Step by Step: Standardized Precipitation Index (SPI) in Google Earth Engine

1. Selecting an area of interest (AOI)

In the following section we will present three different ways to specify the location of your study area. This information is necessary to limit the processing extent of the analysis and avoids redundant calculations. Users can upload location information from a file, import country boundaries provided as GEE FeatureCollections or draw an area of interest by hand.

1.1 Shapefiles

Defining the spatial processing extent with an ESRI-Shapefile (.shp) is the most accurate solution. This is recommended when researching a very distinct study area (e.g. a watershed). Start the import via the "Assets"-tab in the top left corner. On the "New"-drop-down menu select "Shape files'", then select your file. Careful: Make sure also to include the .dbf, .prj and .shx files, as the shapefile relies on them.

Uploading files usually takes a few minutes. You can observe the progress in the "Tasks"-tab on the top right. Once the "Asset ingestion"-task is completed (turns blue) you can import the shape to the script. You might have to refresh the assets tab in case your shapefile does not appear. Under "Assets" click the "Import"-button so the table is listed in the imports section. For the script to recognize this new shapefile, rename it to "AOI" (stands for "Area of Interest").

1.2 Built-in features

GEE so far provides a very limited amount of shapes such as administrative boundaries. If one seeks to perform this analysis on the country level though, there is a suitable data set provided that contains simplified features. In GEE, search for "LSIB'" or "International Boundaries". The FeatureCollection can be imported using its ID ("USDOS/LSIB_SIMPLE/2017"). Select a specific country by filtering the collection by FIPS country code. Here is an example for Costa Rica ("CS"):

var AOI= ee.FeatureCollection('USDOS/LSIB_SIMPLE/2017').filterMetadata('country_co', 'equals', 'CS');

Paste this line of code at the top of your script and make sure this is the only geometry variable in the script. Replace 'CS' with a country code of your choice.

1.3 Hand-drawn polygons

Besides uploading or importing location data, study area boundaries can be created interactively. This is the quickest and easiest option, suitable for exploring and testing the script in different regions. The polygon tool can be activated in the top-right corner of the map pane. Vertices are created with left clicks and the polygon is completed by double-clicking. A geometry can consist of more than one polygon. Once finished drawing the polygon open its setting by hovering over "geometry" in the "Geometry Imports". Change its name to "AOI" and its type to "FeatureCollection". The new AOI will be imported into the "Imports" section at the very top of the script.

2. Selecting a time frame

Besides the area of interest, the user is able to change the time scale of the calculations. This might not be necessary as the script gets the start and end date of the whole data collection and adapts its calculation according to the existing date range.
However, the resulting SVI values might get distorted if the start and end dates are changed, as a shortened lapse of time results in a reduction of the sample size for the calculations.

Beneath the start- and end date variables for the calculation timescale, you can find two variables concerning the start- and end date of the exported images. The export function itself has to be activated by setting it to "true". The strengths of the Google Earth Engine lie in the computing power, less so in the export of large image collections. Therefore, a reduction of the images to be exported or disabling the export function might be useful. Providing a timescale out of which you would like to download the SPI images will add those images to the "Tasks" tab. 

3. Set processing resolution

CHIRPS data comes at a resolution of 0.05° which corresponds to approximately 5550 meter at the equator according to USNA. This figure is also set as the default resolution for the calculations, but it can be changed to any desired resolution. It might be usefull decresing the resolution when working on large AOI, as the GEE might face calculation timeouts otherwise. The resolution affects the calculations for the graphs in the console depicting the precipitation and SPI timelines and the resolution of the exported images. It does not affect the reolution of the images that are shown in GEE's map section.

4. Setting time steps

4.1 Time step for SPI on a basis of one to several months

The first calculation deals with the "common" SPI and will be calculated for one or for several months. The observed number of months can be adjusted by changing the value behind the variable "timestep". For example: Setting the value to '6' will calculate the SPI on a six months basis counting backwards from the most recent date. It is not possible to calculate the SPI-12 (12 months SPI) with this script.

4.2 Time step MODIS related SPI

The second calculation deals with an SPI, whose dates are realted to MODIS's capture dates (the SPI is still based on CHIPRS data). This might help to compare precipitation with vegetation data. The user can apply a relative time "shift". This means that the start date of the SPI calculation is shifted relatively to MODIS's capture dates. A shift of '-5' will lead to an SPI image that starts five days earlier, but also ends five days earlier than the corresponding MODIS image. This might be useful when observing the reaction of the vegation to the precipitation. The default value is '0', which means that the start and end dates of the SPI matches with the start and end dates of MODIS's data. The next variable ("days") deals with the observed number of days for the SPI. The default value is set to '16' as this is the capture rythm of MODIS.

5. Obtaining additional information

It is possible to generate an additional chart in the lower right corner, which displays the timeline of the SPI-n of a specific location. The function is activated if the variable "showInteractiveChart" is set to 'true'. The chart will appear if a location within the AOI is being clicked.