A characterisation of climate variability and trends in hydrological extremes in the Severn Uplands

Hydrological and climatic characteristics were investigated within the Severn Uplands to determine the interconnection between variables with a focus on understanding changes in hydrological extremes. Time series of precipitation and flows over a 30-year period were analysed for trends in extremes. The Mann-Kendall (MK) non-parametric test was used to detect trends at six rain gauges and five flow gauges across the catchment. Maxima, percentiles and N-day maxima of precipitation and flow distributions were investigated to identify changes in extreme intensity, frequency and persistence, respectively. Results indicated some significant statistical trends at annual, seasonal, and monthly scales. The largest number of significant trends in extreme precipitation was identified at mountainous gauge locations where significant increases in extreme precipitation persistence were detected annually and during spring and winter. Additionally, significant increases in extreme precipitation frequency were observed annually and during autumn and winter. Trends in flow identified an increase in extreme intensity on an annual basis, as well as during winter and the month of July, and a decrease in extreme frequency at certain stations during spring. Changes in climate variables were investigated to identify causal relationships with precipitation and flow hydrological extremes. Extremes were found to coincide with certain weather types, generally characteristic of cyclonic conditions and/or orographic enhancement. These were largely correlated with strong positive and negative stages of the North Atlantic oscillation, particularly during winter. Snow cover records indicated rapid decline in the Severn Uplands and a reduction in snow indicated a link to changes in spring hydrological extremes. All changes seemed to be driven by the underlying climate variable of air temperature, which coincided with changes in observed national and global temperature records. Hydroclimatological changes indicated a possible temporal shift in the occurrence of extreme hydrological events, which was likely to have been affected by changes in catchment physical properties. Copyright (C) 2010 Royal Meteorological Society