1. RIVER AND RIVER BASIN

RIVER SYSTEM AS PART OF A GLOBAL WATER CYCLE

A river is a large stream of water flowing in a natural riverbed into another stream, or into a lake or sea. A river starts as a tiny brook, connects with other brooks into a larger stream forming river tributaries, and these tributaries connect into even larger streams, and so on until this large stream of water, as a complex river system, reaches a sea. Rivers are an essential part of the global water cycle, and at the same time, an important part of the freshwater system, which connects surface waters with groundwater. Rivers are the veins of landscapes.

Surface waters are all the waters on the surface of the continents, encompassing springs, streams, rivers, freshwater and salt lakes, floodplains, wetlands, marshes, deltas and freshwater lagoons, while bodies of seawater (the marine environment) include seas and oceans. Intermittent water bodies such as lagoons and estuaries contain brackish water.

RIVER BASIN OR WATERSHED/CATCHMENT, WATER DIVIDE

The network of small streams growing to a bigger river, discharging into a sea, forms the hydrographic network of a catchment, which is also referred to as a river drainage system. The total area including the landscapes from where tributaries catch and drain water into a river (mountains, hills, plains) is called a river basin or watershed. One river basin is separated from another by a line, called water divide, formed along the highest points of a mountain ridge or higher ground on a plain separating two adjacent watersheds.

Running waters feature typical seasonal flow patterns, mainly dependent on climate, weather conditions and precipitation. For example, the glacial hydrological regime in high mountains shows peak flow in summer (snowmelt), while the pluvial regime in the lowlands shows peak flow in winter (rainfall). There are a mean discharge (m³/s), floods, and droughts. Sediment transport is related to flow dynamics, inducing erosion and deposition (accumulation), thus shaping the landscapes (valleys, gorges, plains, meanders, islands, etc). Along the river course, the sediments change from boulders to pebbles to gravel to sand and to mud, due to abrasion processes. Sediments are habitats for aquatic biota (plants, invertebrates, and fish) that are adapted to these natural dynamics (in detailed explained in section 3).

In a river basin, ground waters and surface waters are connected into one water system. The source of a river is a spring, the point at which groundwater hits the impermeable layer of ground and comes running in the open. A river can also start as a mountain torrent that collects all the runoff water. Smaller rivers are connected with a larger river into a larger river system. Large river systems can connect to each other, forming even larger river basins. For example, in the Danube Region, the basins of the Rivers Sava, Drava, Mura, and Tisza are all connected into one large river basin – the Danube River Basin, from which all the water drains into the Black Sea.

The largest water basin on the Earth is the Amazon River Basin, which covers an area of 7.18 million km². In comparison, the Danube River Basin (DRB) encompasses 0.8 million km², representing approx. 10% of continental Europe and extending on the territories of 19 countries, thus making it the most international river basin in the world.

DANUBE RIVER BASIN

In its course of 2,857 km, from the Black Forest to the Black Sea, the Danube flows through 10 countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania, Moldova and Ukraine (Fig.2). The river has 27 large tributaries and over 300 small tributaries, reaching an average discharge of 6,500 m³/s when it meets the sea.

The longest tributary of the Danube is the Tisza River (966 km), followed by the Prut (950 km) and the Drava (893 km). The Tisza River has also the largest catchment (157,186 km²), followed by the Sava River (95,719 km²) and the Siret River (47.610 km²). The Sava is the river with the largest discharge (1.564 m³/s), followed by the Tisza River (794 km m³/s) and the Inn River (738 m³/s). The most important tributaries of the Danube are presented in Table 2.

The Danube Basin also includes a high number of lakes, the largest being Lake Balaton, the largest lake in Central Europe, with a surface of 605 km², Lake Razim, located in the Danube Delta, Lake Romania (surface 392 km²), and Lake Neusidlersee, located at the Austria-Hungary border (surface 315 km²).

In addition to its surface waters, the Danube Basin also includes groundwater. A water divide that separates one river basin from another is usually a ridgeline. Sometimes, however, when the ground is made of permeable rock formation (like in the karst regions), a water divide is formed underground.

DANUBE WATER CYCLE

The water cycle includes several components: precipitation, evaporation, transpiration, and condensation (Fig.3).

In the Danube Basin, the maximum average precipitation amounts to about 3200 mm/year in the western part, in the high mountain regions of the Alps, while the lowest levels are in the lowland area at the Black Sea, with only 350 mm/year (Figure 4; Brilly, 2010).

The temperature plays an essential role in the water cycle. In the Danube Basin, the average temperature shows increasing values from the upper part of the basin (Ulm 20◦C,

Vienna 21 °C) to the middle part (Budapest 23 °C, Beograd 23 °C) and the lower flatland (Bucharest 26 °C) (Brilly, 2010).

The evaporation mainly depends on the available solar energy (sunshine) and water. In the Danube Basin, the highest evaporation is recorded in regions where high amounts of precipitation are combined with high temperatures, such as in the Sava Valley and the slopes of the Carpathians (about 700 mm/yr). The largest part of the catchment has yearly rates of 500–650 mm/yr, while the lowest values are found in the alpine mountains (100 mm/yr), South Carpathian mountains and Danube Delta (400 mm/yr).

DANUBE WATER BALANCE

To present a water balance in a watershed in a simple way, we can use analogy of balancing a household budget with income, savings and expenditures. In hydrology, a water balance is a balance of gains and losses of water described as the flow of water in and out of a water system: precipitation minus evapotranpiration minus changes in storage (due to snow, glaciers, lakes, groundwater) = discharge that fills the rivers.

The basic information on the Danube water balance components (annual mean precipitation, evapotranspiration and ruun off) presented in Table 3 was taken from an article »Water Resources in Danube River Basin« (Prohaska, S., 2013). According to the article, »the highest precipitation is registered in Slovenia (1,308.5 mm) and the lowest in Moldavia (579.8 mm) while the average annual precipitation falls to 784.6 mm in the Danube Basin. … The spatial pattern of evapotranspiration shows considerably lower variation compared to precipitation and it ranges between 343.0 mm in Switzerland and 582.1 mm in Croatia.« The mean annual evapotranspiration in the Danube Basin is 513.5 mm. »The higest spatial distribution is registred for the runoff with values of 916.6 mm in the upper Danube Basin, Switzerland and only 74.0 mm in Moldavia. The average annual runoff in the Danube Basin is 292.2 mm.«