Abstract

The chemical signatures of streams draining coniferous forests in upland Wales, are characterized by high concentrations of 'acidic solutes' such as hydrogen, aluminium, sulphate and nitrate. Attempts to explain such high loadings, must include a full-characterization of the ionic concentrations and water velocities (flux) within those soil horizons through which precipitation must first travel before reaching a stream.

A fourty metre coniferous hillslope-profile at Plynlimon, mid-Wales was instrumented to monitor hydrologic and hydrochemical parameters. Techniques included: neutron moderation, automatic tensiometry, ring permeametry, soil water sampling and automatic streamflow sampling. A total of ten major storm events were investigated.

Two simple analytical techniques based upon (1) Hubbert's Tangent Law and (2) Darcy's and Jaeger's Equation applied to saturated zones, produced similar results. Both techniques indicated the importance of near-surface flow at the O/A and E horizons of the podzolic soils, to the generation of streamflow.

The relationship between the ionic concentrations within each horizon and those within the streamside (or riparian) zones, appear more complex, although the relatively low concentrations of aluminium, and high concentrations of chloride, sodium, and hydrogen within the riparian zones, would again suggest a near-surface source.

The complete hydrologic and hydrochemical datasets are presently being analysed with more complex mathematical models based upon the full Richards and Advection-Dispersion Equations. Preliminary results would suggest that the models are able to simulate streamflow response and solute load, by water and solute movement within the near-surface horizons.