LINKING VARIATIONS IN SHORT-TERM AND MEDIUM-TERM STREAM CHEMISTRY TO RAINFALL INPUTS - SOME OBSERVATIONS AT PLYNLIMON, MID-WALES

Continuously monitored stream water conductivity and pH from the Hafren and Hore streams at Plynlimon, Mid-Wales, is examined. Large inputs of rainwater of high conductivity can be traced to the stream. For events producing low flows (less than about 1 mm h-1), the chemistry of the rainfall is not observed in the stream; for high flow events the rainfall chemistry influences not only the immediate stream response but also the conductivity-flow relationship in subsequent smaller storms. The data indicate that a strong differentiation of water types is present, even after the catchment has been inundated, and that evapotranspiration and dry deposition are important factors in the maintenance of conductivity gradients within the catchment. Comparison of the acidity (pH/Acid Neutralisation Capacity) with the conductivity shows that the acidity in the rainfall is removed by the catchment soils: whereas the rainfall conductivity may affect the stream conductivity directly there is no corresponding relationship for acidity. Rapid chemical reactions, such as ion exchange, control stream acidity but do not affect the conductivity of the stream waters to the same extent.