A Nationally Consistent Approach to Assessing Accumulated Rainfall Rarity

Accumulated rainfall totals are an important variable for a range of hydrological applications, including monitoring and forecasting, and long-term planning. A new report has been published which identifies the most appropriate, nationally consistent approach to quantifying return periods of long duration rainfall.

A comparison of the suitability of nine distribution families for estimating the relative rarity of accumulated rainfall periods across the UK provides opportunities to further improve the accuracy of return period estimation in many areas such as the water resource planning and the Hydrological Summary for the UK, and elsewhere.

Whilst distribution families that are commonly applied in extreme value estimation, such as the generalised extreme value, were demonstrated to be suitable in a lot of cases, overall Pearson Type III outperformed all other assessed distributions. Closer inspection of the performance on accumulation periods of 12 months or less provided further support for the suitability of Pearson Type III, as did the strong performance of Pearson Type III across accumulation periods, start months and regions.

Presentation of return level plots for two potentially appropriate distribution families demonstrated the sensitivity of return period estimates to distribution family, and thus the importance of this question. With this in mind, the approaches presented in Eastman et. al. (2021) provide opportunities to further improve the accuracy of return period estimation and uncertainty quantification.

The full report is now available

Figure 1 Northumbrian 12-month accumulation period beginning in October: return period plots for The Pearson Type III (pe3) and Generalised Extreme Value (GEV) distributions, with associated 95% confidence interval bands, and highlighted return periods corresponding to 48.09, 45.83, and 44.56 mm of rainfall per month. Points correspond to the Weibull plotting positions of the observed rainfall accumulation data.

REF: Eastman, MichaelParry, SimonSvensson, CeciliaHannaford, Jamie. 2021. A nationally consistent approach to assessing accumulated rainfall rarity. Wallingford, UK Centre for Ecology & Hydrology, 42pp.

Historic Droughts: Using the past to inform the future

Lucy Barker (UK Centre for Ecology & Hydrology) presents on a number of cutting edge aspects of drought science.

Climate change projections indicate that extreme events will increase in their frequency and severity in the future.  An improved understanding of the drought events of the past can inform current and future management. In this talk, Lucy demonstrates how reconstructed river flows have enabled consistent, national scale characterisation of historic hydrological droughts and how access to current and historic data can support ongoing drought monitoring activities.

The work originates from a number of projects including Historic Droughts and the About Drought programme and you can view the talk below.

This talk is part of the British Hydrological Society webinar series, “Future Hydrology in a Changing Environment”. You can view past webinars on their YouTube channel.

Interested in the UK water resources situation?

Following the wettest February on record earlier this year, last month is set to be declared the driest May in England for 124 years with some water regions warning of potential drought conditions.

The UK Water Resources Portal allows anyone with an interest in current water resources or drought conditions to explore the data both nationally and locally. The portal makes use of very recently published river flow data from the Environment Agency and rainfall data from the Met Office to show the situation across the UK. Alongside historical data and standardised indices, the Portal allows users to put the current situation into a historical context.

Check out theĀ UK Water Resources Portal

For help using the Portal we have created a YouTube User Guide

The UK Water Resources Portal is one of the outputs from About Drought project. More information and further outputs can be found in the About Drought Handbook.