Adaptation of the Standardized Precipitation Index SPI for drought monitoring in groundwater

The Standardized Precipitation Index (SPI) proposed by Thomas McKee, Nolan Doesken i John Kleist in 1993 is currently one of the most common meteorological index applied in the assessment of drought intensity. It serves for the quantitative assessment ofprecipitation deficit within an agreed time scale. The advantage of the SPI is that it can be applied at different time scales and different climatic conditions. It provides an early warning information regarding droughts and is helpful in the assessment of its severity. The authors propose that the method can be applied in, apart from precipitation, assessments of river flows, depth of snow cover, water resources in reservoirs, soil wetness and groundwater levels. The article proposes an index formula that allows for the assessment of drought in groundwater. Gamma distribution, which is most commonly used in the SPI formula and does not describe the distribution of groundwater level properly, was replaced with the empirical distribution. In this way a new tool with the advantages inheritedfrom its progenitor was defined to assess the intensity of drought in groundwater. The new index has all needed attributes to be used to interpret the current hydrogeological situation on both regional and local scales. It can compete with indicators currently used for this purpose in Poland and abroad. The "new standardized groundwater level index” was applied to analyse the course of a groundwater drought that occurred in Poland in recent years (2015-2016)