Nonlinear quickflow response as indicators of runoff generation mechanisms
Linking quickflow response to subsurface state can improve our understanding of runoff processes that drive emergent catchment behavior. We investigated the formation of nonlinear quickflows in three forested headwater catchments and also explored unsaturated and saturated storage dynamics, and likely runoff generation mechanisms that contributed to their formation. Our analyses focused on two reference watersheds at the Coweeta Hydrologic Laboratory (CHL) in western North Carolina, USA and one reference watershed at the Susquehanna Shale Hills Critical Zone Observatory (SHW) in Central Pennsylvania, USA with available hourly soil moisture, groundwater, streamflow, and precipitation time series over several years. Our study objectives were to characterize 1) nonlinear runoff response as a function of storm characteristics and antecedent conditions, 2) the critical levels of shallow unsaturated and saturated storage that lead to hourly flow response, and 3) runoff mechanisms contributing to rapidly increasing quickflow using measurements of soil moisture and groundwater. We found that maximum hourly rainfall did not significantly contribute to quickflow production in our sites, in contrast to prior studies, due to highly conductive forest soils. Soil moisture and groundwater dynamics measured in hydrologically representative areas of the hillslope showed that variable subsurface states could contribute to non‐linear runoff behavior. Quickflow generation in watersheds at CHL were dominated by both saturated and unsaturated pathways, but the relative contributions of each pathway varied between catchments. In contrast, quickflow was almost entirely related to groundwater fluctuations at SHW. We showed that co‐located measurements of soil moisture and groundwater supplement threshold analyses providing stronger prediction and understanding of quickflow generation, and indicate dominant runoff processes.