On the Flood Forecasting at the Bulgarian Part Of
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BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004 Hydrology Of Prespa Lakes Molnar Kolaneci Hidrometeorological Institute Tirana, Albania Abstract The levels of the Prespa Lakes follow an annual cycle with peak levels in May and June and the low water levels in autumn. Superimposed on this annual cycle are longer period fluctuations caused by particularly wet or dry periods. After 1988 it is observed a continuing and fast decrease of water level in Big Prespa Lake. Up to this year the amplitude of level oscillation from the beginning of the regular observations (1952) was about 4 m with maximum of 852.91-m a.s.l. in 1963 and minimum of 848.91 m a.s.l. in 1978. In 1989 started a fast decrease and is still going on. For the period from 1988-1989 to 1995-1996 (7-8 years) it was observed the amplitude of 4.17 m, the same as it was observed for the period of 36 years (1952- 1988). After a short period of 4 years (1997-2000) of stagnation with small increasing of water level, restarted the decreasing and as consequence the minimal historic water level of Big Prespa Lake is observed in 2002. This phenomenon caused the separation of Small Prespa from Big Prespa Lake. In last years the communication of these two lakes is interrupted. This is a serious problem and constitutes a big threatens for the Prespa Lake ecosystem. Exchanging of hydro meteorological data between the three neighbouring countries and cooperation between professional institutions is the only possible way to define a real water balance of Prespa Lake. Concerning the water inflow from Albanian part, there are not natural surface inflows into both lakes. In Small Prespa in Greek territory there are small temporary streams that inflow into the lake. There are several rivers flowing into Big Prespa Lake: Agios Germanos River in Greek territory and Golema, Kranjska, Brajchinska and Istocna River in Macedonian territory. There are no natural outflows by surface way in all Prespa Lakes. However, there has been considerable human modification of the hydrology of the area, which has to be taken in consideration. So, in 1976 in Small Prespa was created a channel which a water from Devolli River has been is turned in during the wet and withdraw water from the lake for irrigation during the dry period. However, due to the existing artificial channels, the level in Small Prespa Lake is less oscillating. The water balance of Small Prespa Lake shows a good equilibrium between inflow and outflow elements for the long-term period even some of these elements require more accurate estimations. The water balance of Big Prespa Lake demonstrates that the most interesting and in the same time the most difficult element of the outflow by underground way is not yet defined properly. Also in this case the same elements require more accurate estimations. Introduction Prespa Lakes (Big and Small) and Ohrid Lake constitute the biggest water system in Balkan. The surface area of Big Prespa is about 253 km2 and about 47 km2 is the surface area of Small Prespa. Prespa Lakes are situated in an altitude of ca. 850 m a.m.s.l. with high mountains (up to 2000 m) around them. Big Prespa and Small Prespa are linked by a small channel, which traverses the alluvial isthmus that separates the lakes. It is well known, the water communication of all these lakes by both: surface and underground ways. The last one is due to the presence of a deep and very developed karst region between Prespa and Ohrid Lake (Galichitsa Mountain in Macedonia and Dry Mountain in Albania. The fact that three countries, Macedonia, Greece and Albania, share this complex water system, make more difficult a real hydrologic analyse. For example the way of water connection between two Prespa is in the Greek territory, the outflow of Big Prespa is through the well-known Zaveri abyss (Albanian territory) to St. Naumi springs (Macedonian part) and Tushemishti springs (Albanian part), the outflow of Ohrid Lake is Black Drini River in Macedonian part. In all this facts must be taken in account the artificial flow-out- Lakes 1 BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004 pumping water for irrigation from Prespa Lake (Big and Small) in all three countries. This situation needs good collaboration between researchers of the three countries and exchange of scientific information in order to create a Unique Database of this system. Level oscillation in Prespa Lakes The water levels of the Lakes follow an annual cycle with maximum levels in May and June and the low water levels in autumn. This is a typical character of continental water regime. Superimposed on this annual cycle are longer period fluctuations caused by particularly wet or dry periods. After 1988 it is observed a continuing and fast decrease of water level in Big Prespa Lake (see fig.1). Up to this year the amplitude of level oscillation from the beginning of the regular observations (1952) was about 4 m with maximum of 852.91-m a.s.l. in 1963 and minimum of 848.91 m a.s.l. in 1978. In 1989 started a fast decrease and in one year it was observed a falling of more then 1 m of mean annual water level. This decrease is continuing up to minimal historic, which is observed at the end of 1995 and the beginning of 1996. For the period from 1988-1989 to 1995-1996 (7-8 years) it was observed the amplitude of 4.17 m, the same as it was observed for the period of 36 years (1952- 1988). After a short period of 4 years (1997-2000) of stagnation with small increasing of water level, restarted the decreasing and as consequence the minimal historic water level of Big Prespa Lake is observed in 2002. This phenomenon caused the separation of Small Prespa from Big Prespa Lake. In last years the communication of these two lakes is interrupted. Different specialists have discussed the causes of this catastrophic continuing falling of water level in Big Prespa Lake. Until now, three would be the main hypothesis about this spectacular phenomenon: - Tectonic falling of the lake bottom - Widening of underground channels connecting the two lakes - Influence of meteorological parameters For the first hypothesis, the geologic researchers must tell their opinion. Probably the two other hypotheses are more close to the reality. The widening of underground channels hypothesis can be proved by detailed analyse of discharge measurements in St. Naumi and Tushemishti springs for the period 1988-1996, especially for the years1989-1990. In Albanian part the discharge measurements made in Tushemishti springs during this period show net decreasing of water flow, even the number of measurements is scarce. After 1996 seemed that the tendency is going to the normality concerning the spring discharges but not on water levels in Big Prespa Lake (little increasing). For the third hypothesis must be analysed accurately all meteorological parameters for the same period (1988- 1996), even the diagram showing the distribution of the precipitation during all the observation period of Liqenas station (in Albanian part) doesn’t agree with this idea; the fluctuation in long term period has no trend. The fact that in Small Prespa Lake this phenomenon was not observed (see fig. 2), decreases the chances that this hypothesis being true. However the phenomenon we are discussing is extremely complex and none of hypothesizes must have priority above the others. To simplify the question, the problem is to find how are disappeared more then 800 million m3 in Prespa Lake in order to protect this natural ecosystem, in which, we all are common owners. Lakes 2 BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004 Water levels Big Prespa H m. a.s.l. 1952 - 2002 854.0 853.0 Max. historical level 852.91 m a.s.l. (1963) 852.0 851.0 850.0 Mean 849.0 Min 848.0 Max 847.0 Max. Amplitude 8.49 m 846.0 845.0 844.0 Min. historical level 844.42 m a.s.l.(2002) 843.0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Years Figure 1 Water levels of Big Prespa at Liqenas Water level of Micro Prespa Lake H cm 1955-2002 400 350 300 250 200 15 0 10 0 50 Mean Min 0 Max -50 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Years Figure 2 Water levels Small Prespa at Tren Lakes 3 BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004 Rainfall Gorica 1951 - 2002 R m m 14 0 0 12 0 0 10 0 0 800 600 400 200 0 19 5 1 19 5 6 19 6 1 19 6 6 19 71 19 7 6 19 8 1 19 8 6 19 9 1 19 9 6 2 0 0 1 year Figure 3 Distribution of rainfall Gorica station Rainfall Liqenas 1951-2002 R mm 1400 1200 1000 800 600 400 200 0 1951 1956 1961 1966 1971 1976 1981 1986 1991 1996 2001 year Figure 4 Rainfall of Liqenas (Pusteci) 1951-2002 Water balance of Prespa Lakes The method of water balance can be very useful for determining one element when the others are known. In case of Prespa Lake is not possible to define a real water balance without exchanging of hydrometeorological data between three countries and co-operation between professional institutions. The previous studies made on water balance in each country are a good base and will serve as reference points for the future.