Impacts of Climate Change on Rainfall and Flooding Events in Great Britain

There is a general perception that climate change has increased frequency of extreme rainfall and flooding events. It is also well-accepted that such changes and impacts have regional variations and not all regions experience similar levels of change. Even directionally, some regions may see reduced severity and frequency of extreme events. There is also a tendency lately to see every significant flooding event through the lens of climate change, and to attribute it to it. While such perception and attribution may be valid, it is quite important and interesting to see what the data shows. When it comes to rainfall or streamflow, there can be numerous aspects that may be important, for example, the duration of interest. For smaller catchments, sub-daily rainfall intensities are important while for larger watersheds three-day or five-day rainfall may be more relevant. Even a longer period, say of 10 to 20 days, may be useful from the perspective of antecedent conditions that are equally important for rainfall events to translate into flooding. When it comes to streamflow, the effects of anthropogenic climate changes get combined with other changes such as in urbanization or in land use land cover. Changes in river flow regime due to construction and operation of dams and reservoirs along river network further confound the climate change signals in the data. This article presents an in-depth climate change study on a multitude of relevant rainfall and streamflow variables for the Great Britain region. It is interesting that trends are statistically significant for 10%, 12%, and 11% rainfall grids for 24-h, three-day, and five-day rainfall, respectively. Trends in frequency of peak-over-threshold are found significant for 12%, 15%, and 13% rainfall grids for 24-h, three-day, and five-day rainfall. Similarly, trends in the annual maximum flows are significant for 27% of the over 1,000 river gauges analyzed. These trends, in rainfall and streamflow, are in fact in both directions, positive as well as negative, exhibiting a weak spatial coherency. Furthermore, in many instances the trends in rainfall and streamflow are in opposing directions, even for the same smaller (local) catchments. It is quite important to understand all these strong, weak, and at times conflicting signals and make useful inferences.