Influence of forestry, environmental change and climatic variability on the hydrology, hydrochemistry and residence times of upland catchments

Long-term (>25 year) hydrological and hydrochemical data sets for two small afforested catchments at Loch Ard, Central Scotland, are analysed to assess the effects of forest management, environmental change and climatic variability. The two catchments, B10 (0.9 km(2)) and B11 (1.4 km(2)) are dominated by acidic gley soils. Their streams have responsive hydrological regimes and acidic surface waters with pH ranging from similar to 5.5 at low flows to 4.0 at high flows. Partial clear-felling appeared to have limited effects on the catchment water balances, though flood peaks and low flows might be enhanced for short periods of a few years. Felling had a notable, short-term, perturbation on stream water chemistry, with both NO3 and K concentrations being increased. Stream chemistry also exhibited marked variability in response to climatic conditions and changing atmospheric inputs, which resulted in strong inter-annual and seasonal variation of precipitation quantity and quality. These influence annual stream flows and chemistry, with marine-derived Cl and Na being the dominant anion and cation. Emission controls of acidic oxides from fossil fuel burning have reduced SO4 concentrations in precipitation; a trend that has been exacerbated by felling and mirrored in the two streams. This has corresponded to declining levels of inorganic aluminium. In contrast, dissolved organic carbon concentrations have increased possibly in response to warmer drier summers and felling operations. Chemically-based hydrograph separations indicated that groundwater comprised 25-35% of annual runoff in both catchments, with no evidence of any change relating to forest management. Cl data were used to estimate mean catchment residence times by applying lumped parameter models. Both catchments have mean residence times on the order of a few months, though they are about 1 month shorter in B10 which is consistent with differences in soils and topography, though this might also reflect differences in forest management.