Precipitation deficit is a common measure for severity of agricultural drought, for example in the Netherlands, but it is also used internationally. The precipitation deficit addresses the need for drought monitoring, which was expressed by several river basins, including the Drammen, East Anglia and Danube.

The precipitation deficit D is defined as the accumulated difference between precipitation P and (potential) evaporation ET. Both P and ET are needed as daily absolute values. As daily evapo-transpiration for future years was not readily available, we decided to calculate it from incoming radiation and air temperature, which are available from Climate and energy related variables from the Pan-European Climate Database derived from reanalysis and climate projections. The Priestley-Taylor formula was used to calculate daily (potential) evaporation.

From daily precipitation and (potential) evaporation, we calculate the precipitation deficit by taking the sum of the daily differences (evaporation minus precipitation) from April 1st up to the current date. If the precipitation deficit turns negative, we reset it to zero. For drought severity assessment, we take the maximum value per season (April-September). The precipitation deficit is an accumulated index, which takes into account both the intensity and the duration of water scarcity. Low precipitation and high evaporation over a longer period of time will build up a large deficit. Note that the precipitation deficit is based on the potential evaporation. The actual evaporation will be reduced by limited availability of water in vegetation and the soil.

The use of an accumulated indicator implies that a starting date (s=0) is required. In the Netherlands, the starting date is April 1, which is the start of the growing season in this part of the world. For other basins, particularly for southern Europe, the starting date of April 1 may not be appropriate. Moreover, as a result of climate change, droughts may start developing earlier in the year, which would be missed by a precipitation deficit that is starting on April 1. However, this is left for future
studies.

Derived from data from https://cds.climate.copernicus.eu/datasets/sis-energy-pecd

For more information, visit the following page of the STARS4Water Github Organization: https://zenodo.org/records/15240491