| 17 Jul 2026
Contrasting drivers of riparian–stream hydrological dynamics during rainfall events across forest headwater ecoregions
Abstract. Examining how hydrological responses of streams and riparian groundwater interact during rainfall events can help understand runoff generation mechanisms and their links to biogeochemistry. We analysed rainfall–runoff events in four forest headwater catchments spanning semiarid Mediterranean, subhumid Mediterranean, temperate, and boreal ecoregions. Using sub-daily data, we evaluated how hydroclimatic drivers shaped stream and riparian groundwater. We additionally focussed on the slope (slopeLin) and hysteresis (h) of the riparian groundwater–stream relationship, describing coupled responses and connectivity patterns. Rainfall amount and antecedent wetness primarily controlled individual stream and riparian groundwater hydrological response across sites, while the vertical activation of riparian layers, defined by slopeLin, and timing, defined by h, were site-specific. At the Mediterranean sites, deep, low-conductivity layers dominated the lateral connectivity unless sufficient rainfall and wetness allowed activation of shallower flow pathways. At the temperate and boreal sites, shallow, conductive layers were typically active, with temporary disconnection during warm periods at the temperate site and snowmelt-induced activation of upslope sources at the boreal. Clockwise hysteresis, indicating faster riparian than stream responses, prevailed at the temperate and boreal sites. The Mediterranean sites showed weaker or anticlockwise patterns, suggesting activation of other water sources, likely ephemeral tributaries during intense rainfall. Our results show that the activation and hysteresis of riparian–stream connectivity differs across ecoregions, which has implications for solute mobilization. This framework provides a physically grounded perspective for interpreting both rainfall–runoff and concentration–discharge relationships, which can help predict shifts in catchment functioning under changing hydroclimatic regimes.
General comments
The authors of this manuscript offer an interesting comparison of hydroclimatic factors, runoff response, and the relationships between discharge and groundwater levels across four forested catchments subject to different climate forcings.
The manuscript is well structured and clearly written, though it needs some clarification and improvement. While I recognize the value of comparative studies in advancing our understanding of riparian-stream connectivity, I think that four catchments are insufficient to draw broader conclusions about hydrological processes across different ecoregions or climatic conditions. For instance, there is no evidence that the catchments are representative or that the selected period reflects their respective climate. The selected period is very short (<2 years) for two catchments (Upper Rappbode and Krycklan C2), and the rainfall-runoff events may have occurred in years that were wetter or drier than the average. Furthermore, we cannot know whether the observed differences in runoff response across the four catchments are due more to lithology and soils than to climatic factors. The only direct comparison is possible between the two Mediterranean study areas, which have similar lithology and soil textures and overlapping periods for data analysis. I therefore suggest revising the sentences that imply the results from these four catchments are representative of their respective ecoregions.
Finally, several methodological details should be included to clarify the data analysis (e.g., hysteresis sketches/plots, plots showing the relationship between discharge and groundwater table, and a time series of the main hydrometeorological variables).
Specific comments