1. Natural conditions and important structures

1.1. Danube, Danubian Lowland, Geology, Hydroelectric Power Project

Danube, Donau, Dunaj, Duna, ? a poetic river, reappearing in its untouched shape, accompanied by the nostalgic melody of Johann Strauss?s On the Beautiful Blue Danube waltz. The natural evolution of the Danube and the changes resulting from a dynamic development of civilisation, along its banks contributed to the present character of the Danube, which seems to be as untouched as European nature in general (Fig. 1.1).

The historical changes of the Danube system are a consequence of geological development and the often changed climatic relations during Quaternary time. One has to include the changes in the volume and movement of gravel and fine sand in the Danube, a deepening, increasing and meandering of the riverbed, sedimentation and erosion, and frequent floods. The nature has also been affected through intensive felling of forests, preparation of new agricultural land, intensive draining measures, and the construction off irrigation systems and river dikes. At the same time, the changes caused by urbanisation, industrialisation, population growth, transportation and communication systems development, transformation to a modern agriculture based on chemicals, as well as the overall chemical contamination, all have to be taken into consideration (Fig. 1.2).

It is beyond question that the current condition of the Danube and its flood-plain is the result of centuries of human intervention. It is a river that has contributed greatly to the development of the States sharing the Danube basin. It is a river that has been extensively utilised for navigation, water supply, fishing and more recently for hydroelectric power production and other purposes. I is equally beyond question that whenever measures are taken to modify the flow of a river, as contemplated by the Gabčíkovo ? hydroelectric power project, there will be environmental effects, some adverse. This is true of all projects. The same modern technology that has made possible complex river projects has also led to techniques to measure the environmental impacts and to avoid, offset, mitigate, or remedy them. In the EC Fact Finding Mission report [2] it was concluded that "the environmental impacts of reducing the discharge in the Danube are negative, unless proper remedial actions are taken". As will be shown below, such impacts were dealt with and with a great deal of success.

Independent EU experts [1] in November 23, 1992 outlined the state and trends in the area, "Before the 18^th century the Danube split downstream from Bratislava into two almost identical arms. Near Bratislava it was partly a braided river with many small islands, as a result of progressive sedimentation where the Danube entered into the plain. Both arms were however meandering river systems and the Little Danube (Malý Dunaj) still is. Large changes occurred during the 19^th century, when the first regulation works started. Within several decades the system changed into a braided river. Some of the older branches are still present in the landscape" (Fig. 1.3).

"With the past endikements, especially during the last century, flood peaks became steeper and higher, flooding more frequent but in general with a shorter duration. The original zoning in vegetation towards higher grounds and associated forests was largely ?diked? out of the system. Most of the higher, no longer flooded soils, were converted into agricultural lands" [1].

"These river regulation works led to a deliberate and natural cutting off and bundling of river branches into one main, straightened and heavily fortified channel for navigation. This remaining channel is characterised by rapid water level fluctuations and very large stream velocities. The cut off branches, behind the fortified river banks, are only activated at higher discharges. Within the river branches many small weirs and dams were built, so most of them behave like cascade systems at low discharges. The interaction with the side arms so created became limited." According to the experts of the Commission of the European Communities [1], flow in almost all river arms in pre-dam condition existed on an average of only 17 days per year, see Fig. 1.5.

The Gabčíkovo ? Nagymaros Project consists of two parts, or steps, the Gabčíkovo part of the Project and the Nagymaros part of the Project. The Gabčíkovo part of the Project (Fig. 1.4) is situated in the central part of an intermountain depression, the Danube basin, called in Slovakia "Podunajská nížina" (Danubian Lowland) . The Danube basin is filled by Late Tertiary (marine and lacustrine sand, fine sand, clay, sandstone, shales) and Quaternary sediments (river Danube sand and gravel settled in fluvial or lacustrine conditions). The total depth of the Quaternary and Tertiary sediments is 8000 m, with the uppermost Danube River sediments creating a main aquifer of high permeable gravel and sand. The thickness of the river Danube sediments, or the Danubian aquifer, ranges from a few metres at Bratislava to more than 450 m at Gabčíkovo, and goes back to a few metres downstream of Sap in the direction towards Komárno. Beneath this, a system of substantially less permeable aquifers and aquitards exist.

The important factors in the Danube transport and sedimentation processes are the existence of a granite threshold connecting the Alps and the Carpathians in the area of Bratislava, with an outcrop of granites in the Danube River bed. A similar hard rock river threshold, predominantly of andesite rock is situated at Nagymaros (between the cities of Štúrovo-Estergom and Visegrád-Nagymaros), some 160 km downstream from Bratislava. Both thresholds are natural geological hydraulic barriers, steps or thresholds, in the river bed. These are the upstream and downstream geological boundaries of the aquifers and the hydrological barriers naturally damming the Danube River bottom (Fig. 1.4).

Typical for such thresholds are a high gradient of the riverbed, high water-flow velocities and therefore lower navigation water depth, higher erosion downstream of such a threshold, moving fords, meandering of river and river arms, etc. The part of the river at Bratislava, just downstream from such a threshold, is a typical example. The flow velocity is high, the aquifer is shallow but with an extremely high hydraulic conductivity (permeability). Two municipal waterworks are situated at the granite threshold one on each side of the river. The Bratislava waterworks is on the Danube left side, Sihoť island, and is more than 100 years old. The second waterworks at Pečniansky les is on the Danube right side. These waterworks supply Bratislava with drinking water of some 1500 and 600 l/s, respectively. Both river banks in front of these waterworks are natural. And this is the place where the impact of the Gabčíkovo step starts, with a slight increase of the Danube water level.

Just downstream from Bratislava the Danube forms two branches, the Malý Danube in Slovakia and the Mosoni Danube in Hungary. These branches create two analogous islands, "Žitný ostrov" in Slovakia and "Szigetkoz" in Hungary. In the Gabčíkovo part of the Project, between Bratislava and Medveďov, the Danube formed an "inland delta" region, in geological literature expressed as an alluvial fan, through which it once meandered. This "inland delta" has its original typical morphology, i.e. meandering river, coarse sediment accumulation and erosion, changes in river bed gradient, etc. This large alluvial fan consists of an highly permeable extensive aquifer, capable of carrying and transferring high volumes of ground water. The Danube flows on the top of this "fan", see Fig. 1.6. Water from the Danube infiltrates into the fan sediments and flows downward as ground water through the Danubian Lowland, nearly in parallel with the Danube river. In the lower part, where the slope of the river and the surrounding area suddenly decrease to the one quarter of its gradient at Bratislava, the ground water flows back into the Danube river via its own river arms, the Danube tributaries, and the drainage canals (Fig. 1.7). All this occurs because of the lowered permeability, and lowered aquifer thickness downstream from Gabčíkovo, which is a result of changed sedimentation conditions upstream of the andesite hard rock threshold barrier at Nagymaros.

The hard rock granite threshold and the andesite threshold, which naturally dam the Danube river bottom, and the places where the alluvial fan ends (a sudden decrease of river gradient from 40 to 10 cm per kilometre) are also important from the viewpoint of decision making. At these places there have been proposals to situate the hydropower stations known as Wolftal, Nagymaros, and Gabčíkovo, respectively (Fig. 1.4).

According to the mutually agreed plan and Treaty 1977[5] between Hungary and Slovakia, the Gabčíkovo-Nagymaros project is hydrologically connected to the previously planned Slovak - Austrian hydroelectric power plant at Wolfsthal, upstream from Bratislava, and to the project Adony, downstream in Hungary (river kilometre - rkm 1601). The technical proposal is in accordance with the concept of the Rhine-Main-Danube and Danube-Oder-Elbe navigation system and with all hydropower stations and dams on the Danube.

In the German sector of the Danube, some 26 hydroelectric power projects have been completed. In Austria, ten hydroelectric power plants with navigational locks are in operation on the Danube. A chart listing these Austrian plants and the year of construction appears bellow.

Tab. 1.1. List of Austrian hydroelectric power plants on the Danube
 

Power plant Year Jochenstein ? with Germany 1956 Ybbs ? Persenbeug 1959 Aschach 1964 Wallsee ? Mitterkirchen 1969 Ottensheim ? Wilhering 1974

Power plant Year Altenwörth 1978 Abwinden ? Asten 1980 Melk 1983 Greifenstein 1985 Freudenau (Vienna) 1997

1.2. The Gabčíkovo part of the Gabčíkovo ? Nagymaros

The Gabčíkovo part of the hydroelectric power project Gabčíkovo-Nagymaros was based on a combination of flood control, navigational improvements, production of electrical energy and protection of nature. In their working group report [1] independent experts of the Commission of the European Communities, stated on November 23, 1992: "In the past, the measures taken for navigation constrained the possibilities for the development of the Danube and the flood-plain area. Assuming that navigation will no longer use the main river over a length of 40 km, a unique situation has arisen. Supported by technical measures, the river and flood-plain can develop more naturally".

It emerges from the report of the Commission of the European Communities tripartite fact-finding mission [2], dated 31 October 1992, that "not using the system would have led to considerable financial losses, and that it could have given rise to serious problems for the environment".

1.3. The main structures of the Gabčíkovo step

The hydroelectric power station (Fig. 1.8), consisting of four blocks in which eight turbines and generators have been installed. They are all vertical Kaplan turbines, with runners 9.3 m in diameter and a maximum capacity of 90 MW each. The total installed capacity of the hydropower station is 720 MW with an operational discharge of 4000 m³/s. Minimal and maximal discharges are 413 and 636 m³/s per turbine, inversely related to water level differences of 24.0 and 12.88 m, respectively.

Two navigation locks serve passing ships and barges sailing along the Danube. Each lock is 275 m long and 34 m wide. The difference in water levels between the upstream and downstream canal varies from 16 to 23.3 m.

The bypass canal, consist of the headwater section upstream from the navigation locks, a hydroelectric power station, and a tail-race section (outlet canal) downstream from the power station.

The Čunovo reservoir is a part of the original Hrušov-Dunakiliti reservoir, which is situated exclusively on Slovak territory. The area of the originally designed Hrušov-Dunakiliti reservoir is 6000 hectares, and of the Čunovo reservoir approximately 4000 hectares, depending on water level. The operational water level at Čunovo is about 131.1 m a.s.l. (above the Baltic Sea level); the minimal and maximal operational levels are 129 and 131.5 m a.s.l., respectively. Ensured navigational depth is 3.5 m, according to requirement of the Danube Commission.

The intake structure at Dobrohošť supplies the inundation river branch system with water, it enables flood simulations for forestry and ecological purposes. The discharge capacity is up to 240 m³/s.

The original function of the Dunakiliti weir in the Gabčíkovo part of the Project is fully substituted by the Čunovo weir constructed on the Slovak territory and inside of the original reservoir area, upstream from the Dunakiliti weir.

Because at present the construction of the Nagymaros part of the Project on the Hungarian territory has not been built, the Gabčíkovo power station is operated as a run-of-the-river plant in a ?water-level regime?, meaning that the head water level is fixed and the allowed water level fluctuation +/-4 cm at a low flow discharge of up to 1500 m³/s, and +/-15 cm at a higher flow discharge.

1.4. Ecologically and socially important structures and areas

The main parts of the area and of the Gabčíkovo hydroelectric complex having ecological importance and importance to the regional development are shown in Fig. 1.9:

1. The Čunovo reservoir is a new biotope incorporating typical conditions of river and flood-plain ecotopes as, for example, the slowly- and fast-flowing main river beds, through-flowing deep and shallow river branches, flooded areas, and through-flowing lakes with variable depths and diverse flow velocities. The Čunovo reservoir is raising the surrounding ground water level to the level known 30 years ago, before bundling of river branches into one main, straightened and heavily fortified channel for navigation. .
2. Upper part of the Čunovo reservoir includes the original Danube riverbed, suitable for rheophilous species, a long shallow bay, suitable for limnic species, and numerous islands with diverse banks, suitable for macrophytes and waterfowl.
3. Lower part of the Čunovo reservoir includes a deep water area with linear and S shaped hydraulic structures, a waterfowl island, and an area for storing mud and fine sediments in the future.
4. At the ancient city of Šamorín there is projected harbour for yachts and sport vessels.
5. Linear hydraulic structure is designed to ensure sufficiently high flow velocities in front of the waterworks at Šamorín and to maintain high reservoir bed permeability without the deposition of fine sediments at places where ground water recharge towards the waterworks? wells takes place.
6. S-shaped hydraulic structure ensures a partially rotational flow and force sedimentation where it is harmless or advantageous. A function of this structure is also to minimise algae eutrophication.
7. Protected nature areas:

• Protected Landscape: CHKO Dunajský luh (Danube flood-plain), established on the May 1, 1998, as a response to the new hydrological conditions.
• Nature reserve localities: Ostrov Kopáč, Topoľove hony, Gajc, Hetméň, Jurovský les, Ostrovné lúčky.
• Protected sites: Bajdel, Poľovnícky les, Dolný hon, Park v Báči, Park v Rohovciach, Park v Kraľovičových Kračanoch, Park vo Vrakúni, Park v Gabčíkove.
• National nature reserves: Ostrov Orliaka morského, Číčovské mŕtve rameno.
• Nature monuments: Pánsky diel, Kráľovská lúka.
8. After damming the Danube its original river bed has a lower discharge (at present, according to the Agreement between Republic of Hungary and Slovak Republic signed in 1995, discharges are between 250 and 600 m³/s), a lower but more variable and more suitable flow velocities, cleaner water, a narrower river bed and more natural river banks. The river bottom is more stable and more suitable for lithophilous species. There are excellent conditions for nesting and the wintering of waterfowls, especially in severe winters, because the Danube is recharged by warmer ground water infiltrating during the summer from the reservoir. The riverbed resembles a large river arm, similar to the earlier original state before the heavy stony bank stabilisation of the Danube. The abundance of aquatic organisms, mainly the littoral organisms is much increased and the food variety and amount available is much larger than under pre-dam conditions.
9. The seepage canals with on both sides of the reservoir and by- pass canal were designed to channel excess seepage water from the reservoir, to regulate the reservoir-evoked raising of ground water level, and to control the ground water level fluctuation. Water level can be regulated by gates to within a 2 m amplitude. Seepage canals with nearly drinking water quality are suitable new biotopes for some waterfowl, aquatic flora and fauna, and amphibians.
10. The waterworks at Šamorín, under present conditions of increased ground water recharge and raised ground water level, have the discharge capacity of 1200 l/s. Ground water quality was not changed.
11. The Waterworks at Kalinkove, under present conditions of raised ground water level, have the discharge capacity of 600 l/s. Ground water quality was not significantly changed.
12. Perspective water sources locality "Na pieskoch" is an excellent reserve for the future.
13. Waterworks at Rusovce are situated in the area where the ground water level was significantly raised. The ground water quality was, by some parameters, significantly improved, on the area of the waterworks hygiene protection zone, and the discharge capacity is at present at least 2480 l/s.
14. The area of water sports at Čunovo is constructed mainly for wild water sports and the transport of small sport boats between the reservoir and the Danube. It also serves partly as the fish passage between the Danube and reservoir.
15. A polder was filled with gravel to take off the stagnant water body from the area of the waterworks at Rusovce.
16. A bay was filled by gravel, to hinder the concentration of waterborne (and floating rubbish) pollution in front of the Mosoni Danube intake structure.
17. The intake structure for the Mosoni Danube and the small hydropower station was originally designed to provide a permanent and to some extent variable water supply of 20 m³/s into the Mosoni Danube, Zátonyi Danube and Hungarian river branches the whole year. At present it yields up to 40 - 50 m³/s. It is possible to regulate the discharge. In pre- dam conditions the Mosoni Danube was directly supplied with water from the Danube only about 50 days a year, by discharges in the Danube over 3000 m³/s (Fig. 1.5, Fig. 1.14).
18. The raised water level in the Danube improved the discharge control via the intake structure for the Malý Danube.
19. An intake structure at Dobrohošť designed to supply water to the Danube side arms on the Slovak territory takes water from the by- pass canal. The discharge capacity is 240 m3/s. The intake structure supplies the inundation area and river branches with water, and simulates water level fluctuation and floods for forestry and ecological purposes, e.g. the period needed for laying fish eggs.
20. An intake structure to supply side arms on Hungarian territory is situated directly in the Dunakiliti weir is at present not in use. The discharge capacity is up to 200 m³/s.
21. There exist a system of intake structures supplying irrigation canals.
22. Partly sealed bottom of the reservoir serves to diminish the infiltration of surface water directly in front of the waterworks at Kalinkovo.
23. The underwater weir at Dunakiliti, constructed by Hungary in the framework of the Agreement between the Republic of Hungary and Slovak Republic signed in 1995, is designed to raise the Danube water level and to allow direct water connection and flow from the Danube into Hungarian river branches via openings in the river bank. Discharge into branches is regulated by the water level regulation at the Dunakiliti weir. The discharge capacity is over 200 m³/s, according to the river bank opening shape, underwater crest level and water level regulated by Dunakiliti weir.
24. The inundation weir may be used to direct a part of the flood waters into the Danube riverbed and inundation area downstream from the damming of the Danube at Čunovo, usually, if the Danube discharge is over 6,000 m³/s.
25. The bypass weir was designed to channel and regulate flow discharge into the Danube, and to channel ice floes during construction of the Čunovo structures including hydropower station, ship lock and weir. The long-term capacity of the weir is 600 m³/s. At present the weir is used as an auxiliary weir, regulating discharge into the Danube downstream of the damming, and partly as a fish passage. In the future it can be fully adapted as a suitable fish passage.
26. The Čunovo weir is designed to regulate the discharge into the Danube riverbed, the water level in the reservoir, and to release ice floes and reservoir sediments.
27. An auxiliary navigation lock at Čunovo, connecting the reservoir with the Danube, can be used for navigation, for technical purposes, and for smaller and tourist ships.
28. The small hydropower station at Čunovo uses up to 400 m³/s of the discharge from the reservoir into the Danube riverbed.
29. The bypass canal ( diversion, power canal) , which is a continuation of the Čunovo reservoir, directs the water to the power station and serves as a navigation canal. The bypass canal can handle a flood discharge of up to 5300 m3/s. The maximum flow velocity will not exceed 1.5 m/s during flood situations. The main ecological advantage of the bypass canal is that the navigation will no longer use the main river over a length of 40 km. A flood discharge of 5300 m³/s in the bypass canal lowers the discharge in the Danube during a flood situation and protects the Szigetköz area. The bypass canal and the Gabčíkovo navigation locks are the main structures allowing a transfer of navigation away from the main river over a length of 40 km.
30. A system of cascades in the inundation on the Slovak side from Dobrohošť to Gabčíkovo, raises water level and enables the regulation of water levels in river branches of up to 2 m. Together with discharge control at Dobrohošť, it is possible to inundate the flood plain, to simulate a flood, to remove settled organic material from the main branches, and to control the ground water level fluctuation in the flood plain. Similar system has been developed in the Hungarian inundation.
31. A system of hydrogeochemical experimental observation wells, constructed during the PHARE project [3, 4] in 1993, is used to study ground water chemistry and ground water quality processes.
32. The Gabčíkovo Hydroelectric Power Station is producing environmentally clean energy ( 2-2.5 GWh annually) and regulating the water level in the reservoir. The Hydropower Station does not directly influence the level of air pollution; however, production of net energy associated with savings of fossil fuels is contributing to a decrease of Slovak emission of CO₂, SO₂, NO_X, and ash by some 5 - 7% .
33. Dams are popular as cyclistic and touristic routs.

1.5. Discharges and water levels in the Danube, Malý Danube and Mosoni Danube

Discharge and water levels in the Danube, Malý Danube, Mosoni Danube, and river branches have been measured on a number of gauging stations (Fig. 1.10). Fluctuation of discharge and water levels is one of the main characteristics of the Danube. The fluctuation of discharges observed in Bratislava and Komárno are shown in (Fig. 1.11). Simple linear regression lines demonstrate that the long term changes of discharge are, at least in Bratislava, negligible. The annual average discharge in Bratislava is 2.025 m³/s, the minimum measured discharge is 570 m³/s and the highest measured discharge is 10400 m³/s. Predictable 100, 1000 and 10 000 year floods are 10 600, 13 000, 15 000 m³/s, respectively.

The same data of the Danube discharge at Nagymaros are 2421, 590, 8180 m³/s for measured discharge, and 8700, 10 000, 11 100 m³/s for predictable floods. It should nevertheless be noted that the data concerning the measured discharge in Nagymaros are influenced by the occurrence of two disastrous floods in 1954 (Hungarian territory) and 1965 (Slovak territory) during which large areas of the territories were flooded and the part of the discharges were thus dispersed in the region. The retention function of the flood-plain area between Bratislava and Komárno is clearly expressed in the lowering of the peak discharge at Nagymaros gauging station. For example, the highest measured discharge at Bratislava, 10 400 m³/s, was reduced to only 8180 m³/s at Nagymaros.

The water level in the Danube is a result of the discharge, depth and shape of the riverbed, including the flood-plain area, which since the last century is restricted to the area between flood protection dikes. The Danube water levels measured at Bratislava, Rusovce, Gabčíkovo and Komárno are presented in Fig. 1.12. Comparing Fig. 1.11 and Fig. 1.12 it is evident that the discharge fluctuation did not change, but the water level was continuously decreasing. To show better the long term development of water levels, a linear regression line is plotted through the data. Computed changes of discharge and water levels at gauging stations for the last 30 years before putting the Gabčíkovo project in operation, using linear regression, are given on Table 1.2.

The discharge in the Malý Danube (one of two main branches of the Danube) is measured at a gauging station at Malé Pálenisko (Fig. 1.13), Nová Dedinka and Trstice. The discharge in the Mosoni Danube (one of two main branches of the Danube) is measured at a gauging station at Mecsér (Fig. 1.14) and at Dunakiliti.

Table 1.2. Decrease of average discharge and average water levels in the Danube
 

Locality	River km	discharge in m3/s	water level in m
Bratislava	1868.7	12.84	1.32
Rusovce	1855.9		1.10
Gabčíkovo	1819.6		0.20
Medveďov	1805.4		1.05
Kližská Nemá	1792.4		1.14
Zlatná na ostrove	1779.2		0.98
Komárno	1767.1	74.63	0.63

The long-term lowering of the water level in the Danube was one of the factors leading to the decrease of discharges in the Malý Danube and the Mosoni Danube, to lowering of ground water levels, and to changes in the ground water flow directions and velocities. This resulted, among other things, in changes in ground water flow quantities and in a general decrease of the exploitable ground water resources. In the upper part of Žitný ostrov there have also been some other factors influencing the ground water regime; for example, the constructing of the hydraulic blanket around the ?Slovnaft refinery, the development of Petržnaft? refinery, the development of Petržalka suburb on the right side of the Danube, including protective dikes and impermeable wall, and the construction of the waterworks supplying the capital and

1.6. Ground water level regime

There is a basic network of more than 1000 observation wells in the Hungarian and Slovak area where the ground water levels are measured (on the Slovak territory mainly by the Slovak Hydrometeorological Institute (SHMÚ) (Fig. 1.10). Several methods are used to present ground water level data. The most common is the well hydrograph, which visualises the ground water level fluctuation through time and which is the first step to characterising the ground water level changes.

The ground water level fluctuation is basically conditioned by mutual relationship and hydraulic interconnection between the Danube river water and other surface waters with ground water and the relationship between precipitation and evaporation. This ground water level fluctuation is further influenced by various other factors, such as drainage or irrigation of agricultural soils, regulation of water level in seepage canals, etc. It can be seen from Fig. 1.15 that in the region near the Danube the shape of ground water level fluctuation corresponds closely to the water level fluctuation in the Danube. At a larger distance the fluctuation is dependent upon the season and the relationship between precipitation, including snow melting, and evapotranspiration. The irrigation canal network, drainage facilities and melioration have a stabilising effect on ground water levels. The linear regression line is drawn in the figures to show the drop of average ground water levels in a long-term pre-dam period. The long term decrease on both sides of the Danube is evident over a part of the area. The average ground water level represented by the linear regression line for a chosen date is called a reference ground water level. The reference ground water levels were used for ground water level contour map for the years 1962 and 1992, see Fig. 1.16, and these were used for drawing up the ground water level changes between 1962 and 1992 see Fig. 1.17. From these figures a considerable decrease of ground water level, which occurred in the last 30 years (before putting the Gabčíkovo part of the Gabčíkovo-Nagymaros Project into operation) is evident in the upper part of the Danubian lowland.

The impact of the damming of the Danube (putting of the Gabčíkovo part of the Project into operation by means of the dam at Čunovo, which has taken over the role originally destined for the works at Dunakiliti) can be evaluated on the background of the long term development shown on Fig. 1.18a, and in detail on Fig. 1.18b. Fig. 1.18 also represents changes in the trend of the ground water level changes after putting the Gabčíkovo part of the Project into operation. To show the present situation in ground water level, map of average ground water levels for the period between July 1995 and July 1998 (Fig. 1.19), have been plotted. Comparing this map with reference water level contours characterising pre-dam conditions (1992 in Fig. 1.16), Fig. 1.20 was plotted. This map represents the general impact of putting the Gabčíkovo part of the Project into operation on ground water levels.

The changes in the ground water levels observed in the flood-plain area, and generally in the whole region, confirm the positive impact of the Project, in particular on the upper part of Žitný ostrov, and an important positive role of the water supply system for the left side Slovak flood-plain area since 1993, and the right side Hungarian flood-plain area since 1995.The observations support the expectation that, after completion of the water supply facilities for the remaining part of the flood-plain area in the vicinity of the tailrace canal and construction of some underwater weirs in the Danube a positive impact on ground water will occur here too. The measurements of the ground water levels confirm that there is a general trend towards the re-establishment of the situation known some 30 years ago on the greater part of the territory. Such ground water level situation is more natural.

1.7. Ground water level and soil moisture regime

It is evident that as far, as the impact of the Project on soils, agriculture, forestry and environment in general is concerned, the central role belongs to the change in ground water levels and ground water level fluctuation regime and to the changes in the ground water interaction with the soil. This impact is transferred via the aeration zone through capillary transport up to the soil. The soil moisture is strongly conditioned by the availability of precipitation water (rain, snow melting and irrigation water) and water transported from the ground water via capillary rise. This influences the plant transpiration, the soil aeration and temperatures, the vertical transport of nutrients, chemicals and pollutants, and also the long-term development of soils and soil structures.

The capillary rise is determined mainly by the character of sediments or the type of soil, their thickness, the ground water level (depth) and its fluctuation. The capillary transport in gravel deposits is nearly zero. The maximum capillary transport exists in loess (eolian sediments). Good capillary transport exists in finer sediments such as fine sand silt, loam and agricultural soils.

For agricultural production it is important in which sediment and soil horizon the ground water level fluctuates and what the depth and course of ground water level fluctuation is, and mainly whether the ground water level in course of its fluctuation touches sediments with good capillary transport ability or not.

In the area of Szigetköz and Žitný ostrov the most important feature of the interaction of the ground water with the soil is the depth of the boundary between the gravel strata and overlying finer sediments or soils. If the ground water level during the growing season is permanently in the finer sediments overlying the gravel, such depth is optimal from the agricultural production point of view. This optimal depth of ground water level generally ranges from 0.6 m to 2.5 m (for maize slightly more and for barley slightly less). Water logging of soils take place only if ground water level is too shallow, mostly in the depth close to the surface as is usual in some zones in the flood-plain. Usually it occurs if the ground water is at depth of 0 - 0.5 m. In agricultural areas with shallow ground water level the optimal depth of ground water level is ensured by drainage systems. This is the case, for example, in the eastern (lower) part of Žitný ostrov. Shallow ground water level in the flood-plain is welcome; it supports a typical flood-plain biotope and is naturally regulated by the river branches.

General changes