Young runoff fractions control streamwater age and solute concentration dynamics

Abstract

We introduce a new representation of coupled solute and water age dynamics at the catchment scale, which shows how the contributions of young runoff waters can be directly referenced to observed water quality patterns. The methodology stems from recent trends in hydrologic transport that acknowledge the dynamic nature of streamflow age and explores the use of water age fractions as an alternative to the mean age. The approach uses a travel time‐based transport model to compute the fractions of streamflow that are younger than some thresholds (e.g., younger than a few weeks) and compares them to observed solute concentration patterns. The method is here validated with data from the Hubbard Brook Experimental Forest during spring 2008, where we show that the presence of water younger than roughly 2 weeks, tracked using a hydrologic transport model and deuterium measurements, mimics the variation in dissolved silicon concentrations. Our approach suggests that an age–discharge relationship can be coupled to classic concentration–discharge relationship, to identify the links between transport timescales and solute concentration. Our results highlight that the younger streamflow components can be crucial for determining water quality variations and for characterizing the dominant hydrologic transport dynamics.

The Demographics of Water: A Review of Water Ages in the Critical Zone

“…Age distributions with long tails, resulting from heterogeneity of the flow and transport processes, variable flow path connectivity, and/or mixing intensity (Hrachowitz et al, ), are challenging to assess, because the characteristic timescales of hydrological tracer inputs to the system will control which parts of the age distribution can be determined (Benettin, Bailey, et al, ). Moreover, due to the age characteristics of some compartments, particular tracers are useful for one part of the critical zone but cannot be applied for other parts.…”

“…This recharge to groundwater can thus contain noticeable fractions of "old" water. The interactions of saturated and unsaturated zones are generally responsible for hysteresis in the stream water ages when plotted against catchment wetness (Benettin, Bailey, et al, 2017;Hrachowitz et al, 2013;Rodriguez et al, 2018;Yang et al, 2018).…”

Ideas and perspectives: Tracing terrestrial ecosystem water fluxes using hydrogen and oxygen stable isotopes – challenges and opportunities from an interdisciplinary perspective

“…In soils and groundwater, isotopic heterogeneity results from differences in the inputs (precipitation, throughfall, snowmelt), differences in the temporal integration of previous precipitation events (Yang et al, 2016), and differences in the subsequent fractionation from evaporation and transpiration (Benettin et al, 2018). As a general rule, smaller storage-to-output ratios with short residence times generally lead to higher temporal variability.…”

“…Dawson and Ehleringer, 1993) into the models that are used to interpret isotope data. For example, process-based models may help to interpret observations and experimental data (Benettin et al, 2018), assessing the importance, seasonality, and uncertainty in evapotranspiration partitioning (Knighton et al, 2017;, and characterizing water pathways and quantifying the associated travel times at the catchment scale (Kuppel et al, 2018). Clarifications are needed on which parameters to include in a model and on when it might be possible to ignore their influence.…”

Spatial variability in the isotopic composition of water in small catchments and its effect on hydrograph separation

“…These advancements include studies on flow pathways and runoff generation processes, as well as the identification of the sources of plant transpiration (Dawson, Mambelli, Plamboeck, Templer, & Tu, ; Werner et al, ; Penna et al, ) and the effects of vegetation on water fluxes at the catchment scale (e.g., Geris et al, ; McCutcheon, McNamara, Kohn, & Evans, ; Sprenger, Tetzlaff, & Soulsby, ). Long‐term time series of stream isotope data have been used to determine subsurface mixing and to assess the distribution of the times that it took for the water to become streamflow (i.e., transit time) (e.g., McGuire & McDonnell, ; Hrachowitz et al, ; Rigon, Bancheri, & Green, ; Benettin et al, , b; Tetzlaff et al, ; Sprenger et al, ,b; Sprenger et al, ), to determine the fraction of stream water that is younger than a certain age (young water fraction; Kirchner, ; von Freyberg, Allen, Seeger, Weiler, & Kirchner, ; Stockinger et al, ; Lutz et al, ), and to aid the calibration of hydrological models (e.g., Birkel & Soulsby, ; Smith, Welch, & Stadnyk, ).…”