not entirely understood at the catchment scale. Extensive research has demonstrated the utility of tracers for both inferring flow path distributions and constraining model parameterizations. While useful, the common use of linearity assumptions, i.e. time invariance and complete mixing, in these studies provides only partial understanding of actual process dynamics. Here we use long-term (< 20 yr) precipitation, flow and tracer (chloride) data of three contrasting upland catchments in the Scottish Highlands to inform integrated conceptual models investigating different mixing assumptions. Using the models as diagnostic tools in a functional comparison, water and tracer fluxes were then tracked with the objective of exploring the differences between different water age distributions, such as flux and resident water age distributions, and characterizing the contrasting water age pattern of the dominant hydrological processes in the three study catchments to establish an improved understanding of the wetness-dependent temporal dynamics of these distributions.The results highlight the potential importance of partial mixing processes which can be dependent on the hydrological functioning of a catchment. Further, tracking tracer fluxes showed that the various components of a model can be characterized by fundamentally different water age distributions which may be highly sensitive to catchment wetness history, available storage, mixing mechanisms, flow path connectivity and the relative importance of the different hydrological processes involved. Flux tracking also revealed that, although negligible for simulating the runoff response, the omission of processes such as interception evaporation can result in considerably biased water age distributions. Finally, the modeling indicated that water age distributions in the three study catchments do have long, power-law tails, which are generated by the interplay of flow path connectivity, the relative importance of different flow paths as well as by the mixing mechanisms involved. In general this study highlights the potential of customized integrated conceptual models, based on multiple mixing assumptions, to infer system internal transport dynamics and their sensitivity to catchment wetness states.

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

“…This is a reformulation of the Mater Equation for the residence time probability density function (pdf) in time-variable flow systems developed by . A similar description of analogous concepts was also provided by later works [van der Velde et al, 2012;Hrachowitz et al, 2013]. A graphical representation of the physical meaning of equations (2) and (3) is provided in Figure 1.…”

“…[47] The theoretical tools employed in this paper combine several recent advancements [Botter et al, 2010Rinaldo et al, 2011;van der Velde et al, 2010avan der Velde et al, , 2012Hrachowitz et al, 2013] in our understanding of the nonstationary character of catchment TTDs and the role of age distributions in the chemical composition of runoff into a single useful application. Our aims are rather of methodological nature, as the application highlights key features of any model of catchment-scale transport processes.…”

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

“…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.…”

“…Calibration of the models on chloride measurements did not yield as accurate results as those for stable isotopes at S1 and to a higher extent at S2, which may be attributed to the higher effects of evaporative enrichment on chloride. Based on flux tracking methods, Hrachowitz et al (2013) showed that processes such as evaporation can result in considerable biases in TT distribution estimates when using chloride as a tracer.…”

Age-ranked hydrological budgets and a travel time description of catchment hydrology

“…Based on these premises, Fenicia et al (2008), Clark et al (2011), McMillan et al (2012, and Hrachowitz et al (2013) aimed to describe both the spatial organization of the catchment and the set of interactions between processes with an assembly of coupled storages (reservoirs) in the number and the organization necessary to give proper hydrological results without adding unwanted parametric complexity (e.g., Klemeš, 1986;Kirchner, 2006). Despite the simplification efforts, the process of adding physical rigor to their models led to quite complex systems.…”