The Relative Influence of Storm and Landscape Characteristics on Shallow Groundwater Responses in Forested Headwater Catchments

Shallow groundwater responses to rainfall in forested headwaters can be highly variable, but their relative strengths of influences remain poorly understood. We investigated the roles of storms and landscape characteristics on short-term, shallow groundwater responses to rainfall in forested headwater catchments. We used field observations of shallow groundwater combined with random forest modeling to identify the factors that affect shallow groundwater responses and the relative influences of key response drivers. We found that the rainfall thresholds required for groundwater responses were only met by the largest quartile of events, suggesting that most events contributed to unsaturated soil storage or were lost to evaporation. Significantly higher rainfall thresholds and longer response times for south facing catchments as opposed to north facing catchments highlighted the role of insolation in setting antecedent conditions that influenced the groundwater response. During storms, there were significantly larger increases in water table height in catchments dominated by coniferous forests compared to deciduous forests, indicating that local spatial characteristics of hillslopes could be more important factors for groundwater response than catchment wetness. The random-forest analysis revealed that total rainfall amount had the greatest influence on most groundwater responses, but the relative influence of topography and local antecedent wetness was more pronounced as events progressed, indicating a shift in hydrological processes during different stages of the groundwater response. These results have implications for our understanding of runoff generation processes, including processes that determine hydrologic connectivity between stream and hillslopes.