Google Earth Engine Standardized Precipitation Index

The aim of this step-by-step guide, offered on the UN-SPIDER knowledge portal, is to create a standardized precipitation index (SPI) timeline based on daily CHIRPS data. The SPI is used as it highlights the difference to the mean precipitation during a given time and therefore provides information about drought-like conditions. The script will be executed within Google Earth Engine and will work on two independent SPI calculations. The first calculation deals with the "common" SPI, which is calculated on an n-months basis. A SPI, which is calculated for one month usually refers to the description of "SPI-1", for six months "SPI-6" and so on. The second SPI calculation is based on MODIS capture dates. As MODIS provides information about the vegetation, it might be useful to compare its vegetation indices with the SPI. Therefore a 16-day SPI is calculated, whose start date matches with MODIS's start date.

Google Earth Engine (GEE) is a web-platform for cloud-based processing of remote sensing data on a large scale. The advantage lies in its remarkable computation speed as processing is outsourced to Google servers. The platform provides a variety of constantly updated datasets; no download of raw imagery is required. While it is free of charge, one still needs to activate access to Google Earth Engine with a valid Google account.