ERA5–Drought is a global reconstruction of drought indices from 1940 to present.
The dataset comprises two standardised drought indices:
- the Standardised Precipitation Index (SPI)
- the Standardised Precipitation-Evapotranspiration Index (SPEI).
The SPI measures the precipitation deficit that accumulated over the preceding months and evaluates the deficit with respect to a reference period. The SPEI is an extension of the SPI and incorporates potential evapotranspiration to capture the impact of temperature on drought. SPI and SPEI values are in units of standard deviation from the standardised mean, i.e., negative values indicate drier-than-usual periods while positive values correspond to wetter-than-usual periods. Both indices can be used to identify the onset and the end of drought events as well as their severity.
In ERA5–Drought, SPI and SPEI are calculated using precipitation and potential evapotranspiration from the fifth generation of the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalyses (ERA5). ERA5 combines model data with observations from across the world to provide a globally complete and consistent description of the atmosphere. Drought indices are calculated for a range of accumulation windows (1/3/6/12/24/36/48 months) using the reference period from 1991–2020. All data is regridded to a regular grid of 0.25 degrees, making it suitable for many common applications. SPI and SPEI are calculated using both the ERA5 reanalysis (single realisation from the monthly means of daily means(moda) stream) and the ensemble of the reanalysis (10 realisations from the monthly means of daily means for ensemble members (edmo) stream), enabling uncertainty assessment of drought occurrence and intensity. The quality of the derived indices is evaluated using significance testing.
The dataset currently covers 1940 to near-real time and is updated monthly. The consolidated data set is updated 2-3 months behind real time, while the intermediate data set is updated with 1 month of delay. New versions of the dataset are published as settings, such as the reference period, are updated or bug fixes are applied. Bug Fixes will be released using a minor revision (i.e. v1.1), while changes to the reference period will be released as major revisions (i.e. v2.0). Bug Fixes will be published to the Known Issues area on the Documentation tab.
A more detailed description of the ERA5–Drought dataset and comparisons to other drought indices can be found in the associated dataset paper (see Documentation Tab). Information on access and usage examples, e.g. to calculate the area in drought, are provided in these guidelines.
The dataset is produced by ECMWF.
Data description
Data type | Gridded |
|---|---|
Projection | Regular latitude-longitude grid |
Horizontal coverage | Global (90N-90S, 180W-180E) |
Horizontal resolution | 0.25° x 0.25° |
Temporal coverage | From January 1940 to near real time for the most recent version |
Temporal resolution | Monthly |
File format | NetCDF |
Conventions | Climate and Forecast (CF) Metadata Convention v1.6 |
Versions | v1.0 (latest) |
Update frequency | Intermediate dataset updated monthly in near real time. Consolidated dataset monthly updates with 2-3 month delay behind real time. |
Vertical coverage | Surface |
Vertical resolution | Single level |
Main variables
Name | Units | Description |
|---|---|---|
Standardised precipitation evapotranspiration index | dimensionless | The standardised precipitation evapotranspiration index is defined as the deviation from the reference climate and expressed in standard deviations. It evaluates the atmospheric water supply (precipitation, P) against the atmospheric water demand (potential evapotranspiration, PET), calculated as (P–PET). Positive values indicate wet conditions, and negative values indicate dry conditions with respect to the reference climate. |
Standardised precipitation index | dimensionless | The standardised precipitation index is defined as the deviation from the reference climate and expressed in standard deviations. Positive values indicate an excess of precipitation, and negative values indicate a lack of precipitation with respect to the reference climate. |
Related variables
Name | Units | Description |
|---|---|---|
Probability of zero precipitation SPI (P0) | dimensionless | This parameter represents the relative historical occurrence of months without precipitation during the reference period. A minimum of 10 months with non-zero precipitation (P0<0.66) is required to fit a gamma distribution and estimate the SPI. Further quality constraints on the SPI estimates may be introduced by using smaller P0 values. |
Test for normality SPEI (𝛼 = 0.05) | dimensionless | This parameter is a quality parameter that assesses if the distribution of the SPEI over the corresponding calendar months during the reference period follows a normal distribution with mean 0 and standard deviation 1. The test is performed using the Shapiro-Wilks test for normality and a significance level (𝛼) of 0.05. The quality parameter is set to 0 if the resulting p-value of the Shapiro-Wilks test is smaller than 𝛼, indicating deviations from normality and low-quality/unreliable SPEI estimates. The quality parameter is set to 1 if the the resulting p-value of the Shapiro-Wilks test is equal to or higher than 𝛼, indicating high-quality/reliable SPEI estimates. |
Test for normality SPI (𝛼 = 0.05) | dimensionless | This parameter is a quality parameter that assesses if the distribution of the SPI over the corresponding calendar months during the reference period follows a normal distribution with mean 0 and standard deviation 1. The test is performed using the Shapiro-Wilks test for normality and a significance level (𝛼) of 0.05. The quality parameter is set to 0 if the resulting p-value of the Shapiro-Wilks test is smaller than 𝛼, indicating deviations from normality and low-quality/unreliable SPI estimates. The quality parameter is set to 1 if the the resulting p-value of the Shapiro-Wilks test is equal to or higher than 𝛼, indicating high-quality/reliable SPI estimates. This quality parameter may be used in addition to the probability of zero precipitation. |