The influence of climate on the hydrogen-ion budget of upland catchments : a hydrological approach

Clear links are known to exist between the terrestrial characteristics of catchments and the chemistry of their surface waters. During the last decade it has been established that atmospheric pollution, in the form of acidic deposition, can also influence the chemistry of waters draining sensitive upland sites. Furthermore, recent studies have suggested that reductions in these anion loads can have marked consequences for the surface-water quality of acidified catchments. However, many of these field experiments and model estimates have neglected other, potentially important realms of atmospheric influence. This thesis proposes that climate change over periods of up to one century can alter the hydrogen-ion budget of a catchment in two ways. First, variations in the relative frequency of large-scale synoptic features may significantly modify catchment chemical budgets by changing existing spatial and temporal patterns of acidic deposition. Secondly, as each major class of weather type is characterised by distinct precipitation and temperature regimes, the seasonal magnitude and frequency of acidic episodes may also be affected by long-term adjustments to the catchment water-balance. A hydtological perspective was employed in order to investigate these potential hydrochemical relationships. This involved hydrological modelling and hydrogen-ion budgeting, statistical analyses of climatic trends, the application of weather classification schemes, and the generation of synthetic input data from observed and historic meteorological data. These elements were combined by the development of a robust and comprehensive computer package (the Shifting Climate and Catchment Acidification Model, or SeAM) which enables the manipulation of a wide range of atmospheric and catchment properties. The model was calibrated and validated against data obtained from the Beacon experimental catchment in the East Midlands and then transferred to three contrasting watersheds in the Llyn Brianne region, Mid Wales. Using multiple climate and pollution scenarios, modelling experiments revealed that variations in the predominance of three key weather types modified the mean annual wet-deposited acid load by ±20% and the mean surface-water acidities by up to ±15%. Under the most extreme scenario the frequency of daily flows of less than pH 4.5 was increased by +90%. Whilst recognising the simplicity of the hydrologic ally-driven soil model, a feature common to all of the catchments was the exaggeration of existing seasonal discharge and acidity regimes. The sensitivity of individual catchments to a given climate change was found to be highly variable, depending upon the complex interaction of hydrology and catchment characteristics. It was concluded, therefore, that the effect of reduced emissions of acidifying substances on catchment recovery can be modified by climate change. Furthermore, acid-sensitive species occupying marginal sites, could be affected by the changes envisaged under the proposed BASE scenario. This underlines the need for a definition of the term 'critical load' that embraces desirable ecological responses with the required acidic deposition rates, for a given climatic context.