Soil moisture is a key control on runoff generation and biogeochemical processes on hillslopes. Precipitation events can evoke different soil moisture responses with depth through the soil profile, and responses can differ among landscape positions along a hillslope. We sought to elucidate the nature of these responses by estimating changes in water content, response time between peak precipitation and peak soil moisture, and wetting front velocities for 43 storms at 45 locations on three adjacent hillslopes within a headwater catchment of the southern Appalachian Mountains (NC, USA). We used a multivariate modeling approach to quantify the relative influences and the predictability of soil moisture responses by a combination of landscape and storm characteristics. We quantified the lag correlations between hillslope mean soil moisture and catchment runoff to demonstrate how storm properties and hillslope‐scale characteristics may influence runoff at the catchment outlet. Soil moisture responses varied widely, and no consistent patterns were observed among response metrics laterally or vertically along hillslopes. In contrast to other studies, we found that the relative influence of hillslope properties and storm characteristics varied with soil moisture responses and during storms. Antecedent conditions and storm depths influenced the strength of lag correlations between soil moisture and runoff, whereas storm mean intensity was correlated with the lag times. These results highlight the utility of intensive observations for characterizing heterogeneity in soil moisture responses, suggesting, among other things, a need for better representation of the subsurface processes in rainfall‐runoff models. Identifying the relative importance of drivers can be beneficial in building parsimonious hydrological models.

No abstract

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.