Is groundwater response timing in a pre-alpine catchment controlled more by topography or by rainfall?

Groundwater levels in steep headwater catchments typically respond quickly to rainfall but the timing of the response may vary spatially across the catchment. In this study, we investigated the topographic controls and the effect of rainfall and antecedent conditions on the groundwater response timing for 51 groundwater monitoring sites in a 20 ha pre- alpine catchment with low permeability soils. The median time to rise and median duration of recession for the 133 rainfall events were highly correlated to the topographic characteristics of the site and its upslope contributing area. The median time to rise depended more on the topographic characteristics than on the rainfall characteristics or antecedent soil wetness conditions. The median time to rise decreased with Topographic Wetness Index (TWI) for sites with TWI < 6 and was almost constant for sites with a higher TWI. The slope of this relation was a function of rainfall intensity. The rainfall threshold for groundwater initiation was also a function of TWI and allowed extrapolation of point measurements to the catchment scale. The median lag time between the rainfall centroid and the groundwater peak was 75 minutes. The groundwater level peaked before peak streamflow at the catchment outlet for half of the groundwater monitoring sites, but only by 15 to 25 minutes. The stronger correlations between topographic indices and groundwater response timing in this study compared to previous studies suggest that surface topography affects the groundwater response timing in catchments with low permeability soils more than in catchments with more transmissive soils