Multidecadal Repeat-Aridity Exposure Index (MRAEI). This dataset depicts spatial locations that are prone to reoccurrence of atmospheric precipitation levels below the long-term average from 1948-2021 during the warm season (May-September). Arid climatic conditions during the warm season have been identified to predispose forested ecosystems to disturbance with impacts to wildfire area burned and enhance forest vulnerability to bark beetle infestations (Holden et al. 2018 https://www.pnas.org/doi/10.1073/pnas.1802316115; Egan et al. 2023, in press).

The MRAEI is a climate-based indicator that represents locations that have experienced repeat-aridity during warm season throughout the long-term dry cycle that has impacted western U.S. from 2000-2021. This dataset was derived from the standardized precipitation index (SPI; Guttman 1999: https://drought.unl.edu/archive/Documents/NDMC/Workshops/136/Ref/SPI_Guttman.pdf; McKee et al. 1993 https://www.droughtmanagement.info/literature/AMS_Relationship_Drought_Frequency_Duration_Time_Scales_1993.pdf) that was calculated at 4 km2 resolution, respectively for each of 481k grid cells across the conterminous U.S., using the Parameter-elevation with independent slope model dataset (PRISM, Daly et al. 1994) and normalized across 1948-2021 period by fitting Gamma distribution, using observations to compute Cumulative Distribution Function (CDF), then evaluating CDF with inverse Gaussian function to derive final SPI anomaly values (Holden and Hoylman 2023 https://doi.org/10.5281/zenodo.7921890). Then, the temporal frequency of dry anomalies for each site-specific grid cell was calculated from 2000-2021, scaled by severity of dry anomaly with < -3 SPI severe dry as a baseline, and divided by the baseline to represent cumulative percent (%) of years with severe dry equivalence (Egan et al. 2023, in press)