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

Abstract

Abstract. Previous monitoring of the surface water chemistry of the forested catchments at Plynlimon has concentrated on weekly measurements. Hence, peak values and details of the rapid changes in chemistry occurring during events tend to be missed, particularly for smaller catchments. Knowledge of the detailed hydrochemical response is necessary to predict effects on stream ecology and to quantify processes occurring within the catchment. This information is vital if the likely impacts of changes in landuse and pollutant input on episodic hydrochemical variations are to be modelled. A summary of daily and three-hourly chemistry for the River Severn headwaters at Plynlimon, central Wales is presented. Data are from the summer and autumn of 1995 and the winter and summer of 1996. The effects of antecedent conditions, soiltype, landuse and groundwater and tributary inflow on stream episodic hydrochemical response are discussed for a range of spatial scales, including first, second and third order streams. Detailed information on the chemical changes due to a discrete groundwater inflow into the bed of one stream shows that, during episodes, its influence on stream chemistry is overridden by that of event water. A comparison of drainage ditch chemistry with main stream chemistry during storm events is used to consider the importance of tributary inflow at these times. The contrast in results between sites is assessed, to evaluate the effects of clearfelling and soil type on stream event chemistry.

The water quality functioning of the upper River Severn, Plynlimon, mid-Wales: issues of monitoring, process understanding and forestry

“…Hence, to measure the dynamics of the Plynlimon streams, as elsewhere, there has been the need to monitor through events Foster et al, 1997Foster et al, , 2001). However, one of the most surprising aspects of how the water quality changes over short time intervals in the Plynlimon streams came about with an examination of the long-term weekly rainfall and runoff record for chloride, augmented with over three years of daily data at Plynlimon.…”

A hydrochemical modelling framework for combined assessment of spatial and temporal variability in stream chemistry: application to Plynlimon, Wales

“…The high flow end-member may, therefore, be considered to represent an integrated measure of soil water chemistry (Foster et al, 1997) and the same may be considered for groundwater. If changes occur along flow pathways, then the end-member may be considered as representing the water chemistry as the end of the pathway rather than at the beginning.…”

“…Weekly stream chemistry has been monitored at 10 sites on first to third order streams for up to 15 years by the Centre for Ecology and Hydrology (CEH) (Reynolds et al, 1989;Neal et al, 1997b) and continuous flow and pH data are available for the larger streams (Robson, 1993;Hodgson and Evans, 1997). In addition, daily and threehourly episodic storm sampling has been undertaken at 6 sites and one-off low flow samples during the summer of 1996 at 40 sites by Imperial College and CEH working in partnership (Locks, 1996;Foster et al, 1997Foster et al, , 2001. …”

“…It is also scale-, soil-and groundwater dependent (Chapman et al, 1993(Chapman et al, , 1997Neal et al, 1997a. On examination of the temporal variability of chemistry and effects of scale within the Severn catchments, Foster et al (1997) found that first order tributaries, and also the main streams themselves during high flows, may be considered as representing an 'integrated' measure of soil water chemistry. At high flows, this acidic soil-water is the main component of flow and, at other times, there is a more alkaline water source, probably a deeper subsurface component of stream-flow (Hill and Neal, 1997;Neal et al, 1997a).…”

“…Variability of soil type, bedrock mineralisation and landuse across the Plynlimon catchments results in different temporal responses being observed at each monitored site (Reynolds et al, 1986(Reynolds et al, , 1988Foster et al, 1997). This implies that although discharge is the dominant control on hydrochemical variation at a given site, the variability of catchment characteristics between sites has to be considered when modelling the whole area.…”