Drought is a complex natural phenomenon, and its impacts on agriculture are enormous. Early detection of droughts is important for managing emerging crop losses to prevent or mitigate possible related famines, and for dealing with increased fire risk. Satellite imagery helps to monitor precipitation, soil moisture, and vegetation health to support drought early warning systems. It is used to feed monthly drought bulletins and to issue warnings.
Satellite imagery can assist environmental monitoring by detecting changes in the Earth's vegetation, atmospheric trace gas content, sea state, ocean color, and ice fields. By monitoring vegetation changes over time, droughts can be monitored by comparing the current vegetation state to its long term average.
How are droughts monitored from Space?
Meteorological droughts are defined by rainfall deficiency over an extended period of time and can turn into agricultural droughts, which are characterized by a soil water deficiency and subsequent plant water stress and reduced yield. Agricultural droughts can then turn into hydrological droughts, which refer to deficiencies in surface and subsurface water supplies. The different drought definitions imply that several parameters are used to monitor drought: precipitation, temperature, evapotranspiration, soil moisture, and vegetation. These parameters can be observed from space. Using vegetation indices is a popular approach due to its simplicity. Vegetation indices like NDVI and EVI are freely available in near-real-time (cf. Data Application of the Month on vegetation indices). In addition, calculating anomalies or the Vegetation Condition Index indicating the state of the greenness of vegetation in relation to a time-series is straight-forward