Assessment of Flood Monitoring And Forecasting in the Danube river basin 1. In General about the Danube River Basin International cooperation of Danube countries has a long tradition especially as far as the utilization of the Danube River as a natural water-way for navigation and transport is concerned. An intensive economic and social development of Danube countries necessitates optimum water utilization not only in the Danube itself but also in its tributaries – i.e. within the whole drainage basin – for drinking and process water supply, hydropower and navigation purposes. The need to protect population and property from disastrous floods led to an effective cooperation of Danube countries. The Danube with a total length of 2 857 km and a longterm daily mean discharge of 6 500 m3.s-1 is listed immediately after the River Volga (length 3 740 km, daily mean discharge 8 500 m3.s-1) as the second largest river in Europe. In terms of length it is listed as 21st biggest river in the world, in terms of drainage area it ranks as 25th with the drainage area of 817 000 km2. The Danube River Basin (DRB) extends in a westerly direction from the Black Sea into central and southern Europe. The limits of the basin are outlined by line of longitude 8° 09´ at the source of the Breg and Brigach streams in Schwarzwald Masiff to the 29° 45´ line of longitude in the Danube delta at the Black Sea. The extreme southern point of the Danube basin is located on the 42° 05´ line of latitude within the source of the Iskar in the Rila Mountains, the extreme northern point being the source of the River Morava on the 50° 15´ line of latitude. Eighteen countries shared the Danube catchment: significant countries - Germany, Austria, Czech Republic, Slovakia, Hungary, Slovenia, Croatia, Serbia, Bosnia and Hercegovina, Romania, Bulgaria, Moldova, Ukraine – the smallest, almost insignificant parts of the catchement area are in Switzerland, Albania, Italy, Poland, and Montenegro. The source of the Danube tributaries adjoin the source of the Rhine tributaries in the west and northwest; The Weser, Labe, Odra and Visla river basin in the north, the Dnester river basin in the northeast and basins of the rivers flowing into the Adriatic and Aegean Sea in the south. The River Danube drainage basin includes glacier-covered mountains, mid-mountain chainsclad with forests, karst formations devoid of growth, highlands and uplands, table lands plateaus with deeply cavered river valleys, and wide plains and depression parts. About one-third of Danube River basin is mountainous, while the remaining two thirds consist of hill and plains. The mean altitude of the river basin is only 475 m, but the maximum difference in height between the lowland and alpine Peaks is over 3,000 m. The Danube River basin is devided into four parts: the Upper region, the Middle region the Lower region and the Danube delta. 1 The Upper Region extends from the source tributaries to the Gate of Devin at the confluence with Morava. After the confluence of the river Brigach and Breg, the river is called the Danube (Donau in German). Downstream from this point the river follows the fault gap through the german Alp and its well-shaped valley. Major tributaries from the south, including the river Iller, Lech, Isar, Inn, Salzach, Traun, Enns and Ybbs drain the alpine sub-basins and augment the discharge in the Danube substantially. For example, the river Inn has a larger mean annual discharge then the Danube itself at the confluence. Orginally, these mountain tributaries transported large amounts of sediments, but sediment load is now greatly reduced because of the construction of hydraulic works. Major tributaries from the north are the rivers Naab, Kamp and Morava/March. The Morava is the most important and drains the Czech part of the Danube river basin and smaller areas of Slovakia and Austria. The Middle Region is the largest of three regions, extending from confluence with the River Morava at Bratislava to the gorge of the Danube at the Iron Gate dam. Here the Danube enters a flatland region. The sediment-carrying capacity of the river is reduced and gravel and coarse material have been deposited. At times the river separates into branches forming several islands. The major tributaries in the region are from the left; the rivers Váh, Hron, Ipe and Tisza and from the right, the river Leitha, Raab, Drava, Sava and Velika Morava. Down to the confluence with the river Drava the mean runoff rate has increased to about 5,700 m3/s. At Velika Morava, the 117 km long gorge section begins between the Carpatian and Stara Planina (the Balkan Mountains). This gorge has been filled by large reservoirs for hydropower and navigation. The Lower Region is formed by Romanian – Bulgarian lowland and its surrouding upland plateaus and mountains. From the mouth of River Timok to Silistra, the Danube forms the Romanian-Bulgarian border, flowing mostly east. The Danube flows as a wide (800 m), slow-moving River, with extensive, well developed alluvial plains especially on the left bank. The inundated area during floods can reach a width up to 10 km. The right bank is a narrow riverine plain flanked by steep bank. Much of the sediment load of the Danube is deposited behind the Iron Gate Dams, resulting in active riverbank erosion in this floodplain. The Danube Delta covers an area of about 600,000 ha. It was created by the division of the river into tree main branches, forming a triangle with about 70 km long sides. Almost two-thirds of the delta area is seasonally submerged due to the low absolute altitude. The whole of Romanian part of the delta was declared a Biosphere Reserve in 1990 and registred under the Ramsar Convention. Over half of its areas is listed under the World Heritage Convention. Some 15,000 ha of the Ukrainian part of the Deltais protected the rest being used for agriculture. The population in the Romanian part of the delta is only 15 000, while the Ukrainian part has about 68 000 inhabitants. 2 Right bank Left bank 2 Black Sea, 1 000x km2 Black Sea, 1 000x km Increase area of the Danube River basin 3 2. Climate and Hydrological Conditions of the Danube River Basin Climatic conditions of the DRB are influenced by its position in the moderate climatic zone of the Northern Hemisphere, with regular alternating of the seasons of the year. Due to the elongated shape of the DRB in the west–east direction the climatic conditions are variable. In the main contributing areas the complicated orographic structure of the Alpine and Carpathian regions has the most significant impact on climatic variables. Differences extend from the Upper Danube with high Atlantic influence to the eastern territories affected by continental climate. South of the Alps and in the middle Danube Basin, especially in the Drava and Sava basins, the climate is influenced by the Mediterranean. Interaction of the mentioned three main effects can trigger floods in the Carpathian basin in any period of the year. The range of fluctuation of mean monthly air temperatures between the warmest and coldest months increases from the upper Danube basin with 20-21 oC to the confined Carpathian basin with 22-24 oC and to the lower Danube reaches with 26 oC. Average annual air temperature within the basin ranges from -6.2 to 12 oC. The lowest value originates from the Alpine summits, the highest mean annual temperature was observed at the Black Sea coast. In the entire DRB July is the warmest month, January being the coldest one. Winter usually lasts from December to February. The summer is usually hot and lasts from about June to August. The absolute range of recorded temperatures is from -41 oC to 45 oC. The hydrological regime, especially the runoff conditions of the Danube, is substantially influenced by precipitation. Average annual precipitation fluctuates within the range of above 3000 mm in high mountains to 400 mm in the delta region. In upper Danube regions precipitation fluctuates between more than 2000 mm in the upland areas of the Alps and up to 600 – 700 mm in mid-altitude. However, the actual figures can deviate drastically from the long-term mean values. There has been recorded daily precipitation of more than 260 mm at the upper Danube. The catchments of central Danube regions are characterised by similar range. Annual precipitation varies from just above 500 mm in the middle Tisza region to above 2000 mm in the high mountains. Contributing areas of the Upper Drava and Sava in the Julien Alps and in the Kupa spring region have the highest value up to 3800 mm. In the plains of the lower Danube the precipitation is only 500-600 mm, though the lowest annual values are below 400 mm. The number of days with snow cover, the duration and depth of snow increase with the altitude. The Alpine regions have an average such days less than 60 while at elevations above 3000 m more than 190. The shortest duration of snow cover (~10 days) is at the Black sea coast. The snow cover last for only 20-30 days in the Hungarian Plains, 40-60 days in the upper Danube basin, the mean proportion in the total annual precipitation being 10% - 15%. In the Alpine foothills and in high regions of mid-mountains the snow cover lasts 4 more than 100 days (20% to 30 % of precipitation falls in the form of snow). The snow cover remains more than four months in the highest regions of the Alps (above 1500 m a.m.sl.).