Spatial Organisation of Groundwater Dynamics and Runoff Responses in Montane Catchments: Integrating Field Data in a Modelling Framework

Abstract:

We combined multiple tracers with continuous groundwater level monitoring in a modelling framework to understand the spatio-temporal dynamics of influences of groundwater on runoff generation in a 3.2km² catchment in the Scottish Highlands. The montane catchment is underlain by granite and metasediments and has extensive (70%) cover of diverse drift deposits which are up to 40m deep. Flat valley bottom areas fringing the stream channel are characterised by peat soil (0.5-4m deep) which cover about 20% of the catchment and receive drainage from upslope areas. Previous field and modelling work has identified dominant sources of runoff at the catchment-scale and the associated landscape controls on their dynamics and associated transit times. Whilst these studies have emphasised the importance of riparian wetlands as the dominant source of runoff, groundwater discharge provides both an important source of water to these wetlands throughout most of the year, as well as a direct flux into the channel network to sustain the lowest flows. Synoptic hydrogeochemical surveys were carried out on four occasions as flows declined during a 10 year return drought period. Samples were analysed for major anions, cations and water isotopes. Stream chemistry changes showed marked spatial variability implying geochemical differences in the bedrock geology and the distribution of storage in drift deposits. Temporal dynamics inferred heterogeneous montane groundwater bodies contributing to runoff generation differentially during the recession. Modelling confirmed that largest sources of groundwater appear to be located in drifts in the lower catchment where the most marked increase in weathering-derived ions occurred and depleted, non-fractionated isotope signatures implied deeper inflows.