Droughts are complex natural phenomena, and their impacts on agriculture are enormous. Early detection of droughts is crucial for managing emerging crop losses to prevent or mitigate possibly related famines, and for dealing with increased fire risks. Satellite imagery helps to monitor precipitation, soil moisture, and vegetation health to support early drought 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.
Meteorological droughts are defined by rainfall deficiencies over an extended period of time and can turn into agricultural droughts, which are characterized by 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. Different drought definitions imply that several parameters are used to monitor droughts: precipitation, temperature, evapotranspiration, soil moisture, and vegetation. These parameters can be observed from space; the popular approach, due to its simplicity, is using vegetation indices. Vegetation indices like NDVI or EVI are freely available in near-real-time (cf. Data Application of the Month on vegetation indices). In addition, calculating the anomalies or the Vegetation Condition Index indicating the state of the greenness of vegetation in relation to a time-series is straight-forward.