Assessing runoff generation in riparian wetlands: monitoring groundwater–surface water dynamics at the micro-catchment scale

Riparian wetlands (RW) are important variable source areas for runoff generation. They are usually characterised by a combination of groundwater exfiltration—which maintains saturated conditions in low-lying organic-rich soils—and direct precipitation. Both processes interact to generate overland flow as a dominant runoff process. The small-scale details of groundwater–surface water (GW–SW) interactions are usually not well understood in RW. Here, we report the results of a study from an experimental catchment in the Scottish Highlands where spatio-temporal runoff processes in RW were investigated using isotopes, alkalinity and hydrometric measurements. We focused on perennial micro-catchments within the RW and ephemeral zero-order channels draining peatland hollows and hummocks to better understand the heterogeneity in GW–SW interactions. The 12-month study period was dominated by the wettest winter (December/January) period on record. Runoff generation in the RW was strongly controlled by the local groundwater response to direct rainfall, but also the exfiltration of groundwater from upslope. This groundwater drainage is focused in the hollows in ephemeral and perennial drainage channels, but in wet conditions, as exfiltration rates increase, can affect hummocks as well. The hollows provide the dominant areas for mixing groundwater, soil water and direct rainfall to deliver water to the stream network as hollows “fill and spill” to increase connectivity. They also provide wet areas for evaporation which is evident in enriched isotope signatures in summer. Although there is some degree of heterogeneity in the extent to which groundwater influences specific micro-catchments, particularly under low flows, the overall isotopic response is quite similar, especially when the catchment is wet and this responses can explain the isotope signatures observed in the stream. In the future, more longitudinal studies of micro-catchments are needed to better explain the heterogeneity observed.