Comparative estimate of resistance to drought for selected karstic aquifers in Bulgaria

International Journal of Speleology, Dec 2004

Effective management of water resources requires adequate knowledge of groundwater system including the influence of climate variability and climate change. The drought of 1982-1994 in Bulgaria has led to important decrease of springflow and lowering of water levels. Therefore, groundwater demonstrated its vulnerability to drought. The purpose of this paper is to determine relative resistance of selected aquifers in Bulgaria to a prolonged decrease of recharge to groundwater. The drought resistance indicator has been defined for some karstic aquifers based on the method proposed in report of BRGM. The data from National Hydrogeological Network located in the National Institute of Meteorology and Hydrology were processed. For the aim of this study, time-series of discharge for karstic springs were used. Stations with significant impact of human activity on groundwater were eliminated. The results show that most of studied aquifers in Bulgaria have moderate and weak resistance to the drought. They are vulnerable to droughts and need good management for effective use of groundwater resources.

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Comparative estimate of resistance to drought for selected karstic aquifers in Bulgaria

COMPARATIVE ESTIMATE OF RESISTANCE TO DROUGHT FOR SELECTED KARSTIC AQUIFERS IN BULGARIA Tatiana Orehova Effective management of water resources requires adequate knowledge of groundwater system including the influence of climate variability and climate change. The drought of 1982-1994 in Bulgaria has led to important decrease of springflow and lowering of water levels. Therefore, groundwater demonstrated its vulnerability to drought. The purpose of this paper is to determine relative resistance of selected aquifers in Bulgaria to a prolonged decrease of recharge to groundwater. The drought resistance indicator has been defined for some karstic aquifers based on the method proposed in report of BRGM. The data from National Hydrogeological Network located in the National Institute of Meteorology and Hydrology were processed. For the aim of this study, time-series of discharge for karstic springs were used. Stations with significant impact of human activity on groundwater were eliminated. The results show that most of studied aquifers in Bulgaria have moderate and weak resistance to the drought. They are vulnerable to droughts and need good management for effective use of groundwater resources. karst; aquifer; drought; resistance - where is the absolute minimal registered discharge of the spring, value of minimal discharges for the given period. The karstic aquifers are divided into four classes of increasing drought resistance: the average DRIKA ≤ 0,3 0,3 < DRIKA ≤ 0,45 0,45 < DRIKA ≤ 0,6 DRIKA > 0,6 - weak; - moderate; - good; - high. Selection of appropriate stations and general data Bulgaria is rich in karstic springs. They drain elevated massifs of karstified marbles, limestones and dolomites (Antonov & Danchev, 1980; Boyadjiev, 1964) . Important karstic massifs are built from Proterozoic marbles. Sketch map of Bulgaria with the location of the studied springs is presented on Fig. 1. For the aim of this study, perennial karstic springs with long observational period were chosen. Stations showing significant impact of human activity on groundwater were eliminated. Time series of minimal spring discharges for the period 1960 - 2002 were processed. All data originate from National Hydrogeological Network located in the National Institute of Meteorology and Hydrology. The general characteristic of the selected springs is given in Table 1 with indication of their appertainance to the respective territorial unit: • Danube zone – drainage basin of all Bulgarian tributaries to the river Danube (45% of the territory of Bulgaria); • Black sea zone - drainage basin of all Bulgarian rivers with direct discharge to the Black sea (13,8%); • Aegean zone - drainage basin of all South Bulgarian rivers with direct discharge to the Aegean Sea in the territory of Greece and/or Turkey – 41,2% of the territory of the country. For some stations (NN 396, 48, 39a) water level recorder is available, for other stations (NN 59, 59a) water level is measured every day by observers. Measurement of spring discharge (using a current meter) is 12 times annually as usual. Using rating curve the daily data for spring discharge were obtained. For the three chosen stations from the Upper Strouma basin (NN 40, 86, 461) and some springs from Danube zone (NN 394), measurements are made once-twice in a month without daily observations on water level. The springs from the Upper Strouma basin, however, show stable discharge during the year. Seasonal variability of the groundwater regime is discussed by Antonov & Danchev (1980) and Orehova (2002) . As a rule, maximal discharges occur in spring due to snowmelt. For springs in the Danube zone, the most usual case is maximal flows in early spring and minimal ones in October. Minimal springflows are observed in AugustSeptember (48), September-October (39a), February-March (59a), and March-April (59). The three last springs drain Proterozoic marbles from mountain regions (see Table 1). The delay in the extreme for springs 59 and 59a is due to the situation of their watersheds in the high mountain Pirin. The drainage basin of the both springs is located in protected area of Pirin mountain. Pirin National Park is included in the list of UNESCO as a part of World natural heritage. At the spring N 59 the maximum and minimum discharges occur one month later than for the spring N 59a. Most of springs have well defined seasonal cycle. Only springs from the Upper Strouma basin (NN 40, 86 and 461) and spring N 59 show weak seasonal variation throughout the year. Karstic aquifers in Bulgaria were affected by the drought during 1982-94 (Orehova & Station N. N Situation Fore-Balkan Fore-Balkan Fore-Balkan Fore-Balkan Stara Planina Strandja Radomir valley Zemen mountain Konyavo mount. Pirin mountain Pirin mountain Rhodopes Bojilova, 2001a, 2001b; Andreeva & Orehova, 2001) and showed reduction in spring discharge (20-30% on average). Therefore they demonstrate evident vulnerability to drought. Th (...truncated)


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Tatiana Orehova. Comparative estimate of resistance to drought for selected karstic aquifers in Bulgaria, International Journal of Speleology, 2004, pp. 7, Volume 33, Issue 1,