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.

Please see SCs for detailed responses by the authors to the comments made by Referee #2.

“…Our simulations emphasize the time-variant character of water ages and travel times in hydrological systems, as observed at the catchment scale in various recent studies (van der Velde et al, 2012;Benettin et al, 2013;Heidbüchel et al, 2013;Soulsby et al, 2015;Benettin et al, 2017). Further, the age dynamics in the soil waters were driven by the variability in water stored in the soils, supporting an "inverse storage effect" as discussed by Harman (2015).…”

Please see SCs for detailed responses by the authors to the comments made by Referee #2.

“…Our simulations emphasize the time-variant character of water ages and travel times in hydrological systems, as observed at the catchment scale in various recent studies (van der Velde et al, 2012;Benettin et al, 2013;Heidbüchel et al, 2013;Soulsby et al, 2015;Benettin et al, 2017). Further, the age dynamics in the soil waters were driven by the variability in water stored in the soils, supporting an "inverse storage effect" as discussed by Harman (2015).…”

Water ages in the critical zone of long-term experimental sites in northern latitudes

“…Our simulations emphasize the time-variant character of water ages and travel times in hydrological systems, as observed at the catchment scale in various recent studies (van der Velde et al, 2012;Benettin et al, 2013;Heidbüchel et al, 2013;Soulsby et al, 2015;Benettin et al, 2017). Further, the age dynamics in the soil waters were driven by the variability in water stored in the soils, supporting an "inverse storage effect" as discussed by Harman (2015).…”

Chloride Sources and Losses in Two Tile-Drained Agricultural Watersheds

“…The Embarras River watershed was estimated to have 75 to 80% of the land area drained in 1997 (David et al, 1997), and this likely has increased with new patterned systems since that time. The field tile and overall watershed data are similar to results reported by Benettin et al (2013), who stated that chloride in streams comes from both root-zone water with short transport times of weeks to months combined with groundwater chloride with travel times of several years. This would account for the lag times we have observed.…”

Please see SCs for detailed responses by the authors to the comments made by Referee #2.

“…Our simulations emphasize the time-variant character of water ages and travel times in hydrological systems, as observed at the catchment scale in various recent studies (van der Velde et al, 2012;Benettin et al, 2013;Heidbüchel et al, 2013;Soulsby et al, 2015;Benettin et al, 2017). Further, the age dynamics in the soil waters were driven by the variability in water stored in the soils, supporting an "inverse storage effect" as discussed by Harman (2015).…”

Water ages in the critical zone of long-term experimental sites in northern latitudes

“…Our simulations emphasize the time-variant character of water ages and travel times in hydrological systems, as observed at the catchment scale in various recent studies (van der Velde et al, 2012;Benettin et al, 2013;Heidbüchel et al, 2013;Soulsby et al, 2015;Benettin et al, 2017). Further, the age dynamics in the soil waters were driven by the variability in water stored in the soils, supporting an "inverse storage effect" as discussed by Harman (2015).…”

Chloride Sources and Losses in Two Tile-Drained Agricultural Watersheds

“…The Embarras River watershed was estimated to have 75 to 80% of the land area drained in 1997 (David et al, 1997), and this likely has increased with new patterned systems since that time. The field tile and overall watershed data are similar to results reported by Benettin et al (2013), who stated that chloride in streams comes from both root-zone water with short transport times of weeks to months combined with groundwater chloride with travel times of several years. This would account for the lag times we have observed.…”

Please see SCs for detailed responses by the authors to the comments made by Referee #2.

“…Our simulations emphasize the time-variant character of water ages and travel times in hydrological systems, as observed at the catchment scale in various recent studies (van der Velde et al, 2012;Benettin et al, 2013;Heidbüchel et al, 2013;Soulsby et al, 2015;Benettin et al, 2017). Further, the age dynamics in the soil waters were driven by the variability in water stored in the soils, supporting an "inverse storage effect" as discussed by Harman (2015).…”

Water ages in the critical zone of long-term experimental sites in northern latitudes

“…Our simulations emphasize the time-variant character of water ages and travel times in hydrological systems, as observed at the catchment scale in various recent studies (van der Velde et al, 2012;Benettin et al, 2013;Heidbüchel et al, 2013;Soulsby et al, 2015;Benettin et al, 2017). Further, the age dynamics in the soil waters were driven by the variability in water stored in the soils, supporting an "inverse storage effect" as discussed by Harman (2015).…”

Chloride Sources and Losses in Two Tile-Drained Agricultural Watersheds

“…The Embarras River watershed was estimated to have 75 to 80% of the land area drained in 1997 (David et al, 1997), and this likely has increased with new patterned systems since that time. The field tile and overall watershed data are similar to results reported by Benettin et al (2013), who stated that chloride in streams comes from both root-zone water with short transport times of weeks to months combined with groundwater chloride with travel times of several years. This would account for the lag times we have observed.…”