Scale dependence of the episodic hydrochemical response of nested catchments at Plynlimon

摘要

Previous monitoring of the surface water chemistryof the forested catchments at Plynlimon has concentrated on weekly measurements. Hence, peak values and detailsof the rapid changes in chemistry occurring during events tend to be missed, particularly forsmaller catchments. Knowledge of the detailed hydrochemical response is necessary to predicteffects on streamecology and to quantify processes occurring within the catchment. Thisinformation is vital if the likely impacts of changes in landuse and pollutant input on episodichydrochemical variations are to be modelled.A summary of daily and three-hourly chemistry forthe River Severn headwaters at Plynlimon, central Wales is presented. Data are from the summer and autumn of1995 and the winter and summer of 1996. The effects of antecedent conditions, soiltype, landuseand groundwater and tributary inflow on stream episodic hydrochemical response are discussedfor a range of spatial scales, including first, second and third order streams. Detailed informationon the chemical changes due to a discrete groundwater inflow into the bed of one streamshows that, during episodes, its influence on stream chemistry is overridden by that ofevent water. A comparison of drainage ditch chemistry with main stream chemistry during stormevents is used to consider the importance of tributary inflow at these times. The contrast inresults between sites is assessed, to evaluate the effects of clearfelling and soil type onstream event chemistry.