About drought indices

Understanding drought

Drought develops and propagates through different parts of the water cycle. Rainfall deficits, and in some cases, above average temperatures, initially affect atmospheric and climatic conditions, before affecting soil moisture, rivers flows, groundwater levels and the terrestrial environment.

For this reason, the UK Water Resources Portal provides a range of indicators representing different stages of drought development:

It is important to remember, that one of these indicators alone cannot capture every aspect of drought. Using these indices together provides a more complete picture of drought development, severity and impacts across the UK. It is important to note that different indices may show different conditions at the same time because how drought develops at different speeds across different parts of the hydrological cycle.

To support consistent monitoring across the UK, the Portal uses standardised drought indices. These indices are widely used internationally and align with recommendations from the World Meteorological Organization (WMO) for drought monitoring and early warning. Standardisation allows conditions to be compared between wet and dry regions, different river catchments and groundwater systems, and between different times of year. For example, a river in western Scotland and a river in eastern England may have very different typical flows, but standardisation allows users to compare how unusual current conditions are in both locations.

The standardised indices are provided for different accumulation periods (1, 3, 6, 12, 18 and 24 months), so you can look at the deficits (or excesses) in rainfall or river flows over 1, 3 or 12 months etc. depending on your interest. For example, a three-month accumulation period of the SPI in March will tell you about the rainfall deficits or excesses over the January, February and March period. Longer accumulation periods show prolonged drought conditions, while shorter accumulation periods are more sensitive to recent weather. Due to the slow response, or memory, in groundwater data for SGI only one accumulation period is shown for each site.

We calculate the standardised drought indices using the same reference period from 1961 to 2010 to establish long-term average conditions for each month and accumulation period.

The standardised indices are sensitive to aspects of their calculation, for example the choice of statistical distribution or the standard period used to average the data. As such the results could vary when compared with other applications of the SPI, SSI or SGI. For more information on testing appropriate distributions for UK rainfall and river flow data, see Svensson et al. (2017). Note that a statistical distribution is not used in the derivation of the SGI (see Bloomfield and Marchant (2013) for more information).

Meteorological Drought

Meteorological drought occurs when rainfall is below average over a period of weeks, months or years.

Rainfall (SPI)

The Standardised Precipitation Index (SPI) measures how much rainfall has deviated from normal conditions for a particular location and time of year.

The SPI is calculated initially on a 1km grid, based on rainfall data from the UK Met Office. The 1km rainfall is aggregated up NRFA gauged catchments and to river basins from UKCEH's Integrated Hydrological Units (IHU) dataset (groups and areas). The SPI is calculated using the Gamma distribution and a standard period of 1961–2010. For more information on the SPI see McKee et al. (1993).

Climatic Water Balance (SPEI)

The Standardised Precipitation Evapotranspiration Index (SPEI) extends the SPI by considering both rainfall and atmospheric demand for water (potential evapotranspiration, PET), providing an estimate of the balance between water supply (rainfall) and atmospheric demand for water — i.e., whether climatic conditions are becoming wetter or drier.

SPEI is calculated from a simple climatic water balance of precipitation minus potential evapotranspiration. Here, potential evapotranspiration is estimated using the calibrated McGuinness-Bordne method following Tanguy et al. (2018). There are many other ways of estimating PET, but many are more data intensive and therefore less suitable for operational purposes.

The SPEI is calculated initially on a 1km grid, based on rainfall data from the UK Met Office and the temperature-based PET data. The difference between the 1km rainfall and PET is aggregated up NRFA gauged catchments and to river basins from UKCEH's Integrated Hydrological Units (IHU) dataset (groups and areas). The SPEI is calculated using the log-logistic distribution and a standard period of 1961—2010. For more information on the SPEI see Vicente-Serrano et al., (2010).

Hydrological Drought

Hydrological drought develops when meteorological drought persists long enough to reduce river flows and groundwater recharge.

Because catchments and aquifers store water, hydrological drought often develops more slowly than meteorological drought and can continue long after the rainfall has returned. Different catchments and aquifers respond to rainfall deficits at different rates depending on both catchment and climate properties (e.g., Barker et al., 2016).

River Flows (SSI)

The Standardised Streamflow Index (SSI) measures how unusual river flows are relative to historical conditions for the given month and accumulation period for catchments. It is calculated using monthly mean flows for each catchment using the Tweedie distribution where there is more than 25 years of data in the reference period 1961–2010. For more information on the SSI see Vicente-Serrano et al. (2011), and its application in the UK see Barker et al. (2016).

The SSI reflects the combined effects of rainfall, storage and catchment characteristics, making it an important indicator of hydrological drought.

Groundwater Levels (SGI)

The Standardised Groundwater Index (SGI) measures groundwater levels relative to historical conditions at each borehole. Given the memory in groundwater systems, the SGI is not calculated for different accumulations periods like it is for the SPI, SPEI and SSI. For more information on the SGI, see Bloomfield & Marchant (2013).

Groundwater systems generally respond more slowly than rivers, making SGI particularly useful for tracking long-duration droughts and drought recovery.

References

Bloomfield, J P and Marchant, B P. Analysis of groundwater drought building on the standardised precipitation index approach, Hydrol. Earth Syst. Sci., 17, 4769–4787, 2013.

McKee, T B, Doesken, N J, and Leist, J. The relationship of drought frequency and duration time scales, 8th Conference on Applied Climatology, 17–22 January 1993, Anaheim, California, 179–184, 1993.

Svensson, C, Hannaford, J, and Prosdocimi, I. Statistical distributions for monthly aggregations of precipitation and streamflow in drought indicator applications, Water Resources Research, 53, 999–1018, 2017.

Vicente-Serrano, S M, López-Moreno, J I, Beguería, S, Lorenzo-Lacruz, J, Azorin-Molina, C, and Morán-Tejeda, E. Accurate computation of a streamflow drought index, Journal of Hydrologic Engineering, 17, 318–332, 2011.