# Global Drought Layers based on NASA-NEX-GDDP

## Description

The dataset in this folder consists of monthly drought indices (folder Drought_Indices) based on climate 
outputs from NASA Earth Exchange Global Daily Downscaled Projections (NEX-GDDP) (Thrasher et al, 2022).
The indices are provided for historic (1980-2014) and future (2015-2100) runs for four future emission scenarios
SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5. Each subfolder stores a specific Drought Index (or Drought Characteristics)
with its associated accumlation time scale. The timescales of 3-, 6- and 12-months were used in the study. The drought
indices consists of the Standardized Precipitation Index (SPI, Mckee et. al., 1993) and the Standardized 
Precipitation Evapotranspiration Index (SPEI, Vincente-Serrano et. al., 2010). SPI is based on
deviations from precipitation and on a parametric framework from gamma distribution (Mckee et. al., 1993).
SPEI is based on deviations from precipitation minus potential evapotranspiration (PET) and on a parametric 
framework from Generalized Extreme Value (GEV) distribution (Stagge et al., 2015). PET was estimated
through the Penman-Monteith (PM) method. Drought events were identified as time periods of negative SPI/SPEI
that reaches a value of -1 or lower.

A full description of this dataset can be find at:
Diogo S. A. Araujo, Brian J. Enquist, Amy E. Frazier, Cory Merow, Patrick R. Roehrdanz, Gabriel M. Moulatlet, 
Lei Song, Alex Zvoleff, Efthymios I. Nikolopoulos. Global Future Drought Layers Based on Downscaled CMIP6 Models
and Multiple Socioeconomic Pathways. In preparation.

## Drought Indices
SPEI - Standardized Precipitation Evapotranspiration Index
SPI - Standardized Precipitation Index

### Data type
Gridded

### Projection
Regular latitude-longitude grid

### Horizontal Coverage
Global

### Horizontal Resolution
0.25 degrees x 0.25 degrees.

### Temporal Coverage
January 1980 to December 2100

### Temporal Resolution
Monthly

### File format
TIFF

### Folder Naming Convention
[GCM_NAME]/Drought_indices/[DROUGHT_INDEX]_[TIMESCALE]/

### File Naming Convention
[DROUGHT_INDEX]_[TIMESCALE]_[GCM_NAME]_[SCENARIO]_[VARIANT_ID]_[VERSION]_[YEAR][MONTH].tif

## References

McKee, T.B., Doesken, N.J. and Kleist, J. (1993) The Relationship of 
Drought Frequency and Duration to Time Scales. 8th Conference on Applied
Climatology, Anaheim, 17-22 January 1993, 179-184.

Muñoz Sabater, J. (2019): ERA5-Land hourly data from 1950 to present. 
Copernicus Climate Change Service (C3S) Climate Data Store (CDS). 
DOI: 10.24381/cds.e2161bac (Accessed on 25-Jan-2024).

Stagge, James H., Lena M. Tallaksen, Lukas Gudmundsson, Anne F. Van Loon, and
Kerstin Stahl. 2015. Candidate Distributions for Climatological Drought Indices
(SPI and SPEI). International Journal of Climatology, 35(13): 4027-4040.

Thrasher, Bridget, Weile Wang, Andrew Michaelis, Forrest Melton, Tsengdar Lee,
and Ramakrishna Nemani. 2022. NASA Global Daily Downscaled Projections, CMIP6. 
Scientific Data, 9(1): 262.

Vicente-Serrano S.M., Santiago Beguería, Juan I. López-Moreno, (2010) 
A Multi-scalar drought index sensitive to global warming: The Standardized
Precipitation Evapotranspiration Index - SPEI. Journal of Climate, 23: 1696-1718.