Global SPEI database

To calculate the SPEI we used the CRU TS 4.03 dataset. This is the most complete and updated dataset of gridded precipitation and potential evaporation at the global scale, has a spatial resolution of 0.5°, and covers the period 1901−2018.

Details of the SPEIbase and the comparison with other available drought data sets can be consulted in:

• Vicente-Serrano et al. (2010)
• Beguería et al. (2010)
• Beguería et al. (2014)

A number of users of our SPEIbase dataset worried because they could not read our netCDF files in their software. Indeed, some commercial softwares such as Matlab, ArcGIS and Idrisi do not have support for NetCDF4 or did not have it until recently. The main reason for us to use netCDF4 for the SPEIbase is because it allows for data compression, resulting in large file size and internet traffic saving. We did not use any of the other fancy options implemented in netCDF4 such as grouping, compound types or multiple unlimited dimensions. This means that our netCDF4 files conform to the classic model, so they can be converted back to the netCDF3 legacy format.

One way of converting to netCDF3 is by using the nccopy program by unidata. For example, to convert a netCDF-4 classic-model format file foo4c.nc to a classic format file foo3.nc:

nccopy -k classic foo4c.nc foo3.nc.

Another way would be to install the netCDF operators (NCO) toolset from unidata, and then use the ncks utility:
ncks -3 foo4c.nc foo3.nc.

In fact, ncks allows for much greater functionality. For example, if one wants to extract the first 100 times from an SPEIbase file:

ncks -d time,0,100 spei_12.nc output_file.nc

would generate a (smaller) netCDF file with only those timesteps. In a similar fashion, it is possible to use ncks to select a specific geographical region.

The Global 0.5° gridded SPEI dataset is made available under the Open Database License. Any rights in individual contents of the database are licensed under the Database Contents License.

What follows is a human-readable summary of the ODbL 1.0 license. Please, read the full ODbL 1.0 license text for the exact terms that apply.

Users of the dataset are free to:

• Share: copy, distribute and use the database, either commercially or non-commercially.
• Create: produce derivative works from the database.
• Adapt: modify, transform and build upon the database.

Under the following conditions:

• Attribution: You must attribute any public use of the database, or works produced from the database, by citing one or more of the papers referenced in the References section below. For any use or redistribution of the database, or works produced from it, you must make clear to others the license of the original database.
• Share-Alike: If you publicly use any adapted version of this database, or works produced from an adapted database, you must also offer that adapted database under the ODbL.

Dimensions

• longitude: geographical coordinates;
• latitude: geographical coordinates;
• time: months.

Variables

spei

• long_name: "Standard Precipitation-Evapotranspiration Index";
• units: "z values";
• type: float;
• range: -3.0E32 to 3.0E32;
• missing value: 3.0E33f;
• not a number: nan;

longitude

• long_name: "Longitude values";
• cartesian_axis: "X";
• units: "degrees_E";
• type: float;
• interval: 0.5;
• range : -179.75, 179.75;

latitude

• long_name: "Latitude values";
• cartesian_axis: "Y";
• units: "degrees_N";
• type: float;
• interval: 0.5;
• range : -89.75, 89.75;

time

• long_name: "Time coordinate values";
• cartesian_axis: "T";
• units: "Months since 1901-01";
• calendar: "Gregorian";
• type: double integer;
• interval: 1;
• range: January 1901, December 2011;

Format: netcdf

The SPEIdata is provided as single files comprising the whole World for each SPEI time scale. The data format is netCDF.

Metadata

All relevant metadata is self-contained in the .nc files, and can be read and displayed by any software allowing netCDF manipulation.

Suggested tools

Manipulation of netCDF data is usually done at the programming level, and there are several libraries with functions for reading and manipulating netCDF data for a diversity of languages. For more information on the netCDF format and suggested tools, see http://en.wikipedia.org/wiki/NetCDF.

Panoply is a cross-platform netCDF viewer developed in Java (http://www.giss.nasa.gov/tools/panoply/). There are other netCDF viewers, such as ncBrowse, ncview, and nCDF_Browser.

netCDF data can be accessed and manipulated in the (open source) statistical analysis system R using the package netcdf.

Some commercial GIS packages allow reading netCDF data, such as ArcGIS (from version 9.2 on) and IDRISI Taiga.