This index is not threshold based. Instead, it is calculated with 12-month rainfall deficits provided as a percentage of the mean annual climatological total rainfall (1981–2000) for that location. It is therefore a measure of drought severity, not frequency, and higher values indicate more severe drought.

12-month accumulations have been selected as this is likely to indicate hydrological drought - water scarcity over a much longer period of time which heavily deplete water resources on a large scale (as opposed to meterological or agricultural drought, which generally occur on shorter timescales of 3-12 months). However this categorisation is not fixed, because rainfall deficits accumulated over 12-months could lead to different types of drought and drought impacts, depending on the level of vulnerability to reduced rainfall in a region.

This data contains a field for each warming level. They are named 'DSI12' (Drought Severity Index for 12 month accumulations), the warming level, and 'upper' 'median' or 'lower' as per the description below. E.g. 'DSI12 2.5 median' is the median value for the 2.5°C projection. Decimal points are included in field aliases but not field names e.g. 'DSI12 2.5 median' is 'DSI12_25_median'. 

Data defaults to displaying 'DSI12 2.0 median' values, use 'change style' to display other values.

The warming levels used are 1.5°C, 2.0°C, 2.5°C, 3.0°C, 4.0°C, and two baselines are also provided for 1981-2000 (corresponding to 0.51°C warming) and 2000-2017 (corresponding to 0.835°C warming).

What is the data?

The data is from the UKCP18 regional projections using the RCP8.5 scenario. Rather than giving projections for a specific date under different scenarios, one scenario is used and projections are given at the different warming levels. So this data shows the expected Drought Severity Index should these warming levels be reached, at the time that the warming level is reached.

For full details, see 'Future Changes to high impact weather in the UK'. HM Hanlon, D Bernie, G Carigi and JA Lowe. Climatic Change, 166, 50 (2021) https://doi.org/10.1007/s10584-021-03100-5

This dataset forms part of the Met Office’s Climate Data Portal service.