Background
The authors write that "recent flood events have led to speculation that climate change is influencing the high-flow regimes of UK catchments, and projections suggest that flooding may increase in [the] future as a result of human-induced warming."

What was done
Utilizing the UK "benchmark network" of 87 "near-natural catchments" identified by Bradford and Marsh (2003), Hannaford and Marsh conducted "a UK-wide appraisal of trends in high-flow regimes unaffected by human disturbances."

What was learned
The two researchers report that "significant positive trends were observed in all high-flow indicators ... over the 30-40 years prior to 2003, primarily in the maritime-influenced, upland catchments in the north and west of the UK." However, they say "there is little compelling evidence for high-flow trends in lowland areas in the south and east." They also note that "in western areas, high-flow indicators are correlated with the North Atlantic Oscillation Index (NAOI)," so that "recent trends may therefore reflect an influence of multi-decadal variability related to the NAOI." In addition, they state that longer river flow records from five additional catchments they studied "provide little compelling evidence for long-term (>50 year) trends but show evidence of pronounced multi-decadal fluctuations." Lastly, they add that "in comparison with other indicators, there were fewer trends in flood magnitude," and that "trends in peaks-over-threshold frequency and extended-duration maxima at a gauging station were not necessarily associated with increasing annual maximum instantaneous flow."

What it means
In light of their several observations, Hannaford and Marsh conclude that "considerable caution should be exercised in extrapolating from any future increases in runoff or high-flow frequency to an increasing vulnerability to extreme flood events," which word to the wise is something the world's policy makers would do well to carefully consider, especially those residing within the UK.

Reference
Bradford, R.B. and Marsh, T.M. 2003. Defining a network of benchmark catchments for the UK. Proceedings of the Institution of Civil Engineers, Water and Maritime Engineering 156: 109-116.