1.  INTRODUCTION

1.1  Background and Objectives for the Working Group

As a follow-up of the 'Special Agreement for Submission to the International Court of Justice of the differences between the Republic of Hungary and the Slovak Republic concerning the Gabcikovo-Nagymaros Project' a Temporary Water Management Regime for the Danube has to be established and implemented.

In order to provide reliable and undisputed data on the most important effects of the current water discharge and the remedial measures already undertaken as well as to make recommendations for appropriate measures a Group of Monitoring and Water Management Experts for the Gabcikovo System of Locks (Working Group) was established by the Republic of Hungary, the Slovak Republic and the Commission of the European Communities (CEC). The Terms of References for the Working Group are enclosed in Appendix A.

The Working Group is composed of the following five experts:

The five experts were assisted by colleagues as listed in Appendix B. The Working Group had its first formal meeting on September 8-9, 1993 in Bratislava. The second meeting was held in Budapest during the period October 27 November 2, 1993. Field inspections were carried out on October 30 both in Slovakia and in Hungary. In between the two formal meetings comprehensive work on data collection and analyses were carried out by the Slovak and Hungarian experts and interaction with the CEC experts also took place during this period.

The Working Group has to prepare two reports. The present report, which is the first one, comprises an assessment of the impacts of the Gabcikovo project with regard to discharges, surface water levels and quality, sedimentation and erosion, ground water levels and quality, flora and fauna, agriculture, forestry and electricity production. Furthermore, recommendations for strengthening of the monitoring system in the area are given.

The second and final report, scheduled for the beginning of December 1993, will comprise recommendations for the governments for a Temporary Water Management Regime as well as for necessary discharges, water levels and remedial measures to be taken.

The Working Group has obtained most of the relevant data and information requested from the two Governments. The report is based on this information.

1.2  The Gabcikovo Project

The hydraulic structures and their capabilities with regard to water management as per November 22, 1992 are described in ref /1/. Since then developments have taken place with regard to:

A more detailed description of these structures and their water management capabilities will be given in the final report of the Working Group devoted to Temporary Water Management Regime.

2.  DISCHARGE

2.1  Available Data

The amount of discharge data in the area is comprehensive. Daily data from the locations listed in Table 2.1 have been analysed. The locations of the stations are shown on the index map in Fig. 2.1

For many of the stations, especially the ones located at the Danube itself, historical time series exist for several decades. In the data analyses presented below mainly the data from the period 1991 - 93 have been considered. However, also the most important long term trends have been analysed.

The discharge at the bypass weir at Cunovo is estimated from the setting of the gates. The same procedure could in principle be used also for the inundation weir at Cunovo. However, due to the large width of this weir small uncertainties in water level observation would result in very large uncertainties on the discharge through the weir. Therefore, the discharge is not measured, but has to be estimated from e.g. measured discharge data at Rajka.

At Mosoni Danube historical discharge data exist from the mid 1980's onwards.

In order to assess the impacts of rapid changes of operation of the bypass weir at Cunovo and of the turbines at Gabcikovo hourly discharge values have been analysed for a few selected days.

2.2  Data Analyses - Long Term Trends

The long term trend of the Danube discharge can be evaluated from Fig. 2.2, which shows the Danube discharges (and water levels) at Bratislava for the last 40 years. In addition to the actual data the linear regression line is shown in the figure. Evidently, there is no significant long term trend in the Danube discharge.

Historical discharge data from the Little Danube, ref. /2/, show a clear decreasing trend from the mid 1970's to 1992. This is a result of a general decrease in Danube water level at Bratislava, cf. Section 3.2.

2.3  Analyses of 1991-93 Data

Hydrographs of daily discharges have been plotted for all stations, ref. /2,3/, for the period after January 1991. Furthermore, the monthly average values have been calculated.

Selected hydrographs for the main river system are shown in Fig. 2.3, while discharges to the Little Danube, Mosoni Danube, right side seepage canal and intake to the Slovak flood plains are shown in Fig. 2.4.

A summary of 1993 discharge values stations are given in Table 2.2.

From Fig. 2.3 and Table 2.2 it is evident that the Gabcikovo Project has had a very large impact on the discharge regime in the Danube between the weir at Cunovo and the downstream confluence at Palkovicovo. In this reach (the Old Danube) the discharge has in 1993 been reduced to about 20 % as compared to the pre-dam condition. Fig. 2.5 shows the ratio between the Danube discharge at Rajka and Bratislava from January to August 1993. It is noticed that the Rajka discharge during this period has varied between 11% and 50% of the Bratislava discharge.

From Fig 2.4 it appears that the discharge to Little Danube has been increased with about 10 m³/s, representing approximately a doubling as compared to the pre-dam conditions. Similarly, the discharge to Mosoni Danube has been significantly increased. Finally it may be noted that with the water intake from the power canal at Dobrohost to the Slovakian flood plains the water flow through the side arms has been very significantly increased as compared to the pre-dam conditions, which most often were characterized by stagnant water.

An analysis of the uncertainty/consistency of the discharge measurements can be made by considering the following continuity conditions:

Because the discharge through the inundation weir is not measured equation (b) is strictly only applicable for periods with no discharge through the inundation weir. In other situations only the Q_b2 = Q_b3 is valid.

Summary results of this consistency check based on monthly average discharges are given in the second half of Table 2.2. From the table the following findings appear:

The uncertainties, which should be considered in further assessment of these findings are:

These two loss terms have to be subtracted from Q_a1 when comparing with Q_a2. Hence, the discrepancy reduces to approximately 50 m³/s (3%).

On this basis the following conclusions can be drawn with regard to accuracies in the discharge measurements:

2.4  Adequacy of the present Monitoring System

The number and location of discharge measurement stations are adequate.

For obtaining firm conclusions on the discharge uncertainties the following checks are required to be carried out by joint Hungarian-Slovakian teams:

3.  SURFACE WATER LEVEL

3.1  Available Data

The amount of surface water. level data in the area is comprehensive. Data from the locations listed in Table 3.1 have been analysed. Some of the data are average daily values based on raw data from e.g. hourly manual readings or automatic recorders, while at other stations only immediate water level readings are made once per day. The locations of the stations are shown on the index map in Fig. 3.1

For many of the stations, especially the ones located at the Danube itself, historical time series exist for several decades. In the data analyses presented below mainly the data from the period 1991 - 93 have been considered. However, also the most important long term trends have been analysed.

3.2  Data Analyses for Long Term Trends

The long term trend of the Danube water levels can be evaluated from Fig. 2.2 which shows the Danube water levels at Bratislava for. the last 40 years. In addition to the actual water level data the linear regression line is shown in the figure. Evidently, there is a significantly decreasing long term trend in the Danube water levels at Bratislava of about 1.5 m. Between Dunaremete and Gabcikovo the water levels show no clear trend for low flows and a small increasing trend for high flows. The reason for this decrease is described briefly in Section 5.2.

3.3  Analyses of 1991-93 Data

Water level hydrographs have been plotted for all stations, ref /2,3/, for the period after January 1991.

Selected hydrographs for the main river system are shown in Fig. 3.2, while water levels in the Little Danube and Mosoni Danube are shown in Fig. 3.3.

From Fig. 3.2 it is evident that the Gabcikovo Project has had a very large impact on the water level regime in the Danube between Bratislava and the downstream confluence at Palkovicovo. At Bratislava the 1993 water levels during low flow periods have increased by 1-2 m as compared to pre-dam conditions, i.e. the same order of magnitude as the long term decrease during the past 40 years..

From Fig. 3.2 it furthermore appears that the water levels in the 0ld Danube at Rajka and Dunaremete in 1993 have been reduced by 2-4 m to a level 2 m below the lowest ever recorded values. In addition, the characteristic natural dynamics of the water level fluctuations have been changed (reduced) significantly so that the water level is now more or less constant for several weeks.

From Fig 3.3 it appears that the 1993 water levels in Little Danube have been increased with l-2 m as compared to pre-dam conditions. For the Mosoni Danube at Mecser no significant water level changes have occurred.

During the first three months of operation of the weirs at Cunovo unnaturally large and rapid water level fluctuations were generated in Old Danube. Since then the discharge through the weirs at Cunovo have basically been held almost constant except during flood operations where the inundation weir has been used.

Hourly variations in discharges through the turbines at Gabcikovo are shown for three selected days in Fig. 11.2, from where it appear that the discharge variations from hour to hour typically is in the order of 100 - 300 m³/s. Such discharge variation will generate downstream water level fluctuations up to 1 m. Larger water level fluctuations were generated on three occasions, 18-19 June 1993, 26-27 September 1993 and 14 October 1993, with the purpose of assisting large ships through shallow water around Nagymaros.

3.4  Adequacy of the present Monitoring System

In general, the present monitoring of surface water levels appears adequate. However, at present the water levels in the side channels of the flood plains are not monitored on a routine basis on either side of the river.

Furthermore, in areas where rapid water level fluctuations within a day occur it is recommended to substitute the manual observation practice with an automatic recorder.

4.  SURFACE WATER QUALITY

4.1  Available Data

The amount of surface water quality data in the area is comprehensive. The station network is shown in Fig. 4.1, while the analysed parameters are listed in Table 4.1. In addition to this routine program some special measurements of organic pollutants have been made. The routine program has been carried out in Slovakia for l030 years (depending on parameter) and in Hungary for 10-30 years (depending on parameter).

In addition to the above network a program of surface water quality monitoring in the reservoir has been initiated in 1993. This monitoring comprises the key parameters for assessment of eutrophication conditions.

4.2  Data Analyses for Long Term Trends

All the time series of the Slovakian data are plotted in ref /2/. The Hungarian Data Report /3/ provided data from only one station.

The Danube water quality can according to Hungarian classification be categorized as 1st class regarding the majority of the components, as 2nd class regarding Ph, orthophosphate, nitrate, BOD and 3rd class with regard to bacteria and some heavily degradable substances such as e.g. hydrocarbons.

Due to high oxygen content, low organic carbon contents and the very small quantities of fine grained sediments the surface water quality is generally well suited for river bank infiltration, which is the major source of water supply along the Danube between Bratislava and Budapest.

The water quality of the side branches differs from that of the main Danube channel due to the much lower velocities and periods and places with stagnant water. In drier years a negative trend has been observed with high pH, high organic matter and low oxygen contents.

The major sources of water pollution on the Slovak section of the Danube are river Morava and Bratislava.

4.3  Analyses of 1991-93 Data

With exception of November - December 1992, when sudden. changes of regime and a high flood event occurred, no significant changes in surface water quality parameters as compared to pre-dam conditions can be detected after damming the Danube. (See also Section 8.3).

4.4  Adequacy of the present Monitoring System

The present monitoring system appears adequate as far as the Danube and the reservoir is concerned. However, there is a need for monitoring of surface water quality in the side channels in the flood plains.

5.  SEDIMENT TRANSPORT AND SEDIMENTATION/EROSION

5.1  Available Data

The data available on sediment transport and erosion aspects are limited.

In Slovakia, a comprehensive monitoring and analyses program was carried out in the 1950's and 1960's, but limited field data have been collected since then. During the period September 1992 to August 1993 a sediment transport field monitoring program was conducted in connection with the "Danubian Lowland - Ground Water Model" PHARE project. This program comprised amongst other regular measurements of suspended transport at Bratislava and Medvedov as well as analysis of bed material at different locations in the reservoir.

In Hungary river cross sections are regularly surveyed for the main Danube. Such measurements have also been made in 1993 for comparison with pre-dam conditions (September 1992). However, these data are presently only partly processed. Furthermore, Hungary usually carry out suspended sediment sampling in three sections (Rajka, Dunaremete, Medve) each year simultaneously with the discharge measurements. Occasionally, bed load measurements are carried out in the same sections. In 1992 this was done once, while in 1988-89 there were 13 measurements in this river stretch. Bed material sampling was done approximately for every 1-1.5 km of the river (evidence sections).

5.2  Data Analyses for Long Term Trends

The main channel has been significantly lowered due to erosion caused by a combination of several man made factors:

Until the damming of the Danube, erosion took place between Bratislava and Dunaremete. Similarly, sedimentation occurred downstream of Sap/Palkovicovo.

In some places the river bed has been lowered more than two meters since the 1960's, leading to lower ground water levels, occasional drying out of river branches (e.g. down-stream of Bratislava) and less flushing of most river branches. The lowering of the riverbed during the past 30 years has been particularly large between Bratislava and Rajka. It is estimated to be about 0.8 meter at Gabcikovo and near Bratislava about 1.5 meter.

According to Hungarian measurements the quantity and concentrations of suspended load on the Danube reach at Rajka has shown a decreasing trend during the past 30 years, see ref /1/.

5.3  Analyses of 1991-93 Data

Qualitatively, the effect of the damming at Cunovo is highly significant on the sedimentation/erosion balance in the Old Danube. Two counteracting processes are important. On the one hand, most of the transported material of the river have already settled upstream in the reservoir (all bed load and 60% of the suspended load according to Slovakian predictions, ref /2/). On the other hand, the water velocity has been reduced very much. The sedimentation is therefore likely to continue and result in finer bed sediments. This will have some implications for the permeability of the river bed.

For quantitative analyses few data exist enabling some tentative but no firm conclusions regarding the impacts due to the Gabcikovo Project.

During the November 1992 flood approximately 2-3 mill m³ of sand and gravel material were eroded in the first 500 m downstream the inundation weir where the bed protection works were not yet completed. This material has been deposited downstream in the Old Danube. The 2-3 mill m³ may be compared to the amount of dredged material (for navigation purposes) between Cunovo and Medvedov, which according to ref /2/ is 2.9 mill m³ over the last 45 years. As a preliminary conclusion from the Hungarian surveys of river cross sections it can be stated that in some parts of the Old Danube river bed downstream Cunovo (around 1850 rkm and 1811 rkm) there is 1-2 m high freshly sedimented material. This is supported from the observations made during the Working Group's field visit, where a new island of deposited material could be seen in the middle of the river bed.

In Fig. 5.1 suspended sediment concentrations taken in 1993 are compared to pre-dam data. At the two stations in the Old Danube, Rajka and Dunaremete, virtually the same sediment concentration levels have been found after the damming as compared to the pre-dam conditions. However, as the discharges have been very much reduced a significant change in the relationship between discharge and sediment concentration is noticed. Finally, it appears that the impacts for the station downstream the confluence, Medve, may not be significant on the present (small) data basis.

5.4  Adequacy of the present Monitoring System

There is a need for establishment a permanent sediment transport measurement programme comprising both bed load and suspended load measurements at the following locations:

Furthermore, there is a need for establishment of permanent programs for monitoring river bed and reservoir topography.

6.  GROUND WATER LEVEL

6.1  Available Data

The amount of ground water level data in the area is comprehensive. The network of observation stations is shown in Fig. 6.1. On the Slovak side monitoring is carried out on a weekly basis for about 300 stations and by use of automatic recorders for more than 100 stations. On the Hungarian side the monitoring is carried out mostly by use of automatic recorders. For many of the observation wells historical time series exist for several decades.

6.2  Data Analyses for Long Term Trends

The ground water regime is to a large extent determined by the permeability of the river channels and the variations in river water table. In the reach between Bratislava and Komarno an estimated 10 - 20 m3/s infiltrates to gravel aquifers on the Slovakian side and 8 - 15 m3/s between Rajka and Medve on the Hungarian side. This constitutes one of the largest ground water resources in Central Europe. Due to the very large permeabilities in the gravel aquifer the ground water flow rates are very high (1 - 3 m/day).

The depth of the ground water table, shown in ref /1,2,3/, ranged in the pre-dam condition from more than 5 meters close to Bratislava to around 1 m at Medve. The trend over the past 30 years is illustrated in Fig. 6.2 showing river water level at Bratislava together with ground water levels from five we11s located with distances from 0.8 km to 12 km on a transect perpendicular to the Danube on the Slovakian side.

As indicated in Fig 6.2 and documented in ref /1,2/ the ground water levels have decreased ranging from about 2 meters around Bratislava to about zero at Komarno. This decrease is due to erosion of the river bed.

A very important feature of the ground water regime is the large ground water level fluctuations generated by the dynamics of the river water table. This is illustrated in Fig. 6.2, where the fluctuations in the wells located close to the Danube are largely determined by the fluctuations in the river water levels, while ground water level fluctuations further away from the river mainly depend on the annual variation in recharge from rain and snow. Because Szigetköz is narrower than Zitny Ostrov the ground water fluctuations over the entire Szigetköz is dominated by the Danube.

6.3  Analyses of 1991-93 Data

Ground water level hydrographs have been plotted for all stations, ref /2,3/, for the period after January 1991. Due to the large and rapid ground water level fluctuations generated by the Danube it is not possible to accurately assess the impacts of the Gabcikovo projects by direct comparison of observed ground water levels for short periods before and after the damming. Therefore, both the Hungarian and the Slovakian experts have developed regression models for computations of ground water levels corresponding to the pre-dam condition.

Selected hydrographs are shown in Figs. 6.3 and 6.4. Fig 6.3 shows the ground water levels at three wells in Hungary. The three wells are located at 50 m, 400 m and 4000 m distance from the Danube, see map. At the two wells located closer to the Danube (Rajka and Lipot) the ground water levels were in the beginning of 1993 reduced by 1.5 - 2.0 m corresponding to the decrease in the Danube water level. After May 1993 the reduction at Lipot decreased to about 1.0 m. At Darnozseli the reduction is initially about 0.4 m and after May gradually changes to about 0.2 m. The timing of this reduced impact coincides with inundation of the side channels in the Slovak flood plains. However, the exact reason for this reduced impact (i.e. increased ground water levels) after July 1993 has not been documented. In all cases the ground water level fluctuations have been reduced significantly.

Fig. 6.4 shows similarly the ground water levels at three wells in Slovakia. The good agreement between observed and computed values in the pre-dam conditions is noted. The effect of the damming is clearly seen in all three wells. At well 694, located close to the reservoir, the ground water level has increased by 2-3 m. At the other two wells, located 10-15 km away from the reservoir in the flood plain area and just behind the intake canal, respectively, the ground water level initially decreased significantly. However, after discharging water into the side channels in the Slovakian flood plain from May 1993 onwards the ground water levels have increased above those corresponding to pre-dam conditions. This demonstrates that a considerable recharge now takes place from the side channels. This has become possible because the running water has removed the fine material, previously clogging the bed of these river arms. In all cases the fluctuations háve been reduced significantly.

For the Slovakian area the ground water levels measured on March 3, 1993 are shown in Fig. 6.5 together with calculation of the changes at that time of ground water levels due to the Gabcikovo Project. Similarly, conditions on June 30, 1993 are shown in Fig. 6.6. Hungary was not able to produce similar maps. Instead a map previously presented, ref /4/, showing an estimate of the ground water level changes as per February 8, 1993, is given in Fig. 6.7.

By comparison of Fig. 6.5 and 6.6, which represent conditions before and after putting water to the side channels on the Slovakian flood plain, it is evident that a good hydraulic connection between the side channels and the ground water system has been established. Thus, a substantial ground water recharge takes place from the side channels resulting in up to 1.5 m increased ground water levels.

From the most recent map in Fig. 6.6 it is noticed that the ground water levels on all the Slovakian territory have increased or have not been affected. The increases have mainly occurred in the upstream area close to the reservoir, i.e. in the area which have been most negatively affected by the long term trend of decreasing ground water levels.

As a similar comprehensive analysis has not been made for the Hungarian data, the conclusions with regard to impacts. on ground water levels in Hungary are less certain. However, by considering Fig. 6.4 and 6.7 it appears that the ground water levels have also increased close to the reservoir (Rajka - Dunakiliti region). In the middle of Szigetköz between Dunakiliti and Asvanyraro the ground water levels have decreased in areas close to the Danube.

6.4  Adequacy of the present Monitoring System

The present monitoring of ground water levels is adequate.

7.  GROUND WATER QUALITY

7.1  Available Data

The amount of ground water quality data in the area is comprehensive. The network of observation stations are shown in Fig. 7.1.

In Slovakia a systematic monitoring has been carried out since 1983 on a bimonthly basis. After the damming of the Danube an extended monitoring programme with fortnightly sampling has been made in a number of wells located close to the Danube. Under the ordinary monitoring programme the following parameters are analysed: TDS₁₀₅, O₂, BOD₅, COD_Mn, Fe, Mn, NO₃, SO₄2-, Cl- and PO₄3-. Under the extended monitoring analyses are made for more than 100 parameters including heavy metals and organic micropollutants. The Slovakian Data Report (ref /2/) shows plots of all data from the ordinary monitoring programme plus a summary of data from a single well under the extended monitoring programme.

In Hungary a large amount of data is being collected on a fortnightly basis. 23 parameters are measured. The Hungarian Data Report (ref /3/) shows no ground water quality data.

7.2  Data Analyses for Long Term Trends

The ground water quality in the area dominated by the infiltration from the Danube is generally in a good state. Thus, the quality of the ground water abstracted from the water works located close to the Danube is generally excellent.

For the areas farther away from the river, where the ground water recharge partly originate from infiltration in agricultural and industrial areas, there are some problems with ground water pollution from point sources (e.g. from Slovnaft oil refinery starting in the 1960's, landfills and dumping sites) and from agrochemicals.

The data from the Slovakian ordinary monitoring program reveal with a few exceptions no long term trends. In a couple of wells the NO3- concentrations show an increasing trend and in the Rusovce area, where examples of an increasing trend for TDS105 and a decreasing trend for the Mn concentration can be found.

7.3  Analyses of 1991-93 Data

In general no ground water quality changes can be identified after the damming of the Danube. One exception is the Rusovce area where decreases in TDS105 and NO3- can be found. These changes can be explained by a changed flow pattern in this area, which now receives its water from infiltration in the reservoir, while it in the pre-dam conditions was flowing from the inland towards the river.

The extended monitoring programme of ground water quality in Slovakia shows occurrence of organic micropollutants in some of the observation wells but not in any of the production wells used for abstraction of drinking water. There is no trend over time in these concentrations, which originate from old pollution of the ground water system.

According to the Hungarian Data Report (ref /3/) no significant changes have been detected in the ground water quality.

7.4  Adequacy of the present Monitoring System

The present monitoring of ground water quality appears adequate. However, because of the relatively slow ground water transport process, it should be emphasized that the intensive monitoring must continue for the coming years, especially with regard to areas close to the Danube where. the infiltration conditions have been changed. Thus it cannot be guaranteed that a permanent situation, whether changed or unchanged as compared to pre-dam conditions, is obtained all over the area the first few years.

8.  FLORA AND FAUNA

Biological field research has been done for several decades in the influenced area. There exist general overviews about the occurence of 1.000 plant species on the Slovakian and 820 plant species on the Hungarian territory. Approximately 2.800 animal species are detected on the Hungarian territory. It can be expected, that the same amount of species are found by Slovakian investigations.

It is considered that these numbers are significantly lower than the real numbers, because not all the taxa of fauna and flora were investigated (and could be investigated without enormous efforts).

8.1  Available Data

There exist investigations on the occurence of the different taxa with quite different methods. Biomonitoring stations are shown in Fig. 8.1 and described in Table 8.1 and Table 8.2 for the Hungarian and Slovakian areas; respectively.

Concerning higher (vascular) plants the Slovakian Data Report (ref /2/) provides a phytocoenological map of the forests from 1960. The Hungarian Data Report (ref /3/) shows species lists of single investigation plots.

Hungary and Slovakia present in their data reports an overview of the zoobenthos and zooplankton in the main channel and in the arm system with species lists. There is an estimation on species number, biomass and the saprobity (a biologically based water quality scale ranging from 1 to 4 with 1 representing the best condition) in the Slovakian data report. The Hungarian report deals with the abundance of the different species in a 20-1-sample.

From the fish species both the Slovakian and Hungarian reports list 65 as the present number. The Slovakian report contains an estimation of the ichtyomass in different habitats.

There are less data about the terrestrial fauna. Species lists exist in Slovakian report from Collembola, Acarina, Araneae, Chilopoda, Carabidae, Staphylinidae, Curculionidae, Rhopalocera, amphibians, reptiles, bats and mammals.

Used Methods:

Hungary can provide data about the following taxa: Mollusca, Rotatoria, Crustacea, Odonata, Heteroptera, Neuropteroidea, Coleoptera (61 families), Trichoptera, Lepidoptera, Acarina (Oribatida), Pisces, Amphibia, Aves, Mammalia (small mammals).

8.2  Data Analyses for long Term Trends

It can be estimated that forestry and agriculture together with regulation measures in the Danube and construction of dikes have caused changes in flora and fauna in former times but the data base does not allow to analyze the long term trends for most of the taxa. On the other hand in some cases it provides a good basis for analyzing the trend in the past (e.g. the phytocoenological map of 1960) and for monitoring the development in the future (e.g. investigation plots for higher plants on the Hungarian territory if these are fixed in the field).

Long term analysis with a good data base can be done with fish species. From 56 native fish species 4 are now extinct, 13 species were introduced by man.

8.3  Analyses of 1991-1993 Data

On the one hand the data base and on the other hand the long response time of natural systems only aliow to quantify the influence of Variant C structures on flora and fauna on single aspects:

Biomass data from nonvertebrates were in general not taken as basis for analyzing short term changes because these values normally do not allow a clear diversion of effects of the Variant C structure and parameters like temperature,amount of light or amount of rainfall.

8.4  Adequacy of the Present Monitoring System

At present a huge amount óf data are collected. There is a need for application of more quantitative and semiquantitative methods that deal with (under monitoring aspects) good indicator taxa in suitable sites. The present monitoring system should be strengthened at least with the following investigations:

The monitoring plots should be mapped in detail (1:100 1:1.000). Investigation plots l0xl0m or 25x25m should be durable marked in the field so that the survey can be repeated on the exactly same place.

9.  AGRICULTURE

The annual rainfall and evaporation amounts are of the same order of magnitude; however with significant different seasonal variations. Thus it is required with some additional water supply to. the vegetation during the summer season. This extra water supply has traditionally been possible throughout the area by vertically upwards flow in the capillary zone from the ground water table to the root zone. The necessary conditions for this are that the ground water table is not too deep and that no (capillary breaking) gravel layer is located in between.

9.1  Available Data

For assessing the possible changes in capillary water supply for the agricultural production data is required on depth to ground water table and depth to gravel layer.

This information is provided by the Slovakian Data Report (ref /2/), while the Hungarian Data Report (ref /3/) does, not provide any data nor analysis on this issue.

9.2  Data Analyses for Long Term Trends

Due to the general decline of the ground water table in large parts of the area during the past 40 years the conditions for capillary water supply to the root zone have decreased and the irrigation. water requirements have increased correspondingly.

9.3  Analyses of 1991-93 Data

Due to the increase of ground water tables in large parts of the Slovakian area the conditions have improved. According to an estimate given in ref /2/ the requirements for irrigation from external sources is expected to decrease by about 25% as compared to the pre-dam conditions.

No specific analyses on this issue is provided in ref /3/ for the Hungarian area. In part of the Hungarian agricultural area ground water levels have increased, while in other areas close to the main Danube the ground water levels have decreased. However, without specific analyses the impacts on agriculture in Hungary cannot be predicted with certainty.

9.4  Adequacy of the present Monitoring system

The present monitoring system is adequate as far as irrigation requirements are concerned.

10.  FORESTRY

10.1  Available Data

For the Slovak territory (ref /2/) there exists a map of groups of forest types (according to Zlatnik) from 1960 in the scale 1:50,000 derived from maps in the scale 1:10,000. This represents the state before starting construction works. The distribution of groups of forest types (in before and after construction of the Gabcikovo dam is shown. The summarized mean annual increment of thickness, tree height, mean losses of leaves and leaf area index from 24 monitoring areas are shown for 1991 and 1992.

The Hungarian Data Report (ref /3/) shows results of measurements of perimeter growth of 117 trunks all located in the Dunasziget area. Additional data from the Dunakiliti area could be provided.

10.2  Data Analyses for long Term Trends

Forest types change under natural conditions according to changes of the site parameters. In economically used (commercial) forests these changes are modified by selection of planted tree species. The first so-called profit forms of poplars were planted at the end of the 19th century. However these measures might represent the site conditions.

10.3  Analyses of the 1991-1993 Data

Changes in distribution of groups of forest types and its tree species composition before (1976) and after (1993) construction of the Gabcikovo dam are shown in Table 10.1 and in Fig. 10.1. for the Slovakian area. Similar figures for the Hungarian forest areas are shown in Table 10.2.

In a short time view changes in composition of tree species caused by changes in site conditions cannot be observed. Other indices must be used for monitoring short term changes.

Therefore, in the Slovakian Data Report the leaf area index is used as important production-ecological characteristic and the loss of leaves in the middle of August are used. They show a significant increase of the leaf area index from 1991 to 1993 near the reservoir (Rusovce) and a decrease of the mean loss of leaves in August in the same area. There are no comparable data from the area between the reservoir and Sap/Palkovicovo and from outside the influenced area (to exclude climatical effects).

Changes are not due to Gabcikovo System constructions, but to economic considerations.

In the Hungarian Data Report it is estimated (without evident data base) that roughly 5 % of the floodplain trees already died. The perimeter growth data were taken in Dunasziget area and near Dunakiliti. Both show a significant decrease in 1993. There are no comparable data from outside the influenced area (to exclude climatical effects)

10.4  Adequacy of the Present Monitoring System

The present monitoring system should be strengthened in the following way:

11.  ELECTRICITY PRODUCTION

The electricity production at Gabcikovo is shown in Fig. 11.1. The same figure shows the ratio between the Gabcikovo electricity production and the total electricity consumption in Slovakia as a graph.

The electricity produced at Gabcikovo goes into the Slovakian electricity distribution network.

An illustration of the hourly variation of electricity production and the discharge through the turbines is shown for three typical days in Fig. 11.2.

12.  OTHER PARAMETERS

The international navigation through the shiplocks at Gabcikovo has functioned since its opening on November 9, 1992. During this period the locks have been closed for 14 days.

The numbers of ships and shiplock openings are shown in the Slovakian Data Report (ref /2/). The monthly average numbers of openings of shiplocks and of ships passing have been shown for January - August 1993 in Table 12.1. It is seen from the table that for the first eight months of 1993 there has been in average 12 openings of ship locks per day and in average 31 ships have passed the locks per day.

13.  SUMMARY ASSESSMENT OF IMPACTS FROM THE GABCIROVO PROJECT

The impacts of the Gabcikovo Project to the various parameters described in the above Chapters 2-12 may be categorized in two main groups, namely those where major general impacts have been identified and those where for various reasons no major general~impacts as compared to the pre-dam conditions can be detected from the available data.

13.1  Aspects where major general impacts have been observed

As described in the above Chapters significant general impacts have been identified for the following parameters:

The impacts are summarized in the following.

Discharge

The Gabcikovo Project has had a very large impact on the discharge regime in the Danube between the 'dam at Cunovo and the downstream confluence at Sap/Palkovicovo. In this reach (the Old Danube) the discharge has in 1993 been reduced to in average about 400 m³/s corresponding to about 20 % as compared to the pre-dam condition.

As an effect of the project the discharges in the Little Danube and the Mosoni Danube have been increased by 10 - 20 m³/s, so that Mosoni Danube now permanently carries discharge. Finally, water is being provided to the side channels on both the Slovakian and Hungarian flood plains through new structures.

Surface water level

The Gabcikovo Project has had a very large impact on the water level regime in the Danube between Bratislava and the downstream confluence at Sap/Palkovicovo.

At Bratislava the water levels during low flow periods have increased by 1-2 m as compared to pre-dam conditions, i.e. to a level corresponding to the situation 40 years ago.

In the upstream part of the Old Danube the 1993 water levels have been reduced by 2-4 m as compared to pre-dam conditions, and have thus reached a level 2 m below the lowest ever recorded values. In addition, the characteristic natural dynamics of the water level fluctuations have been changed (reduced) significantly.

Sediment transport and sedimentation/erosion

Significant erosion occurred downstream the Cunovo structures under the November 1992 flood event. This material has been deposited downstream in the Old Danube. Sedimentation of fine material/silt can be seen in the Old Danube. Most likely, sedimentation of the total bed load and a substantial part of the suspended load have occurred in the reservoir. However, there are presently not sufficient data to quantitatively assess such impacts.

Ground water level

The Gabcikovo Project has had a significant impact on the ground water levels in the region. Initially the ground water levels were reduced very much close to the Old Danube and increased significantly close to the reservoir. The inundation of the side channels on the Slovak flood plains has subsequently significantly increased the ground water levels in the Slovak area.

In June/July 1993 the situation in Slovakia shows that over the entire area the ground water levels have increased or have not been affected. The increases have mainly occurred in the upstream area close to the reservoir, i.e. in the area which have been most negatively affected by the long term trend of decreasing ground water levels. On the Hungarian side, where comprehensive impact assessments have not been completed, it appears that the ground water levels have also increased close to the reservoir (Rajka Dunakiliti region). Furthermore, it appears that in the middle part of Szigetköz between Dunakiliti and Asvanyraro the ground water levels have decreased in the areas close to the main Danube.

On both sides the ground water level fluctuations have been reduced significantly.

Electricity production

The Gabcikovo hydropower plant has produced 150 - 200 Gwh/month in 1993. This corresponds to about 10% of Slovakia's electricity consumption.

13.2  Aspects where minor impacts have been observed or no significant impacts can be detected from the available data

As described in the above Chapters no general significant impacts could be detected on the basis of the available data for the following parameters:

For some of the parameters significant local changes have been identified. For other parameters the data availability and/or the time period have not been sufficient to derive firm conclusions. Finally, for some parameters no significant general impacts are expected.

The conclusions found for each of the parameters are summarized in the following.

Surface water quality

With exception of November - December 1992, when sudden changes of regime and a high flood event occurred, no significant changes in surface water quality parameters as compared to pre-dam conditions can be detected after damming the Danube.

Ground water quality

In general, no significant ground water quality changes can be identified after the damming of the Danube. One exception is the Rusovce area where some parameters (e.g. Tota1 Dissolved Solids and nitrate) have changed due to changes in the flow pattern. No changes in concentrations of heavy metals nor organic micropollutants have been detected.

Flora and fauna

Due to insufficient data availability and due to the long response time of natural systems with regard to flora and fauna, no major general impacts have been identified. However, on the following aspects significant impacts have occurred: removal of about 4,500 ha floodplain forest under the construction phase; saprobity values in the Old Danube slightly increased; and some changes in the occurrence of certain species have been noticed.

Agriculture

Due to increases of ground water tables on the Slovak territory a slight increase in the capillary water supply for Slovakian agricultural areas has taken place. In Hungary, where comprehensive assessments have not been made, the impacts on agriculture are uncertain.

Forestry

The leaf area index and the perimeter growth are positively correlated and the loss of leaves in the middle of August is negatively correlated to increase of ground water levels. Hence, the impacts on forestry depends on the impacts on ground water levels, i.e the forestry has been positively influenced in Slovakia and negatively in Hungary.

14.  RECOMMENDATIONS FOR STRENGTHENTNG OF MONITORING SYSTEM

A comprehensive monitoring programme has been carried out on a routine basis for many years (decades) in both countries. Furthermore, extended programs have been established to monitor in greater details the conditions after the damming of the Danube.

The Working Group finds the present monitoring system, i.e. the extended programs in both countries, generally to be relevant and recommend it to be continued. In many fields the present monitoring system appears adequate, while in other fields there are clear needs for a strengthening.

The strengthening comprises two different components, namely:

14.1  Aspects requiring more Measurements

Surface water levels

There is a need for a new monitoring program on measurements of surface water levels in the side channels on the flood plains both in Hungary and in Slovakia. At stations, where substantial fluctuations within a day occur, it is recommended to substitute manual measurement practises with automatic recorders.

Surface water quality

There is a need for a new monitoring programme on measurements of surface water quality in the side channels on the flood plains both in Hungary and in Slovakia.

Sediment transport and sedimentation/erosion

There is a need for establishment of a permanent sediment transport measurement programme comprising both bed load and suspended load measurements at the following locations:

Furthermore, there is a need for establishment of permanent programs for monitoring river bed and reservoir topography.

Ground water quality

The intensive monitoring must be continued for the coming years, especially with regard to areas close to the Danube where the infiltration conditions have been changed. Depending on the development of the measured ground water quality parameters it may be required to add more observations in the future.

Flora and fauna

The present monitoring system should be strengthened with the following investigations:

The monitoring plots should be mapped in detail (1:100 1:1.000). Investigation plots 10x10m or 25x25m should be durable marked in the field so that the survey can be repeated on the exactly same place.

Forestry

The present monitoring system should be strengthened with the following investigations:

14.2  Aspects requiring coordination Efforts

For obtaining firm conclusions on the discharge uncertainties the following checks are required to be carried out by joint Hungarian-Slovakian teams:

Budapest, 2. November 1993

  Jan M. van Geest

  Jens Christian Refsgaard

  Johann Schreiner

  Igor Mucha

  Gabor Vida

15.  REFERENCES

/1/  Working Group of independent Experts on Variant C of the Gabcikovo-Nagymaros Project. Final Report. Budapest, November 1992.

/2/  Data Report prepared by Slovakia as input to the present Working Group. Volumes on

/3/  Data Report prepared by Hungary as input to the present Working Group. Volume on

/4/  Hydrographic data taken in the period taken after the closure óf the Danube. Ministry of Environment and Regional Policy, Hungary, Budapest, May 1993.