Official URL: https://nora.nerc.ac.uk/id/eprint/529618/

Accumulated rainfall totals are an important variable for a range of hydrological applications, including monitoring and forecasting, and long-term planning. For more than three decades, the National Hydrological Monitoring Programme (https://nrfa.ceh.ac.uk/nhmp) at UKCEH has produced the monthly Hydrological Summary for the UK. A key component of this publication, a strategically important output with a wide readership, is a table of rainfall accumulations over a range of durations to present, and an assessment of the rarity of wet and dry periods in the context of rainfall data back to 1910. However, there is a need to update the methodology used to derive rainfall return periods, and to assess the rainfall frequencies more accurately, particularly from a low rainfall perspective. The aim of this report is to identify the most appropriate approach to quantifying return periods of long duration rainfall. The Data and Methods sections describe the data used to test the range of distributions and goodness-of-fit measures. Results are presented which identify the most appropriate distribution for nationally consistent applications, before the key findings are summarised