Welsh Journals

SLOPE AND CHANNEL RUNOFF PROCESSES IN UPLAND CATCHMENTS: INTERFACES BETWEEN PfíECIPITATION AND STREAMFLOW ACIDITY MALCOLM D. NEWSON CAMBRIA Newson, Malcolm D. 1986. Slope and channel runoff processes in upland catchments: interfaces between precipitation and streamflow acidity. Cambria, Vol.13 (2), pp 197-212. ISSN 0306-9796. Whilst environmental interest in stream acidification is relatively recent there is a considerable climatological, pedological and hydrological literature which points to controls and processes acting in connection with this phenomenon as it affects the British uplands. A review of this literature is used to help explain acidification phenomena monitored in the Plynlimon catchments, mid-Wales. Forested catchments have a lower mean pH of runoff waters and a steeper fall of pH values during floods, especially those following dry spells. Professor M.D. Newson, Department of Geography, University of Newcastle, Newcastle, NEl 7RU. (Revised April 1985). INTRODUCTION Most upland soils and waters in Britain are naturally acid; so is precipitation. It is difficult, therefore, to detect the influence of the recorded increase in atmospheric sources of acidity since the industrial revolution. The present paper sets down the hydrological processes which translate rainfall to runoff in the uplands, drawing evidence of both runoff processes and stream acidification from the Institute of Hydrology's long-running investigations in the headwaters of the Wye and Severn on Plynlimon, mid-Wales. The output of most dissolved chemicals in upland waters is dominated by atmospheric sources but involves every pathway in the ground phase of the hydrological cycle, even though many pathways have relatively little influence because of low solubility or rapid flows. At the present stage of debate on stream acidification there is considerable advantage in understanding the cascade of processes linking an acid input with a more or less acid output. Until now, water quality analysts have been mainly