Quantifying controls on rapid and delayed runoff response in double-peak hydrographs using Ensemble Rainfall-Runoff Analysis (ERRA)

Abstract. Double-peak hydrographs are widely observed in diverse hydrological settings, but their implications for our understanding of runoff generation remain unclear. Previous studies of double-peak hydrographs in the extensively instrumented Weierbach catchment have linked he first peak to event water and the second, delayed and broader peak to pre-event water. Here we use Ensemble Rainfall-Runoff Analysis (ERRA) to quantify how precipitation intensity and antecedent wetness influence groundwater recharge and double-peak runoff generation at the Weierbach catchment (Luxembourg). The spiky first peak can be attributed to a rapid response directly linking precipitation to streamflow via near-surface flowpaths. Relative to this first peak, the second peak is delayed (peaking ~1.5 days after rain falls), lower (~1/3 the height of the first peak), and broader (declining to nearly zero in ~10 days), and can be attributed to a groundwater-mediated pathway that links precipitation, groundwater recharge, and streamflow. The sum of these two runoff responses quantitatively approximates the whole-catchment runoff response. Under wet conditions, the first peak increases nonlinearly (particularly above precipitation intensity of 2 mm h^-1) and the second peak becomes higher, narrower, and earlier with increasing precipitation intensity. Under dry conditions, the first peak increases nonlinearly with precipitation intensity (particularly above 4 mm h^-1), and groundwater recharge also responds to precipitation, but no clear second peak occurs regardless of precipitation intensity. The lack of a second peak under dry conditions plausibly arises from groundwater loss to evapotranspiration and from limited connectivity between groundwater and the stream, rather than from a lack of groundwater recharge. Almost no runoff response occurs at precipitation intensities below ~0.8 mm h^‑1 under wet conditions and ~1.5 mm h^‑1 under dry conditions. After a precipitation-related threshold that initiates the first peak and a catchment wetness threshold that initiates the second peak, higher precipitation intensities amplify the first peak nonlinearly and trigger a larger and quicker second peak.