An introduction to drought indices WMO-ETSCI Workshop, Barbados 17 February 2015 Acacia Pepler Australian Bureau of Meteorology "Drought" by Tomas Castelazo - Own work. Licensed under CC BY 3.0 via Commons - https:// commons.wikimedia.org/wiki/file:drought.jpg#/media/file:drought.jpg
Why do we care about drought? Agriculture and food security Water security National GDP Interactions with other events: Increase flooding from heavy rain events Increase the likelihood/impacts of extreme heat & bushfires Dust storms and air pollution
Drought in the Caribbean 2015 was the driest year on record for eastern Caribbean including Barbados Water shortages in Antigua, Barbados, St Kitts & Nevis, St Lucia, Cuba, Guyana, Puerto Rico Agriculture losses in Belize ($millions), Dominican Republic (down 11%), Haiti (30% reduction in 2015 harvest) Tropical Storm Erika in Dominica: 1.3 billion EC$ damage and losses. Related to strong El Nino event Expected to persist until May
Types of Drought
Indicators of drought An indication of some sort of drought stress or deficiency River and Streamflow levels Soil Moisture information Reservoir storage Crop status/yields Vegetation Health/Stress Ground water Snow pack
Indices of drought We want: A way to assess & compare the intensity of a drought Applicable to a range of climates and time scales Able to be computed in real-time and for a long historical record We have: Precipitation, temperature Less often: wind speed, evaporation, radiation, soil moisture,
Simple drought indices Rainfall anomalies/deciles Consecutive dry days Normalised Differential Vegetation Index (NDVI)
More complex health indices Soil moisture anomalies/deficits Palmer Drought Severity Index (PDSI) Keetch-Byram Drought Index (KBDI) Mount s Soil Dryness Index (SDI) Surface Water Supply Index (SWSI) Standardized Precipitation Index (SPI) Standardized Precipitation-Evapotranspiration Index (SPEI)
Standardized Precipitation Index (SPI) Developed by McKee et al. in 1993 Recommended as the common meteorological drought index by the WMO Uses precipitation only simple! Easily calculated over multiple time scales for different drought types Based on the normalised local precipitation distribution easily comparable across regions
Standardized Precipitation Index (SPI) Gamma distribution
Standardized Precipitation Index (SPI)
Standardized Precipitation Index (SPI)
Standardized Precipitation Index (SPI) Drought events can be defined as the period where SPI is below -1 This can be used to calculate the duration and accumulated intensity for comparison to past events Weakness: No temperature/evaporation
Standardized Precipitation-Evapotranspiration Index (SPEI) Developed by Serrano, Begueria and Moreno Builds on the SPEI to include the effects of evaporation/temperature, which can enhance water deficiencies Normalised anomaly of P-Eto Difference between total precipitation (available water) and highest possible evapotranspiration (atmospheric water demand) Correlates well to more complex measures e.g. PDSI at long timescales Freely available software: http://digital.csic.es/handle/10261/10002
Standardized Precipitation-Evapotranspiration Index (SPEI) Evaporation can be calculated many ways, varying complexity
Timescales matter! SPI, SPEI calculated over various timescales These indicate different drought impacts 1-3 months: Precipitation, river flows, upper level soil moisture 6-9 months: Reservoir storage, agricultural impacts 12-24 months: Groundwater levels, long-term drought A short wet period in the middle of a long-term drought may not indicate an end to the drought
Timescales matter! 8-month rainfall deficiencies 39-month rainfall deficiencies
Timescales matter! 3-month SPI 12-month SPI
Timescales matter! 3-month SPEI 12-month SPEI
http://rcc.cimh.edu.bb/spi-monitorjanuary-2016/ Drought indices in practice
Drought indices in practice http://rcc.cimh.edu.bb/long-range-forecasts caricof-climate-outlooks/
Drought indices in practice http://sac.csic.es/spei/ map/maps.html
Thank you!