| Results for long term water quality monitoring are described for the headwaters
of the principal headwater stream of the River Severn, the Afon Hafren. The results are
linked to within-catchment information to describe the influence of conifer harvesting on
stream and shallow groundwater quality. A 19-year record of water quality data for the
Hafren (a partially spruce forested catchment with podzolic soil) shows the classic
patterns of hydrochemical change in relation to concentration and flow responses for upland
forested systems. Progressive felling of almost two-thirds of the forest over the period of
study resulted in little impact from harvesting and replanting in relation to stream water
quality. However, at the local scale, a six years’ study of felling indicated significant
release of nitrate into both surface and groundwater; this persisted for two or three years
before declining. The study has shown two important features. Firstly, phased felling has
led to minimal impacts on stream water. This contrasts with the results of an experimental
clear fell for the adjacent catchment of the Afon Hore where a distinct water quality
deterioration was observed for a few years. Secondly, there are localised zones with
varying hydrology that link to groundwater sources with fracture flow properties. This
variability makes extrapolation to the catchment scale difficult without very extensive
monitoring. The implications of these findings are discussed in relation to strong support
for the use of phased felling-based management of catchments and the complexities of within
catchment processes.
Keywords: deforestation, water quality, acidification, pH, nitrate, alkalinity, ANC,
aluminium, dissolved organic carbon, Plynlimon, forest, spruce, Afon Hafren, podzol |