Drought monitoring is based on the analysis of a series of drought indicators, representing different components of the hydrological cycle (e.g. precipitation, soil moisture, reservoir levels, river flow, groundwater levels) or specific impacts (e.g. vegetation water stress) that are associated with a particular type of drought. The indicators generally represent statistical anomalies of the current situation with respect to the long-term climatology at a given location and period of time, and so they provide a measure of the probabilistic severity of a given event.