The effect of record length on the analysis of river flow trends in Wales and central England

The detection of trends in hydrological variables is directly affected by the length of time series available for analysis and is of key importance to the global FRIEND community. Long-term (<= 50 years) seasonal gauging station records from Wales and the English Midlands are analysed for different time spans up to 2001 to demonstrate the effect of record length on linear trend analysis. Non-parametric Mann-Kendall trend methods are applied to time series of different flow quantiles, and for different seasons, thereby assessing trends across the streamflow spectrum. Key differences are quantified between trends of varying record lengths, but the dominant trend is for streamflow increases at high and low flows over time periods greater than 30 years. The implications for the FRIEND community are widespread and support the need to maintain gauging station networks and maximise instrumental records.