Climate Hazard InfraRed Precipitation with Stations (CHIRPS)-SPI data are produced using CHIRPS data from the Climate Hazards Group at the University of California, Santa Barbara, and use statistical methods (described below) to identify when precipitation is above or below normal for each cell in a 0.05° grid of the world between 50°S and 50°N (and all longitudes). This information can be used in early warning systems to identify anomalous precipitation that could lead to droughts or floods. The base CHIRPS incorporates 0.05° resolution satellite imagery with on-the-ground precipitation station data to create gridded rainfall time series, measured in millimeters of precipitation/time period. Satellite data are derived from both microwave and infrared sensors. On-the-ground precipitation station data come from the Agromet Group of the Food and Agriculture Organization of the United Nations (FAO) and the Global Historical Climate Network (GHCN). A detailed description of the methodology behind CHIRPS can be found in this Nature article: http://www.nature.com/articles/sdata201566 http://dx.doi.org/10.3133/ds832. When new precipitation data are released, an SPI is calculated for each grid cell. The result is a global grid of the same resolution as the precipitation data, with the units of data in that grid converted from millimeters of precipitation into standard deviations away from the seasonal mean. At least 30 years of historical precipitation data should be used to determine the median rainfall amount and the variability of rainfall for each cell over the chosen time frame, and these values are then used to convert the new precipitation data into the SPI. Resource Watch features SPIs calculated over 1-, 2-, 3-, 6-, or 12-month time frames. SPIs of differing time frames will correlate to different degrees with other observed phenomena, as is explored in this article: http://www.tandfonline.com/doi/pdf/10.1080/2150704X.2016.1264020. Resource Watch shows only a subset of the data set. For access to the full data set and additional information, see the Learn More link.
Standard Precipitation Index (SPI)
Two CHIRPS products are produced operationally: a rapid preliminary version and a later final version. The preliminary CHIRPS product is available, for the entire domain, 2 days after the end of a pentad (2nd, 7th, 12th, 17th, 22nd, and 27th of the month). The preliminary CHIRPS uses 2 station sources: GTS and Mexico. The final CHIRPS product takes advantage of several other station sources and is complete sometime in the 3rd week of the following month. Final products for all times/domains/formats are calculated at that time.
Funk, C.C., P.J. Peterson, M.F. Landsfeld, D.H. Pedreros, J.P. Verdin, J.D. Rowland, B.E. Romero, G.J. Husak, J.C. Michaelsen, and A.P. Verdin. 2014. "A Quasi-global Precipitation Time Series for Drought Monitoring." U.S. Geological Survey Data Series 832. 4 pp. http://dx.doi.org/10.3133/ds832. ftp://ftp.chg.ucsb.edu/pub/org/chg/products/CHIRPS-2.0. Accessed through Resource Watch, (date). www.resourcewatch.org.