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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.

The study area in the upper part of the Osam River basin (central North Bulgaria) is characterized by highly variable mountain relief and diversified geological and hydrogeological conditions. Mesozoic clayey-terrigenous sediments are widespread in the area. Triassic and Jurassic carbonate rocks are developed locally, mainly in the upper part of the river basin. The mountainous landscape is dominated by old-forest vegetation communities. Both fissured-porous and karst groundwater systems are developed in the area. The groundwater is recharged mostly from precipitation and forms baseflow of streams. The aim of the study is to evaluate the groundwater contribution to the river flow for the catchment area of the Cherni Osam River and the Beli Osam River, forming the upper course of the main Osam River. For this purpose, two baseflow separation methods are used (local minimum method and digital filter of Eckhardt). Daily data for the river stream gages for the period 2009–2015 were processed. The baseflow for the studied catchments shows higher value for the Cherni Osam River basin and almost twice as lower value for the Beli Osam River basin. These results are most probably largely related to both the karst area and the forest cover. The digital filter developed by Eckhardt is considered as the most reliable for baseflow separation for the study area.

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