DOI: 10.5285/a754cae2-d6a4-456e-b367-e99891d7920f
title: Historic Standardised Precipitation Index time series for IHU Hydrometric Areas (1862-2015) v2

description: Standardised Precipitation Index (SPI) data for Integrated Hydrological Units (IHU) Hydrometric Areas (Kral et al., 2015; https://doi.org/10.5285/3a4e94fc-4c68-47eb-a217-adee2a6b02b3). SPI is a drought index based on the probability of precipitation for a given accumulation period as defined by McKee et al. [1]. SPI is calculated for different accumulation periods: 1, 3, 6, 9, 12, 18, 24 months. Each of these is in turn calculated for each of the twelve calendar months. Note that values in monthly (and for longer accumulation periods also annual) time series of the data therefore are likely to be autocorrelated. The standard period which was used to fit the gamma distribution is 1961-2010. The dataset covers the period from 1862 to 2015. NOTE: the difference between this dataset with the previously published dataset 'Standardised Precipitation Index time series for IHU hydrometric areas (1961-2012)' [SPI_IHU_HA] (Tanguy et al., 2015;  https://doi.org/10.5285/5e1792a0-ae95-4e77-bccd-2fb456112cc1), apart from the temporal extent, is the underlying rainfall data from which SPI was calculated. In the previously published dataset, CEH-GEAR (Tanguy et al., 2014; https://doi.org/10.5285/5dc179dc-f692-49ba-9326-a6893a503f6e) was used, whereas in this new version, Met Office 5km rainfall grids were used (see supporting documentation for more details). Within Historic Droughts project (grant number: NE/L01016X/1), the Met Office has digitised historic rainfall and temperature data to produce high quality historic rainfall and temperature grids, which motivated the change in the underlying data to calculate SPI. The methodology to calculate SPI is the same in the two datasets.

This release supersedes the previous version, https://doi.org/10.5285/d8655cc9-b275-4e77-9e6c-1b16eee5c7d5, as it addresses localised issues with the source data (Met Office monthly rainfall grids) for the period 1960 to 2000.

[1] McKee, T. B., Doesken, N. J., Kleist, J. (1993). The Relationship of Drought Frequency and Duration to Time Scales. Eighth Conference on Applied Climatology, 17-22 January 1993, Anaheim, California. 

description: SPI is calculated as originally defined in McKee et al. [1]. SPI is based on the cumulative probability of a given rainfall amount occurring at a location. The historic rainfall data of the station is fitted to a statistical distribution. For this dataset, the statistical distribution used is the gamma distribution, which has been extensively used and is recommended as a default choice for Europe by Stagge et al. [2]. The L-moments method was used to estimate the gamma distribution parameters, as the maximum likehood method was failing to fit a realistic distribution in some isolated cases (more details on the choice of methodology in Tanguy et al., in preparation). To calculate SPI, the R package SCI was used, but modified to use L-moments (instead of Maximum Likelihood). The input data used is the monthly rainfall grids from the Met Office 5km gridded rainfall product provided within Historic Droughts project (grant number: NE/L01016X/1), which has been area-averaged over each of the Integrated Hydrological Units (IHU) Hydrometric Areas (Kral et al., 2015; https://doi.org/10.5285/3a4e94fc-4c68-47eb-a217-adee2a6b02b3). In this version of the dataset (version 2), the monthly rainfall grids from 1960 to 2000 was derived from the Met Office 5-km daily rainfall grids, to address some localised issues that were found in the Met Office monthly rainfall grids.

[1] McKee, T. B., Doesken, N. J., Kleist, J. (1993). The Relationship of Drought Frequency and Duration to Time Scales. Eighth Conference on Applied Climatology, 17-22 January 1993, Anaheim, California. 
[2] Stagge, J. H., Tallaksen, L. M., Gudmundsson, L., Van Loon, A. F. and Stahl, K. (2015), Candidate Distributions for Climatological Drought Indices (SPI and SPEI). Int. J. Climatol. doi: 10.1002/joc.4267