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Investigating young water fractions in a small Mediterranean mountain catchment: both precipitation forcing and sampling frequency matter
  • +3
  • Francesc GallartOrcid,
  • María Valiente,
  • Pilar Llorens,
  • Carles Cayuela,
  • Matthias SprengerOrcid,
  • Jérôme Latron
Francesc Gallart
Orcid
CSIC
Author Profile
María Valiente
University of the Basque Country - Bizkaia Campus
Author Profile
Pilar Llorens
Institute of Environmental Assessment and Water Research. CSIC
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Carles Cayuela
IDAEA
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Matthias Sprenger
Orcid
IDAEA
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Medium
Jérôme Latron
IDAEA-CSIC
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Peer review status:ACCEPTED

07 Nov 2019Submitted to Hydrological Processes
11 Nov 2019Submission Checks Completed
11 Nov 2019Assigned to Editor
11 Nov 2019Reviewer(s) Assigned
26 Nov 2019Review(s) Completed, Editorial Evaluation Pending
30 Nov 2019Editorial Decision: Revise Major
14 Jan 20201st Revision Received
16 Jan 2020Submission Checks Completed
16 Jan 2020Assigned to Editor
16 Jan 2020Reviewer(s) Assigned
11 Mar 2020Review(s) Completed, Editorial Evaluation Pending
20 Mar 2020Editorial Decision: Revise Major
29 Apr 20202nd Revision Received
30 Apr 2020Submission Checks Completed
30 Apr 2020Assigned to Editor
30 Apr 2020Reviewer(s) Assigned
05 May 2020Review(s) Completed, Editorial Evaluation Pending
06 May 2020Editorial Decision: Revise Minor
08 May 20203rd Revision Received
09 May 2020Assigned to Editor
09 May 2020Submission Checks Completed
09 May 2020Reviewer(s) Assigned
11 May 2020Review(s) Completed, Editorial Evaluation Pending
12 May 2020Editorial Decision: Accept

Abstract

The young water fraction (Fyw), the proportion of water younger than 2-3 months, was investigated in soil-, ground- and stream waters in the 0.56 Km2 sub-humid Mediterranean Can Vila catchment. Rain water was sampled at 5-mm rainfall intervals. Mobile soil water and groundwater were sampled fortnightly, using suction lysimeters and two shallow wells, respectively. Stream water was dynamically sampled at variable time intervals (30 minutes to 1 week), depending on flow. A total of 1,529 18O determinations obtained during 58 months were used. The usual hypothesis of rapid evapotranspiration of summer rainfall could not be maintained, leading to discard the use of an “effective precipitation” model. Soil mobile waters had Fyw up to 34%, while in ground and stream were strongly related to water table and discharge variations, respectively. In stream waters, due to the highly skewed flow duration curve, the flow-averaged young water fraction (F*yw) was 22.6%, whereas the time-averaged Fyw was 6.2%. Nevertheless, both F*yw and its exponential discharge sensitivity (Sd) showed relevant changes when different 12-month sampling periods were investigated. The availability of Sd and a detailed flow record allowed us to simulate the young water fraction that would be obtained with a virtual thorough sampling (F**yw). This showed that underestimation of F*yw is associated with missing the sampling of highest discharges and revealed underestimations of F*yw by 25% for the dynamic sampling and 66% for the weekly sampling. These results confirm that the young water fraction and its discharge sensitivity are metrics that depend more on precipitation forcing than on physiographic characteristics, so the comparisons between catchments should be based on mean annual values and inter-annual variability. They also support the dependence of the young water fraction on the sampling rate and show the advantages of flow-weighted F*yw. Water age investigations should be accompanied by the analysis of flow duration curves. In addition, the simulation of F**yw is proposed as a method for checking the adequacy of the sampling rate used.