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)