Groundwater drought investigations – Case study from Pomurje (NE Slovenia) for period 1981 – 2016

There is no uniform definition for groundwater drought. The identification of the phenomena in literature is common defined by w ter deficiency in water body compared to normal water conditions (Sheffield & Wood, 2011). Slovenia is water abundant country and therefore the usage of the term drought often brings up the dilemma. Is it correct to talk about groundwater drought in water abundant aquifers where there is no measurable consequences on living species at very low groundwater stages compared to longterm data? Can the notion of drought be unique defined for different climatic zones, aquifer dimensions and times of groundwater retention in aquifers? Moreover, what is the influence of different aquifer boundary conditions on groundwater quantity status in areas with same climate conditions? The first part of the research deals with comparison of three different approaches for drought index calculation with aim to determine optimal index for groundwater drought assessment. The index chosen was analogous to calculation of the standardized index of precipitation quantity where the input variable is groundwater level. World Meteorological Organization (WMO) and Global Water Partnership (GWP) (2016) also recommends the approach used. The second part of the research is focused on comparison of the standardized groundwater index values (SGI) distribution in time and space for 12 groundwater-measuring stations of alluvial aquifers of Pomurje region in NE of Slovenia in period 1981-2016 where precipitation and surface water discharge values of the area are also taken into consideration. Temporal and spatial distribution of SGI is furthermore considered as the consequence of natural and/or anthropogenic impact. Established s tatistical methods used in the analyses include cluster, correlation and cross correlation methods. Based on the results of the study conclusions are summarized as follows: Groundwater drought is a heterogeneous phenomenon, which can not be solely determined on the basis of the current climatic characteristics of investigated area. - Groundwater drought can be influenced by boundary conditions apart from direct infiltration of precipitation, which are not necessarily synchronized. - Groundwater drought usually have lag time concerning the input parameters of groundwater recharge and can last long after the input parameters that caused the drought are no longer significant. - In addition to the intensity, the length of the groundwater drought play the important factor in drought identification. - Anthropogenic influences, whether local or regional, have the impact on groundwater quantity status and should be considered in groundwater drought studies.