Drought Indicators
The African Drought Monitoring and Advisory system is based on the analysis of combined spatial patterns of precipitation, soil moisture and greenness vegetation anomalies. This analysis results in the Combined Drought indicator (CDI) and the latter acts as a traffic light pointing to the areas at risk of agricultural drought, areas where the vegetation has already been affected by drought and areas of no drought conditions. In this venue, the CDI classification scheme defines four primary drought classes (“No drought”, “Watch”, “Warning” and “Alert”).
What is a drought
Drought is an extreme hydrological event, and drought advisory is a monitoring platform that uses different datasets to model the different components that affect hydrology, such as surface water, groundwater, precipitation, temperature, and soil moisture content. Drought advisory also looks into other effects associated with drought events, such as vegetation water stress and changes in vegetation cover.
How it works
The Drought indicators generally evaluate the current hydro-logical conditions against the long-term averaged climatological conditions of the specific areas, and therefore they give a measure of of the probabilistic severity of a given event.
Drought Indicators Monitored
Standardized Precipitation Index (SPI)
The SPI is a statistical index comparing the total precipitation received at a particular location during a period of n months with the long-term precipitation distribution for the same period of time at that location (reference period is 1981-2010). This indicator measures anomalies of accumulated precipitation during a given period (e.g., 1, 3, 12, 24,48 months), and is the most commonly used indicator for detecting and characterizing meteorological droughts. Resolution is 0.25 decimal degrees in EDO, 1 decimal degree currently in ADMA system.
Soil Moisture Anomaly (SMA)
Soil moisture anomaly maps are computed on a 30-day moving window at a spatial resolution of 0.1 decimal degrees and updated every 10 days. This indicator measures anomalies of daily soil moisture (water) content and is used to measure the start and duration of agricultural drought conditions. The resolution is 5 km in EDO, 0.1 decimal degree in the ADMA system
Anomaly of Vegetation Condition (FAPAR Anomaly)
The FAPAR Anomaly indicator is computed as deviations of the satellite-measured biophysical variable Fraction of Absorbed Photosynthetically Active Radiation (FAPAR, sometimes written as fAPAR or FPAR), composited for 10-day intervals, from its long-term mean values. This indicator measures anomalies of satellite-measured FAPAR (Fraction of Absorbed Photosynthetically Active Radiation) and is used to highlight areas of relative vegetation stress due to agricultural drought. Resolution is 0.0416 decimal degrees in EDO and 0.0833 decimal degrees in Mukau.
Combined Drought Indicator (CDI)
Drought Indicator (CDI) is derived from the combination of SPI, SMA and fAPAR, to identify areas with the potential to suffer agricultural drought, areas where the vegetation is already affected by drought conditions, and areas in the recovery process to normal conditions after a drought episode.
Normalised Difference Vegetation Index (NDVI)
NDVI is a metric used to measure vegetation density and assess changes in plant health using sensor data. It is calculated as a ratio between the red (R) and near-infrared (NIR) values in traditional fashion: (NIR - R) / (NIR + R).