Topographic effects on flow path and surface water chemistry of the Llyn Brianne catchments in Wales

Topographic shape is a watershed attribute thought to influence the flow path followed by water as it traverses a catchment. Flow path, in turn, may affect the chemical composition of surface waters. Topography is quantified in the hydrological model TOPMODEL as the relative frequency distribution of the index ln(atanB), where a is the upslope area per unit contour that drains past a point and tanB is the local surface slope. Spatial distributions of ln(atanB) were calculated for eight catchments in Wales on a 25 m x 25 m grid. Among the catchments, mean observed stream H+ concentration during high flow periods was highly correlated with the mean of the ln(atanB) distribution. The steady-state gain of a transfer function (time series) model relating H+ to discharge was positively correlated with the mean of the ln(atanB) distribution. These results suggest that during high flow periods, both the average stream acidity and the magnitude of fluctuations in H+ are conditioned by the topographic shape of the catchment. By performing a sensitivity analysis on TOPMODEL, we also show that as the mean of the ln(atanB) distribution for a catchment increases, so does its theoretical likelihood to produce significant quantities of surface and near-surface runoff. Our observed results in the Llyn Brianne catchments are consistent with this theoretical expectation in that surface or near-surface runoff is often higher in acidity than are deeper sources of hillslope runoff.