Drought forecasting is based on forecasted gridded groundwater time series (EFAS, IFS-SEAS) for the whole of Europe and at regional scale. Two approaches are used, that are the Standardized Groundwater Index (SGI) and the threshold level method (TLM).
The SGI provides the drought index in groundwater as the number of standard deviations from the median (e.g. SGI=-2 says that the groundwater is 2 standard deviation from the median). For the SGI, we do not aggregate the groundwater over periods with different lengths.
The TLM forecasts the severity (deficit volume), duration, onset, and termination of the drought in groundwater in probabilistic terms. The drought occurs when the groundwater is below the threshold. Summaries of total drought area (SGI and TLM), total drought duration (SGI and TLM), and total drought deficit volume (TLM) are provided.
The probabilistic forecasts are presented as 25th percentile, median, and 75th percentile of the 51 ensemble members.