THE REGIME OF WATER LEVEL OF LAKES DURANKULAK AND

assoc. proff. Nelly Hristova Sofia University “St. Kliment Ohridski” Sofia, Email: [email protected]

Abstract This paper presents the results from analyses of water regime of Lakes Durankuk and Shabla, which are the best-preserved coastal wetlands in Bulgaria with international importance. The lakes are Ramsare Convention Sites. Their water balance is different, but the regime is similar – with high level from November till June and law levels - from July till October. The deference between input flow and output flow for tow lakes is reason for the different variations of monthly levels – not so much for Durankulak Lake and large for Shabla Lake.

Keywords: lake, wetlands, water regime

Introduction There are in Bulgaria some wetlands with international importance – Srebarna, Durankulak, Shabla, Burgaski lakes. The more of them (except Srebarna) are coastal lakes. This report presents the water regime of two wetland systems – Durankulashka and Shablenska, which are Ramsare Convention Cites, part of the list of Birdlife International and protection zones. Lakes Durankulak and Shabla are in north-eastern Bulgaria. They are situated on the coast and they are coastal firths (Fig.1). The area of Duranculak complex is 20 425.3 dka and includes some lakes – Durankulak lake which is the biggest (area 2 529 ha, average depth – 1.2 m, maximum depth – 4 m, water volume – 2.5.103 m3), Orlovo (Kartaliisko) marsh (it is naturally separated from the lake by a narrow mud shallow and has been additionally separated from the rest of the aquatic area by an artificial dike), Small lake and Korugiol. Shabla complex includes two lakes – Shablensko (area 594.7 dka, average depth – 4.6 m, maximum depth – 9.5 m, water volume – 3600.103 m3) and Ezeretsko (area 314.2 dka, average depth – 3.5 m, maximum depth – 9.5 m, water volume – 2,5.103 m3) which are connected by artificial canal and sepatated from the Black sea by a sandbar. Shabla complex includes a hyper saline lake Shablenska tuzla (area 0.3 km2; average depth – 0.6 m,) – lagoon, which is separated from Black sea by a sandbar (80 m) and dunes.

Natural features Climate The Durankulak and Shabla Lake Systems belong to the Dobrudga microclimatic region, where the climate is transitory-continental (Velev, 1990). The average temperature is 11 oC (Tabl. 1). The hottest month is July (+21.0 - +23.0 oC), the coldest – January (0.6 oC). The annual precipitation sum in the region is the lowest in Bulgaria and varies between 438 and 480 mm (Tabl. 2). There ate two maximum in regime of precipitation - November and June and two minimums - in August and March (Fig. 2). About 40 % is in the form of snow, which cover the area from December till February.

Table 1. Average monthly and annual temperatures Station І ІІ ІІІ ІV V VІ VІІ VІІІ ІХ Х ХІ ХІІ Annual temperature Shabla 0,6 2,0 4,4 9,8 15,7 20,1 22,5 22,1 18,0 13,2 8,9 3,8 11,8

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Figure 1. Scheme of lakes Durankulak and Shabla ( http://www.natura2000bg.org)

Photos 1 Lock of Durankulak Lake Photos 2. Orlovo (Kartaliisko) marsh

Table 2. Average monthly precipitation sums (mm) Station І ІІ ІІІ ІV V VІ VІІ VІІІ ІХ Х ХІ ХІІ Annual sums Durankulak 34 30 24 35 44 45 36 29 29 43 51 38 438 Shabla 42 36 29 35 40 42 34 30 36 47 60 49 480 Balchik 35 33 30 34 40 45 31 40 25 39 50 38 440

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mm oC 70 25

60 20 50

15 40

30 10

20 5 10

0 0 I II III IV V VI VII VIII IX X XI XII

Figure 2. Graph of temperature and precipitation for Shabla

Geology and hydrogeology. The typical rocks in the region are sediments. The Quaternary is represented by sands (wich distribution is mainly to the sand strip and the beach), clays (which occur in the bottom parts of the lake and the river valleys) and loess (which spread on wide extends of the lake’s catchment area), the Miocene - by Sarmatian limestones (which are of paramount importance for the accumulation of the underground waters in the lake’s catchment area). The underground waters are of punctum - karst type and non-pressure character. The thickness of the water-saturated limestone is different – between 35-50 m (in the western part) and 120-150 m (in the east part, near the Black Sea). The level of the underground waters is from 10-12 till 50-60 m. The underground flow is from west to east (Antonov, Danchev, 1980). Relief. The relief around the lakes is plain. The typical is karst topography – limestone pavements and etc. There are a lot of dry valleys. Hydrographic features. The catchments of Durankulak and Shabla Lakes are part of Dobridga region, which is without surface water. Their areas are respectively 476 km2 and 220 km2. The hydrographic network includes short dry gulches. There are some big gulches in the catchments of Durankulak - Neikovsko (length 72 km and watershed 430.8 km2), Giorinsko and Granichno and in the catchments of Shabla – Shablenska (length 25.6 km and watershed 118.4 km2) and Ezeretska (length 20.3 km and watershed 101.5 km2) rivers (Hristova, Zelezov, 2005). The flow in their beds is transient and due to rainfall or thaw. That’s why the gulches have not so important for the feeding of the lakes and why the groundwater is very important for the water regime. Soils. The main soil type in region is chernozem (black soils) – the carbonate, the proper and the leached. There are diluvia-meadow soils also. They are distributed around lake and cover the beds of valleys. Flora. Some type’s vegetation covers the region. The water plants and the read occupy littoral parts, the xerophytic vegetation, the trees (acacia, polar, black pine) and bushes – small part of the region. Anthropogenic activities The most important anthropogenic activities for the hydrological regime of lakes are water consumption. The underground waters are used for supplying drinking and for irrigation. There are two irrigations and one drinking water station in the natural site of Durankulak, seven pumping stations in the site of Shabla and a lot of drainage canals. One of these canals finish in the Durankulak Lake and it makes possible confiscation of water from the lake. The water supply in Durankulak complex is 6.5.103 m3 and in Shabla complex - 14-15.103 m3 per year.

The regime of water levels Water levels fluctuate naturally as a result of changes in precipitation, snow melt, groundwater inflow, evaporation, and outflow. That’s why the elements of water balance are so important for the regime during the year and for a lot of years. Water balance The water balance is comprised of: 1) water supply from precipitation, surface water and inflow from underground waters and 2) water loss from outflow to Black sea, transpiration and evaporation. The inflow from the underground water is constantly and the biggest for the lakes - 9.2.106 m3/y (79 % from total inflow) for Durankulak Lake and 21.7.106 m3/y (95 % from total inflow) for

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Shabla Lake. The water supply from precipitation is biggest during October-December and it is 2.0.106 m3/y for Durankulak and 0.3.106 m3/y for Shabla Lake (Fig. 3 and Fig. 4). The surface flow is respectively 0.47.106 m3/y and 0.44.106 m3/y. The main outflow from lakes is the outflow to Black sea – 7.1.106 m3/y for Durankulak and 7.1.106 m3/y for Shabla (Fig. 3 and Fig. 4). Water loss from evaporation and transpiration is for Durankulak 2.3.106 m3/y and for Shabla Lake – respectively 0.9.106 m3/y and 1.9.106 m3/y (Fig. 3 and Fig. 4).

INFLOW TO DURANKULAK lAKE OUTFLOW FROM DURANKULAK LAKE 4% 17% 20%

61% 79% 19%

underground w ater rainfall surface flow outflow to Black sea evaporation transpiration

Fig.3 Water balance of Durankulak Lake

INFLOW TO SHABLA LAKE OUTFLOW FROM SHABLA LAKE

3% 2% 4% 9%

87% 95%

underground w ater rainfall surface flow outflow to Black sea evaporation transpiration

Fig.3 Water balance of Shabla Lake

The fluctuation of annual water level The average of annual water levels for Durankulak Lake is 43.9 sm and for Shabla Lakes – 119.5 sm. The fluctuations are big - between 27.5 (1954) and 71.2 (1981) sm for Durankulak and between 40.4 (1964) and 157.4 sm (1967) for Shabla Lake. Coefficients of variation are respectively 0.82 and 0.74. There are trend in the annual level of two lakes – the water level is higher after 1970 year for Durankulak Lake and after 1965 year for Shabla Lake (Fig. 4).

240

190

140

90

water level, sm 40

-10

-60

2 6 4 51 59 72 79 950 958 963 964 971 978 1 19 195 1954 1955 1956 1 19 1960 1962 1 1 196 1967 1968 1970 1 19 197 1975 1976 1 19 1980 1982 1983

Shabla Durankulak

Fig.4 Water levels for period 1950-1983

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The ratio between annual maximum and minimum of water level for Durankulak is 3.25. It is 4.84 for Shabla and shows, that the fluctuations are bigger. The reason is that there are some rivers, which flow input in lake during the period with precipitation. The average of highest water levels for Durankulak Lake is 49.3 sm and the fluctuations are in diapason 23.2 -75.5 sm. The absolutely high level is 98 sm (March and April, 1981). Coefficient of variation is 0.54. The highest level increase after 1970 year like the annual levels. The average of highest water levels for Shabla Lake is 119.5 sm and their variations are between 66.7 -169.9 sm. The absolutely high level is 224 sm (March, 1967). The chronology of high water levels is similar of annual water level – positive trend after 1965 year. The average of law water levels for Durankulak Lake is 39.3 sm, the fluctuations are from 15.6 and 66.6 sm, and the coefficient of variation is 0.55. The trend of law water levels is positive. The average of law water levels for Shabla Lake is 108.3 sm, the fluctuations are between 42.2 and 144.8 sm, and the coefficient of variation is 0.52.

The regime of water level The regime of water level for Durankulak Lake and Shabla Lake changes seasonally. It is sensitive to temporary surface water and not dependent from precipitation (Fig. 5).

Water level and presipitation for Water level and presipitation for Shabla Durankulak Lake Lake

70 7 160 7 60 6 140 6 50 5 120 5 100 40 4 4 80 3 30 3 60 20 2 2

water level, sm 40 sm precipation, water level, sm level, water 10 1 precipation, sm 20 1 0 0 0 0 ІІІІІІІVVVІ VІІ VІІІ ІХ Х ХІ ХІІ І ІІ ІІІ ІVVVІ VІІ VІІІ ІХ Х ХІ ХІІ

Fig. 5 The dependence between water level and precipitation

The regime of two lakes is similar (Fig. 6 and 7). The water level begins to rise in November, when the precipitation is highest. Then levels continue to increase till March for Shabla Lake and till April for Durankulak Lake (Table 3 and Table 4, Fig. 7). The levels are high till June and they are 40-65 sm for Durankulak and 105-145 sm for Shabla Lake. The period with law levels is from July till October, when the evaporation is high. The minimum of water level for Durankulak Lake is September, for Shabla Lake – in August. sm 160 140 120 100 80 60 40 20 0 I II III IV V VI VII VIII IX X XI XII

Durankulak Lake Shabla Lake

Fig.6 The water level of two lakes

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Durankulak Lake Shabla lake

65 160

60 140 55

50 120

45 100 XI XII I II III IV V VI VII VIII IX X 40 XI XII I II III IV V VI VII VIII IX X

35 80 water sm level, water level, sm 30 60 25

20 40

Fig.7 Periods with high and law water levels

Table 3 Water levels for Durankulak Lake (sm) Water months level І ІІ ІІІ ІV V VІ VІІ VІІІ ІХ Х ХІ ХІІ Нaverage 49.4 55.9 57.0 60.1 60.4 51.3 35.3 26.1 24.5 27.2 32.7 40.4 Hmах 53.4 61.9 63.5 62.6 63.4 56.2 45.8 32.3 28.7 30.7 36.8 45.7 Hmin 43.1 48.2 51.3 55.4 53.4 44.7 30.6 22 21.8 24 28.4 35.8

Table 4 Water levels for Shabla Lake (sm) Water months level І ІІ ІІІ ІV V VІ VІІ VІІІ ІХ Х ХІ ХІІ Нav. 131.8 139.7 142 138.8 127 105.1 77.8 58.8 64.68 88.7 107.7 122.6 Hmах 139.3 152.3 148.9 147.3 137.4 121.9 94.4 67.5 72.5 100.7 119.4 132.8

Hmin 126.3 137.4 139.5 133.6 119.4 89.2 63.3 50.8 54.8 75.5 99.6 115.6

The frequency of monthly maximums for Durankulak Lake is biggest – 27.6 % in April (Table 5). It is high in May too. The picture for frequency of monthly maximums for Shabla Lake is a little different – the biggest frequency is in March – 50.1 % (Table 6). So, all monthly maximums (without some) of Shabla Lake are registries in two months – March and April. The frequency of monthly minimums is in the summer. It is biggest in September for Durankulak Lake (37.9 %) and for August (55.9 %) for Shabla Lake.

Table 5 Frequency (%) of monthly maximums and minimums for Durankulak Lake months І ІІ ІІІ ІV V VІ VІІ VІІІ ІХ Х ХІ ХІІ Нmax 3.5 17.2 13.8 27.6 24.4 10.0 - - - - - 3.5

Hmin ------31.0 37.9 17.3 10.3 3.5

Table 6 Frequency (%) of monthly maximums and minimums for Shabla Lake months І ІІ ІІІ ІV V VІ VІІ VІІІ ІХ Х ХІ ХІІ Нmax 2.9 5.9 50.1 35.3 2.9 ------2.9 Hmin 2.9 - - - - 2.9 5.9 55.9 26.6 2.9 - 2.9

Conclusions Lakes Durankulak and Shabla are coastal wetlands. They have got the similar natural features in their catchments, but different water balance and fluctuations. The input flow from groundwater for Shabla lake is more then Durankulak Lake, but the monthly and annual fluctuations of water level is biggest then Durankulak lake. There are tow reasons for that: the big input flow from dry gulches and anthropogenic activities. The period with high level is from November till June. The period with law levels - from July till October.

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Acknowledgements The present was supported by Ministry of education and science.

References Antonov, Hr., D. Danchev. Podzemnite void v Bulgaria. (1980). S. Tehnika. Hristova N., Zelezov G. (2005). Hydrographic picture of Ludogorsko-Dobrudzanski region. A nnuaire de l Universite de Sofia “St. kliment ohridski”, Faculti de geologie et geographie, livre 2 – geographie, tome 98. http://www.natura2000bg.org Ivanov K. (1964). Ezerata v Bulgaria. Trudove IHM, volume XVI, pp. 21-23 Management plan „Durankulak lake natural complex”. (1998).Bulgaria, Varna. Management plan „Shabla lake natural complex”. (1998).Bulgaria, Varna. Velev St. (1990). Klimatat na Bulgaria. DI “Narodna prosveta”.

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