Sensitivity of groundwater levels to low-frequency climate variability in a large watershed

Groundwater level (GWL) can vary over a wide range of timescales. Previous studies highlighted that low-frequency variability (interannual (2-8 years) to decadal (>10 years)) originating from large-scale climate variability, represents a significant part of GWL variance. It remains an open question, however, how GWL, including extremes, may respond to changes in large-scale climate forcing, affecting precipitation variability. Focusing on the Seine River basin, this study therefore aims to assess how GWLs respond to changes in interannual to decadal climate variability. We implemented an empirical numerical approach, which enables an assessment of the GWL sensitivity to changes in precipitation variability over a range of timescales (up to decadal), using the Seine hydrosystem as a case study. The approach consists in: i) identifying and modifying the spectral content of precipitation in the low-frequency range; ii) using these perturbed precipitation fields as input in the physically-based hydrological/hydrogeological CaWaQS model for the Seine River basin to simulate the corresponding GWL response; iii) comparing the spectral content, mean, variance and extremes of perturbed GWLs with reference (i.e. unperturbed) GWLs. Two interannual (2-4 yr and 5-8 yr) and one decadal (15 yr) timescales were modified individually by either increasing or decreasing their amplitude by 50 %. This led to six scenarios of perturbed low-frequency precipitation variability, which were subsequently used as CaWaQS inputs to assess the GWL response. Results indicated increased (decreased) GWL up to 5 m when low-frequency precipitation variability increased (decreased) by 50 %. This led to an increased occurrence of groundwater floods (droughts) with increased severity and decreased occurrence of groundwater droughts (floods) with decreased severity, respectively. These results indicate: i) how using biased climate data, in terms of low-frequency variability, leads to large deviations in the GWL simulation, ii) to what extent potential changes in low-frequency climate variability may affect future GWL, and particularly drought and flood occurrence and severity.