Groundwater regime variability during the 19821994 drought period

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The orographical, climatic and hydrological conditions influence groundwater occurrence, recharge intensity and regime. Our previous studies concerning the drought period in Bulgaria confirmed the sensitivity of karst springs and shallow groundwater to climate variability (Orehova & Bojilova 2001; Gerassimov et al. 2004a). For these studies the examples of springs and observational wells from the NHGN were used.

The impact of the 1982-1994 drought period on groundwater was observed all over the territory of the country. The consequences of drought for the springs were comparable: 20-30% reduction of discharge. The results are similar for the springs draining karstic massifs of different geological ages: Precambrian marbles, Triassic dolomite and limestone, Upper Jurassic and Cretaceous limestone.

Table 2. Statistical structure of discharge (h, mm) and precipitation (P, mm) of the drought period 1982-1994

T lïïl TTT TTT1 T ~TT TTTT TTTT TTTT TTTT 1TTT ÎTTT TTTT 11 0.01 0.05 0.1 0.5 1 2 5 1 0 2 0 30 40 5 0 6 0 70 80 90 95 98 99 99.5 99.9

probabilites, %

Fig. 3. Probability distribution curves for springs 48 and 63 located in the Black Sea basin.

Table 2. Statistical structure of discharge (h, mm) and precipitation (P, mm) of the drought period 1982-1994

Drainage

X13

X13/X106

0"13

S13/

basin

(mm)

(mm)

(mm)

s106

Danube

h

98.3

0.638

36.1

0.642

P

633.6

0.865

82.2

0.791

Black Sea

h

112

0.733

41.9

0.710

P

598.7

0.93

68.1

0.600

Aegean

h

182.6

0.719

52.7

0.791

P

658.7

0.873

67.2

0.626

Total for

h

138.2

0.695

41.7

0.747

Bulgaria

P

640.2

0.877

66.5

0.639

From Gerassimov et al. (2004a).

From Gerassimov et al. (2004a).

Table 3. Deviations of average values (e, %) for spring discharge and hydrological zones (river runoff ) in relation to their 37-year values

Object (spring or hydrological zone)

1960-1996*

1960-1981

1982-1994

1985-1994

Spring 25, Zlatna Panega

_

13.5

- 21.7

- 22.2

Spring 396, Musina

-

16.8

- 28.1

- 45.0

Danube hydrological zone

-6.0

20.5

- 32.3

- 39.0

Spring 48, Kotel

_

12.5

- 20.5

- 24.9

Spring 63, Malko Tarnovo

-

9.9

-15.9

- 20.0

Black Sea hydrological zone

4.9

14.8

- 29.9

- 38.9

Spring 59, Jazo-Razlog

-

14.7

- 22.9

- 25.9

Spring 39a, Beden

-

14.6

- 19.3

- 30.0

Aegean hydrological zone

-3.9

14.6

- 25.3

- 34.0

Total river runoff

-3.9

17.0

- 27.7

-35.8

*In relation to the period 1890-1995 (106 years).

*In relation to the period 1890-1995 (106 years).

The effect of the 1982-1994 drought period on the groundwater regime was studied (Orehova & Bojilova 2001). Representative springs and wells with long observation periods from the three main hydrological zones were chosen. Most of the observational stations refer to shallow groundwater with recharge from rainfall and snowmelt. A limited number of stations represents deep aquifers. In this study the influence of the drought period on shallow porous and karstic aquifers was investigated. These aquifers are potentially most vulnerable to droughts.

The processed data give evidence for a major drop of recharge to the aquifers in Bulgaria during the 1982-1994 drought period and also for a decrease of groundwater resources. The time series of discharge for springs and groundwater level for observational wells give important information on the groundwater regime in Bulgaria. Some of them have 40-year long observational periods and reflect the influence of climate variability on groundwater.

Assessments for karst springs

For quantitative comparative estimations, the periods investigated were the 37-year longer period 1960-1996, and three shorter periods of 22 years (1960-1981), 13 years (1982-1994) and 10 years (1985-1994). The percentage deviations of the shorter periods in comparison with the longer period are calculated. To make an assessment, deviations for the same periods for the river runoff in the three main hydrological zones are obtained. In Table 3 some results for the chosen karstic springs are given.

The chronological structure of the investigated periods in reduced variables C = (X _ X)/sx with reference to the values of mean and standard deviation (X, sx) is presented for the three hydrological zones and selected springs in Figures. 4-6. From Table 3 and Figures 4-6 we draw the following conclusions.

• The shift of the groundwater regime in Bulgaria during the drought period is similar to that for

Fig. 4. Chronological graphs for spring 396 near Musina village and Danube hydrological zone.

spring

i i

Black Sea basin

sd+

oo o cn at

CO cn to cn cn

Fig. 5. Chronological graphs for spring 48 near Kotel town and Black Sea hydrological zone.

river runoff; the corresponding deviations are in the same range.

• The drought period 1982-1994 and especially the shorter component 1985-1994 are characterized by deep depression of the groundwater.

• The chronological structure of spring discharge and groundwater levels during the drought period resembles that of river runoff.

During the 1982-1994 drought period the recharge to the groundwater diminished. Orehova & Benderev (2004) applied two techniques to quantify the multi-annual recharge of Kotel spring basin. The reduction of recharge during the drought period is also assessed (around 70 mm). Such estimations require detailed knowledge of relief features, geological and hydrogeological settings, and hydrological data on springs draining the karst massif.

Despite the long observation period, no natural regularities can be settled now concerning the regime of springs in different hydrological and climatic zones. This inference is most likely related to highly varied conditions of selected karst basins.

From the two springs of the Danube zone, spring no. 396, Musina, is the most vulnerable to climate variability. We suppose that it is related to preponderant recharge due to precipitation. The precipitation in its drainage basin, having no possibility to generate surface runoff, percolates through the karst forms. This is the cause of the high vulnerability of karst waters in this region to drought. Permanent river recharge for the Zlatna Panega karst basin from the Vit River is a reason for the relatively high resistance to drought of the spring Glava Panega.

The two springs from the Black Sea basin show a similar reaction to drought. In both cases karst formations perform a retention function. This function is most visible in the region of Strandja mountain (Malko Tarnovo town), where the level of karstifi-cation is lower.

Fig. 6. Chronological graphs for spring 59 Jazo, and Aegean hydrological zone.
Table 4. Absolute and relative declines of the groundwater levels for the drought period

River basin

Observational

Hav (m)

s (m)

Hav (m)

DH (m)

DH/s

well no.

1960-2000

1982-1994

Tundja

271

2.09

0.351

2.38

-0.29

- 0.827

Tundja

273

7.02

0.968

7.72

-0.70

- 0.723

Maritza

526

7.86

0.329

8.08

-0.22

- 0.699

Maritza

287a

2.92

0.655

3.43

-0.51

- 0.779

Hav, multiannual average water level; s, standard deviation of Hav (1960-2000).

Hav, multiannual average water level; s, standard deviation of Hav (1960-2000).

The two springs from the Aegean Sea basin drain karst basins in mountain regions. In the case of Razlog karst basin, the flow distribution is stable during the whole year due to considerable snow cover and an extended period of snowmelt.

Assessments for observational wells

To investigate groundwater variability in shallow porous aquifers, observational wells in proluvial-alluvial formations were selected. Two regions from the central part of the country are presented. In Table 4 the examples of observational wells in Kazanlak kettle (Figure 7) and Upper Thracian valley are given. The numbering of the chosen wells is in accordance with the system used in NHGN. The wells are located in the Eastern Aegean basin under similar climatic conditions. The

Kazanlak kettle occupies the inside graben orientated in a west-east direction and is bounded to the north by Stara Planina mountain and to the south by Sredna Gora mountain. The kettle is filled with proluvial-alluvial deposits in which an aquifer is formed. The groundwater level in the terrace is hydraulically connected to the Tundja River.

The Upper Tracian valley is the largest superimposed depression in Bulgaria. The valley is filled in with aquiferous Pliocene and Quaternary deposits of different genesis. The recharge is mainly due to river water, surface runoff and rainfall water. The Maritza River is the main collector in the valley. The Upper Tracian valley is characterized by more complicated conditions than the Kazanlak kettle.

For the chosen observational wells in proluvial-alluvial deposits the average drawdown of water level is in the range 0.2-0.7 m for the period

Fig. 7. Tundja River basin.

Well 271 Well 273 sd+ sd-

Fig. 8. Groundwater level variability for observational wells from the Kazanlak kettle, Tundja river basin.
Fig. 9. Groundwater level variability for observational wells from the Upper Thracian valley, Maritza river basin.

1000

8 400 o

Fig. 10. Monthly distribution of discharges for two dry years (1994, 2001). The reference period for average monthly data (Qav) is 1961-1990.

Fig. 10. Monthly distribution of discharges for two dry years (1994, 2001). The reference period for average monthly data (Qav) is 1961-1990.

10 11 12

10 11 12

1982-1994 (see Table 4). These values depend not only on reduction of recharge to the aquifer, but are subject to the influence of variable hydrogeological setting and parameters as well. In the last column of Table 4, relative deviations are presented in a comparable range. This feature can be seen in Figures 8 and 9, where the chronological structures of water level for selected observational wells are presented in reduced variables.

Mild winters and groundwater

Mild winters were analysed by Andreeva et al. (2003). Mild winters in Bulgaria, according to the classification given in the paper, are characterized by temperatures above normal, and precipitation about normal or below. Droughts during the cold period for the years had a strong negative influence on groundwater. Examples of monthly distribution of discharge for two dry years are presented in Figure 10 for hydrogeological station no. 48 (Kotlenski springs). The reference period for average monthly data (Qav) is 1961-1990.

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