Water temperature of Plynlimon streams

Water temperature data were collected from five stations in the upper Severn system. Temperatures were compared between a stream with a 335 ha catchment after it had flowed for c. 1.5 km through clear felled land and a stream with a 347 ha catchment after it had flowed for c. 2.5 km through coniferous forest. The results suggest that the effect of forest cover was to lower the annual mean water temperature by c. 0.4 degrees C, mainly in summer and through depression of both daily maxima and daily minima, though mainly the former. There was no clear evidence of temperature elevation in the afforested stream in winter. It is important to note that these conclusions depend on several assumptions that cannot be substantiated objectively. There is some evidence that water temperatures in some parts of the upper Severn system may be influenced by groundwater inputs.