Long-term trends in hydro-climatology of a major Scottish mountain river

The River Dee, in North East Scotland, is a mountainous river strongly influenced by patterns of snow accumulation and melt from the Cairngorm Mountains. Analysis of this river's flow record from 1929-2004, the longest in Scotland, supports anecdotal evidence that river extreme flows are increasing. There was no detectable change in the overall annual flow patterns. However, an analysis of seasonal data suggested a shift towards increased flows in spring (March-May) and decreased flows in summer (June-August) over the 75 years of the record. Flows in spring exceeded 29 m(3) s(-1) for 50% of the time over the earliest part of the record (1930 to 1954), whereas in the last 25 years of the record (1979 to 2004) 50% of the flows exceeded 35 m(3) s(-1). Precipitation is increasing in the spring and decreasing in July and August. If these trends continue they have important implications for water management in the Dee, with a potential increase in flood risk in spring and the increased possibility of drought in summer. Combined with this increase in flows the river appears to be more responsive to precipitation events in the catchment. In large heterogeneous catchments with a marginal alpine/high latitude climate it is difficult to assess the amount of precipitation falling as snow and its relative accumulation and ablation dynamics on daily to seasonal time scales. Changes in the temporal pattern of coherence between flow and precipitation are thought to be linked to changing snow patterns in the upland part of the catchment. A decreased amount of precipitation occurring as snow has led to higher coherence. We also show that in responsive systems it is important to record river flows at an hourly rather than daily time step in order to characterise peak flow events.