Storm flow and baseflow response to reduced acid deposition: using Bayesian compositional analysis in hydrograph separation with changing end members

In this paper, we present the analysis of long-term (since 1989) hydrochemical data from two small (ca 1 km(2)) catchments in Central Scotland. Both catchments have experienced marked reductions in acid deposition. Time-series analysis of stream water alkalinity, although systematically changing as a result of recovery from acidification, was used to conceptualize how the composition and contribution of different hydrological sources responded over the study period. Nonlinear curve fitting methods allowed the temporal changes in concentration-discharge relationships to be sufficiently well described to assess the impact of reduced acid deposition on storm flow and baseflow hydrochemistry. A Bayesian compositional analysis was applied to facilitate chemically based hydrograph separation. This allowed temporal variation over longer time periods in catchment-scale hydrological source contributions (specifically groundwater) to be estimated. Although these showed no systematic trend, they did differ between the two catchments, most likely as a result of small, but significant differences in the riparian soil cover. Understanding such changes to high and low flows over time is of paramount importance as such flow extremes have the most relevance to applied problems, particularly those related to environmental change. Copyright (C) 2010 John Wiley & Sons, Ltd.