14 Jun 2022
14 Jun 2022
Status: this preprint is currently under review for the journal HESS.

Frequency domain water table fluctuations reveal recharge in fractured aquifers depends on both intense and seasonal rainfall and unsaturated zone thickness

Luca Guillaumot1, Laurent Longuevergne1, Jean Marçais2, Nicolas Lavenant1, and Olivier Bour1 Luca Guillaumot et al.
  • 1Univ Rennes, CNRS, Geosciences Rennes - UMR 6118, F-35000 Rennes, France
  • 2INRAE, UR Riverly, F-69625 Villeurbanne, France

Abstract. Groundwater recharge is difficult to estimate, especially in fractured aquifers, because of the spatial variability of the soil properties and because of the lack of data at basin scale. A relevant method, known as the WTF method, consists in inferring recharge directly from the water table fluctuations (WTF) observed in boreholes. However, the WTF method neglects the impact of lateral groundwater redistribution in the aquifer, i.e. assumes that all the WTF are attributable to recharge. In this study, we developed the WTF approach in the frequency domain to better consider groundwater lateral flow, which quickly redistributes the inpulse of recharge and mitigates the link between WTF and recharge. First, we calibrated a 1D analytical groundwater model to estimate hydrodynamic parameters at each borehole. These parameters were defined from the WTF recorded for several years, independently of prescribed potential recharge. Second, calibrated models are reversed analytically in the frequency domain to estimate recharge fluctuations (RF) at weekly to monthly scales from the observed WTF. Models were tested on two twin sites with similar climate, fractured aquifer, and land use but different hydrogeologic settings: one has been operated as a pumping site for the last 25 years (Ploemeur, France) while the second has not been perturbed by pumping (Guidel). Results confirm the important role of rainfall temporal distribution to generate recharge. While all rainfall contribute to recharge, the ratio of recharge to rainfall minus potential evapotranspiration is frequency dependent, varying between 20–30 % at periods <10 days, 30–50 % at monthly scale, and reaching 75 % at seasonal time scales. We further show that the unsaturated zone thickness controls the intensity and timing of RF. Overall, this approach contributes to better assess recharge and enable to improve the representation of groundwater systems within hydrological models. In spite of the heterogeneous nature of aquifers, parameters controlling WTF can be inferred from WTF time series.

Luca Guillaumot et al.

Status: open (until 30 Sep 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2022-201', Ty P. A. Ferre, 04 Aug 2022 reply
    • AC1: 'Reply on RC1', Luca Guillaumot, 27 Sep 2022 reply
      • RC3: 'Reply on AC1', Ty P. A. Ferre, 27 Sep 2022 reply
  • RC2: 'Comment on hess-2022-201', Anonymous Referee #2, 26 Aug 2022 reply
    • AC2: 'Reply on RC2', Luca Guillaumot, 27 Sep 2022 reply

Luca Guillaumot et al.

Luca Guillaumot et al.


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Short summary
Recharge, defining the renewal rate of groundwater resources, is difficult to estimate at basin scale. Here, recharge variations are inferred from water table variations recorded in boreholes. First, results show that aquifer-scale properties controlling these variations can be inferred from local boreholes. Second, groundwater is recharged by both intense and seasonal rainfall. The short-term contribution appears overestimated in recharge models and depends on the unsaturated zone thickness.