Monthly drought index for the conterminous United States: 6-month and 36-month Standardized Precipitation Evapotranspiration Index (SPEI) for 10 climate scenarios, 1950-2070

Drought is an important stressor affecting forests and rangelands. The Standardized Precipitation Evapotranspiration Index (SPEI) is a meteorological drought index that can be used to investigate trends in drought over time. The SPEI allows for comparisons among locations for historical as well as future conditions, and can be computed over multiple time scales, making it useful for monitoring drought in different ecological contexts. We calculated the 6-month and 36-month SPEI, which assigns values for a given month by comparing the cumulative climatic water balance (precipitation minus potential evapotranspiration, or PET) for the previous 6- or 36-month period to the same cumulative 6- or 36-month water balance for all months in a reference period (defined here as 1950 to 2005). Because of the 6- and 36-month lags in the calculation, the first month in these data sets with data are July 1950 (6-month SPEI) and January 1953 (36-month SPEI). This data publication consists of 10 sets of geoTIFF (TIF) raster files with monthly values for the SPEI for the period 1950-2070. We calculated the SPEI using temperature and potential evapotranspiration (PET) data from the MACAv2-METDATA for five global climate models for a historical modeled period (1950-2005) and a future period (2006-2070). For the future period, two Representative Concentration Pathways (RCPs), RCP 4.5 and 8.5 were used. The five global climate models used were: MRI-CGCM3, HadGEM2-ES, IPSL-CM5A-MR, CNRM-CM5, NorESM-M. The result was a total of ten future climates. Those are the same ten climates used in the 2020 Resources Planning Act (RPA) Assessment. The spatial extent and resolution of the data match those of the MACAv2-METDATA, covering the conterminous United States at a grid cell size of approximately 4 kilometers (1/24 degree) on a side.
These data were developed to assess the exposure of U.S. forests and rangelands to meteorological drought in the 2020 Resources Planning Act (RPA) Assessment (https://www.fs.usda.gov/research/inventory/rpaa), Chapter on Disturbances to Forests and Rangelands.
These data were published on 03/06/2023. Metadata updated on 05/18/2023 to include reference to newly published article and again on 10/17/2023 to include reference to published RPA Assessment. For more information about these data, see Costanza et al. (2023, https://doi.org/10.1002/ecs2.4525).