Monitoring Drought from Space

Monitoring Drought from Space

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FAO’s agri stress index system uses satellite data to detect hotspots in every region where crops may be affected by drought

The Food and Agriculture Organization (FAO) of the United Nations has developed the Agriculture Stress Index System (ASIS) to detect agricultural areas with a high likelihood of water stress (drought) at the global level. Based on earth observation data, ASIS will support the vegetation monitoring activities of the FAO-Global Information and Early Warning System (GIEWS).

The idea behind ASIS is to mimic the analysis that a remote sensing expert will do and simplify the results for the end-users. ASIS will provide a map every 10 days in which the GIEWS officers will detect ‘hot spots’ for every region where crops may be affected by drought during the growth season. To ensure that the data is not falsified by external factors, the officers will verify the ‘hot spots’ with auxiliary information, for example by contacting the Ministry of Agriculture of the affected country or by monitoring prices of the commodities.

Detecting drought
ASIS uses the Vegetation Health Index (VHI), which is derived from the Normalized Differenced Vegetation Index (NDVI). VHI was developed at the United States National Environmental Satellite, Data and Information Service (NESDIS) and has successfully been applied in many different environmental conditions around the globe, including in Asia, Africa, Europe, North America and South America.

The first step of ASIS is to elaborate a temporal average of the VHI assessing the intensity and duration of the dry period(s) occurred during the crop cycle at pixel level. ASIS is based on 10-day (dekad) satellite data of vegetation and land surface temperature from the METOP-AVHRR sensor at 1 km resolution.

The second step is the calculation of the percentage of agricultural area affected by drought (pixels with VHI<35 – a value identified as critical in previous studies) to assess the extent of the drought. finally, the whole administrative area is classified according to the percentage of area affected.

vhi can detect drought conditions at any time of the year. for agriculture, however, the most interesting period is the one most sensitive for crop growth (temporal integration), so the analysis is performed only between the start and end of the crop season. asis assesses the severity (intensity, duration and spatial extent) of the agricultural drought and indicates the final results at administrative level given the possibility to compare it with the agricultural statistics of the country.

the asis database contains 30 years of agricultural hot spots, starting with the year 1984 when the sahel was severely affected by drought. since metop imagery is only available for 2007 or later, the flemish institute for technological research, fao’s partner in scientific and technical development, simulated metop data into the noaa-avhrr time series through an ‘inter-calibration’ study to obtain a time series from 1984 to 2013. the most affected year by drought of the time series is 1989 where most of the agricultural land suffered from water scarcity. at the end of the first 10-day period of july 2014 the most significant hot spots of the main crop season 2013 />2014 were detected in Nicaragua, Colombia and Venezuela.

From the global version of ASIS, which was designed to detect agricultural hot spots on the globe, standalone versions can be developed to monitor agricultural drought at country or regional level. The standalone versions would be calibrated with local agricultural statistics and they would use specific parameters, coefficients and masks of the main crops of the country or region. The standalone ASIS is under development funded by the EU/FAO Programme on ‘Improved global governance for hunger reduction’.

Courtesy: FAO

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