Sources of baseflow in larger catchments – Using tracers to develop a holistic understanding of runoff generation

SummaryDespite increasing awareness about the importance of low flows and droughts, spatial and temporal variations in the quantity and quality of baseflow generated in larger catchments are poorly constrained. Here, tracers are used in conjunction with hydrometric analysis to examine the sources of baseflow in the 1849 km2 catchment of the River Dee in Scotland. Geochemical tracers showed that the chemical composition of baseflow evolved downstream, but mainly reflected the dominance of sources in montane tributaries in the upper catchment. These findings were corroborated by hydrometric analysis which showed that the upper 54% of the catchment contributed 71% of baseflow in the lower river. Baseflows in smaller (<10 km2) headwater sub-catchments exhibited highly variable hydrochemical characteristics which were averaged at larger scales. Analysis of δ18O implies that well-mixed groundwater sources with similar isotopic signatures dominated baseflow generation throughout most of the catchment, probably indicating that groundwater in the lower slopes of montane headwaters provide a major source of baseflow. Periods of baseflow were found to be dynamic; despite the river system being in overall recession, the channel network exhibited contrasting responses to relatively small (<10 mm) isolated rainfall events in different parts of the catchment. Moreover, diurnal variation in flows in sub-catchments with a high proportion of coverage by peat soils is apparent, which may reflect the daily variation of evapotranspiration as a control on baseflow. Although a relatively stable period in the hydrological year, baseflows in larger catchments result from a complex suite of distributed hydrological processes. Further research is needed to understand how baseflows sustain water supplies and aquatic ecosystems, if managers are to be able to protect these catchment services from environmental change.