ERA5–Drought is a global reconstruction of drought indices from 1940 to present. It provides monthly updates for a set of indices that facilitate the identification and quantification of drought conditions. To date, the dataset is focused on meteorological drought indices that are calculated from atmospheric and land surface variables retrieved from the ECMWF ERA5 reanalysis; ERA5 combines model data with observations from across the world to provide a globally complete and consistent description of the Earth’s climate and its evolution in recent decades and is considered a good proxy for observed atmospheric conditions. The dataset is produced on a regular grid of 0.25 degree globally, extends back to 1940, and is regularly updated as ERA5 data become available.
To date, the dataset comprises two drought indices:
SPI and SPEI are two of the most commonly used drought indices. The SPI measures the precipitation deficit that accumulated over a certain time window, usually 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. Both indicators can be used to identify the onset and the end of drought events as well as their severity.
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. The magnitude of the SPI and SPEI is an indicator of the severity of event.
Both indicators are calculated using 1-, 3-, 6-, 12-, 24-, 36-, and 48-month accumulation periods; the time window considered is indicative of the potential impact of the drought.
Name | Units | Description |
Standardised preciptation index | dimensionless | The standardized 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. |
Standardised preciptation evapotranspiration index | dimensionless | The standardized 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). Analogous to the SPI, positive values indicate wet conditions, and negative values indicate dry conditions with respect to the reference climate. |
Description of related quality variables
In addition to the drought indices, quality parameters are provided. For the SPI, at least 10 months with non-zero precipitation are required, but further constraints may be applied using the corresponding probability of zero precipitation (P0) parameter. For both the SPI and the SPEI, an additional quality flag is provided and indicates if the standardisation was successful at significance level 𝛼 = 0.05.
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. |