| Upland streamflow generation has traditionally been modelled as a simple
rainfall-runoff mechanism. However, recent hydrochemical studies conducted in upland Wales
have highlighted the potentially important role of bedrock groundwater in streamflow
generation processes. To investigate these processes, a detailed and novel field study
was established in the riparian zone and lower hillslopes of the Hafren catchment at
Plynlimon, mid-Wales. Results from this study showed groundwater near the river behaving
in a complex and most likely confined manner within depth-specific horizons. Rapid
responses to rainfall in all boreholes at the study site indicated rapid recharge pathways
further upslope. The different flow pathways and travel times influenced the chemical
character of groundwaters with depth. Groundwaters were shown to discharge into the stream
from the fractured bedrock. A lateral rapid flow horizon was also identified as a fast
flow pathway immediately below the soils. This highlighted a mechanism whereby rising
groundwater may pick up chemical constituents from the lower soils and transfer them
quickly to the stream channel. Restrictions in this horizon resulted in groundwater
upwelling into the soils at some locations indicating soil water to be sourced from both
rising groundwater and rainfall. The role of bedrock groundwater in upland streamflow
generation is far more complicated than previously considered, particularly with respect
to residence times and flow pathways. Hence, water quality models in upland catchments
that do not take account of the bedrock geology and the groundwater interactions therein
will be seriously flawed.
Keywords: bedrock, groundwater, Hafren, hillslope hydrology, Plynlimon, recharge, soil
water, streamflow generation |