A D Benderev1 T V Orehova1 E K Bojilova2

1 Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 24, 1113 Sofia, Bulgaria (e-mail: [email protected])

2National Institute of Meteorology and Hydrology, Bulgarian Academy of Sciences, Tzarigradsko chaussee 66, 1784 Sofia, Bulgaria

Abstract: Groundwater regime in Bulgaria is influenced by climate variability. The impact is evident especially for karst water. A time series analysis of spring discharge for selected karst basins was performed. The impact of the 1982-1994 drought period on groundwater regime was detected. For springs that drain open and mountainous karst, the impact of climate variability is similar to that on surface waters. In fact, the difference in degree of influence of the drought period is related to the specific geological structure of the karst massifs and recharge conditions. Furthermore, the porous waters are characterized by a weaker reaction to such an effect. In general, groundwater use during the 1982-1994 drought period was impacted by climate variability due to limited resource availability.

Climate variability is of importance for the territory of Bulgaria, and particularly its effect on groundwater. This influence depends on the general characteristics of the groundwater parameters as well as the specific natural conditions in the country. Long-term observations of groundwater allow deviation of the groundwater regime to be assessed with respect to climate influence.

For the aims of this research, data from the National Hydrogeological Network (NHGN) are processed. The NHGN of Bulgaria has been in operation since the late 1950s. Our study concerns a long-lasting drought period (1982-1994) in the territory of the country. Discharge data and water levels in wells on a monthly and annual basis are used for the analysis in accordance with the needs of water resource planning.

Studies similar to ours have been developed to estimate droughts in the whole of Europe using monthly and annual temporal scales (Alvarez Rodriguez & Estrela Monreal 2000). Applying the clustering technique, ten groups were summarized for the 1901-1996 period. According to Alvarez Rodriguez & Estrela Monreal (2000), north Bulgaria belongs to the Middle and Lower Danube basin, and south Bulgaria to the Eastern Mediterranean basin.

Various studies have concluded that in the last few decades the drought situation in many European regions has become more severe, due to an increase in frequency, duration or intensity of low flows (Bernhard & Doll 2001; Demuth & Stahl 2001; DVWK 1998). A further increase driven by global and climate change impacts is expected, in particular for southern European areas (Bernhard & Doll 2001; Watson et al. 1997).

Drought is a phenomenon that is not constrained by international boundaries and can therefore afflict many countries simultaneously. As low flows and droughts commonly cover large areas for long time periods, it has been suggested that these events should be studied within a regional context (Demuth & Stahl 2001; Tallaksen 2000).

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