Earth Observation by GIEWS

The Global Information and Early Warning System (GIEWS) is the leading source of information on food production and security for every country in the world, whether or not it is an FAO member. Over the years, a unique database on global, regional, national and subnational food security has been maintained, refined and continuously updated.

Global Information and Early Warning System on Food and Agriculture (GIEWS) monitors the condition of major food crops across the globe to assess production prospects. To support the analysis and supplement ground based information, GIEWS utilizes remote sensing data that can provide a valuable insight on water availability and vegetation health during cropping seasons. In addition to rainfall estimates and the Normalized Difference Vegetation Index (NDVI), GIEWS and FAO NRC Division have developed the Agricultural Stress Index (ASI), a quick-look indicator for early identification of agricultural areas probably affected by dry spells, or drought in extreme cases.

The seasonal indicators are designed to allow easy identification of areas of cropped land with a high likelihood of water stress (drought). The indices are based on remote sensing data of vegetation and land surface temperature combined with information on agricultural cropping cycles derived from historical data, and a global crop mask. The final maps highlight anomalous vegetation growth, and potential drought, in crop zones during the growing season.

The satellite data used in the calculation of the mean VHI and the ASI is the 10-day (dekadal) vegetation data from the METOP-AVHRR sensor at 1 km resolution (2007 and after). Data at 1 km resolution for the period 1984-2006 was derived from the NOAA-AVHRR dataset at 16 km resolution. The crop mask is a modified version of an EC-JRC data set that compiles several different sources of land cover data, including GlobCover V2.2, Corine-2000, AfriCover, SADC data set and USGS Cropland Use Intensity Data Set.

The vegetation indicators (NDVI anomaly, VCI and VHI) provide alternative measures of the relative vegetation health. These indices can be used to monitor areas where vegetation may be stressed, as a proxy to detect potential drought. The precipitation indicators present a global analysis of the absolute (mm) and relative (%) rainfall levels per dekad, in addition to the long-term average precipitation levels (mm).

All three vegetation indicators are based on 10-day (dekadal) vegetation data from the METOP-AVHRR sensor at 1 km resolution (2007 and after). Data at 1 km resolution for the period 1984-2006 was derived from the NOAA-AVHRR dataset at 16 km resolution. Precipitation estimates for all African countries (except Cabo Verde and Mauritius) are taken from NOAA/FEWSNet, while for the remaining countries data is obtained from ECMWF.