PART 3

 

Ground Water Regime

The impact of the temporary technical measures and discharges into the Danube and Mosoni branch of the Danube, realised according to the Agreement, and the impact of the water supply on the ground water regime and ground water levels in the hydrological year 2003 was monitored by 265 observation wells on the Slovak and the Hungarian territories. These monitoring objects are situated in the area of Žitný ostrov and in the Szigetköz region. The situation of the observation networks on both sides is shown in Fig. 3-1.

The evaluation of ground water level data in 2003 in local scale was done by the Parties themselves and is given in the respective National Annual Reports. The jointly elaborated regional evaluation given in this report was prepared according to the jointly constructed ground water level equipotential lines. The equipotential lines were constructed for comparison of the ground water levels in the influenced area in the current year and the ground water levels before construction of bottom weir and introducing the water supply into the river branches on the Hungarian side.

3.1. Joint evaluation of the ground water regime

Considering the course of water levels in the Danube river during the hydrological year 2003 it can be stated that it was an atypical year. In general highest ground water levels were induced in the winter period as a consequence of extremely water rich months, while in the vegetation period continuous decrease of natural water level was observed. However, where water supply was applied – in the Slovak and Hungarian river branch system and along the Mosoni Danube – the ground water level was artificially raised in comparison to the natural water level.

This evaluation is focused onto the area influenced by the water supply. The ground water level position depends mainly on the water level fluctuation in the Danube, water level position in the reservoir and water level fluctuation in river branches. For computing the ground water level differences three hydrological situations were chosen in the period before and after introducing the water supply. The hydrologic situations were selected to characterise the low, average and high flow rate conditions in the Danube, corresponding to flow rates approximately 1000, 2000 and 3000 m³s^-1. However, due to atypical flow rate regime of the Danube in 2003 the chosen hydrological situations are not fully comparable. The climatic conditions and evapotranspiration at low and high flow rates were different.

The selected dates and the corresponding flow rates in the Danube at Bratislava-Devín gauging station are the following (Tab. 3-1, Fig. 3-2, Fig. 3-3a, Fig. 3-3b):

Table 3-1

Low flow rates period in the hydrological year 2003 occurred in the second half of the vegetation period, while normally it occurs during the winter period. For the evaluation purposes the hydrological situation at the end of a period with low flow rates in August 2003 was chosen. The average flow rates period was chosen in the first decade of June, in a comparable hydrological situation as the period chosen in 1993. The high flow rates period was chosen at the beginning of December 2002.

Maps of equipotential lines were jointly constructed for the selected dates using the measured ground water levels (Figs. 3-4 to 3-6). In wells where the water level is measured once a week the ground water level for the selected dates were computed by linear interpolation. In the all other wells the daily average values were used. The altitudes of the ground water levels are given on maps for each observation object used for the equipotential line construction. For the equipotential lines construction computed surface water level data in the Danube were used as well. These levels were computed by calibrated model, using river morphology data and measured water level data on given stretch. The other surface water levels were not used for construction of the equipotential lines. The constructed equipotential lines represent general ground water levels and do not show local influences of the channels or river branch systems.

Differences between ground water levels for selected comparable hydrological situations are expressed in Figs. 3-7 to 3-9.

The evaluation is mainly focused to the area influenced by the technical measures and discharges according to the intergovernmental Agreement and by the water supply realised on the Hungarian side. In this sense the inundation and the flood-protected area on the Hungarian side and partly the inundation area on the Slovak side represent the influenced area.

Low flow rates conditions

Comparing the low water hydrological situations (approximately 1000 m³s^-1) in periods before and after realising the technical measures and releasing discharges according to the Agreement (2003 versus 1993) it can be stated, that in consequence of increased water level in the Danube old riverbed and water supply into the right side river branch system and into the Mosoni Danube, on whole part of influenced area significant increase of ground water levels occurred, both on the flood-protected area and inundation area as well (Fig. 3-7). In the upper and middle part of the Szigetköz area significantly higher ground water levels were created at low flow rate conditions. Significant increase of ground water level occurred also along the Mosoni Danube as well. In the lower part of the Szigetköz, around the Bagoméri river branch system, where the water supply is still not solved, the ground water levels were lower than the ground water levels in 1993. Lower ground water levels were recorded at the whole lower part of the Žitný ostrov as well, due to extremely low flow rate in the Danube in the vegetation period. However, the most significant decrease of ground water levels occurred along the tailrace channel and downstream the confluence of the tailrace channel and the Danube old riverbed. The riverbed erosion influences this region.

In general, the increase of ground water levels is about 0.5-1.2 m in comparison to ground water levels in 1993. It occurs mainly in the area affected by the water supply realised on the Hungarian side. There is increase of ground water levels in the inundation area on the Slovak side as well, which is evoked by the water supply on the Slovak side, however this is independent on the measures according to the Agreement. The ground water level fluctuation during the year in the upper and middle part of the Szigetköz region was around 0.5-1.5 m. In the lower part of the Szigetköz it changed between 1.5-3 m. Due to extremely low flow rates in the Danube the ground water level in observed wells approached the lowest recorded levels.

On the left side of reservoir a decrease of ground water levels can be observed. This is connected with lower water level in the reservoir comparing to the 1993 and decrease of permeability of the reservoir bottom as well. However, the ground water levels at present are much higher than before constructing the dam. The decrease of ground water levels on the left side of reservoir is significantly lower in comparison to the situation before two big floods on the Danube in 2002. After floods in the hydrological year 2002 the permeability of the reservoir bottom significantly increased and the ground water levels around the reservoir almost returned to their position after fulfilling the reservoir.

The decrease of ground water levels downstream of the Gabčíkovo hydropower station is caused by partial deepening of the tailrace canal and erosion of the Danube riverbed downstream of the confluence (at mouthing of the Bagoméri river branch). However, the decrease in 2003 is more significant due to extremely low flow rates in the Danube in the vegetation period.

Average flow rate conditions

Comparing the ground water levels in the period before realising the technical measures and after increasing the discharges into the Danube according to the Agreement and after introducing the water supply on the Hungarian side, at average flow rate conditions in the Danube (approximately 2000 m³s^-1), the result is similar to that at low water conditions. Significant increase of ground water levels occurred in both, the upper and middle part of the Szigetköz area. The increase of ground water level is about 0.3-1 m (Fig. 3-8). The most significant ground water level increase occurs in upper and middle part of the Hungarian inundation area where it reaches 0.5-1 m. Comparing to the year 1993 the ground water levels along the Danube in the lower part of the Szigetköz (Bagoméri river branch system) were slightly lower and were influenced by the low flow rates in the Danube, however on the rest of the area they remained unchanged. The decrease of ground water levels is caused by riverbed erosion in this Danube stretch. On the Slovak territory no impact of the technical measures according to the Agreement appears. Higher ground water levels in the left side inundation area reflect the different water supply regime into the river branch system in 1993 and 2003. Decrease of ground water levels can be observed around the left side of the reservoir. This results from lower water level in the reservoir comparing to the 1993 and decrease of permeability of the reservoir bottom as well, but in general the ground water levels are higher than before damming the Danube. On a large part of the Slovak side there were no change in ground water levels observed.

High flow rate conditions

When comparing the high flow rate conditions in the Danube (approximately 3000 m³s^-1) for the period before (1993) and after (2003) realising the measures according to the Agreement, decrease of ground water levels can be seen along the Danube, both in the Szigetköz and Žitný ostrov regions (Fig. 3-9). The decrease in the inundation area reaches 0.3-0.8 m. This difference is caused by the difference in the discharges released to the Danube old riverbed in 1993 (1020 m³s^-1) and in 2003 (392 m³s^-1) and due to comparison different hydrologic situations. On most of the area no change is documented.

Changes in the area around the reservoir on the Slovak side are related to the lower water level in the reservoir and decrease of permeability of the reservoir bottom.

3.2. Conclusions

In spite of unfavourable hydrometeorological conditions in 2003 (low flow rates in the Danube during the whole vegetation period, and insufficient precipitation amount) no significant changes were observed in the area affected by the water supply in comparison to the previous years.

In general it can be stated that the water supply into the right side river branch system plays an important role in influencing ground water levels over the Szigetköz region. As a result of the measures realised according to the intergovernmental Agreement significant increase of ground water levels occurred for low and average flow rate conditions in the Danube. However, no significant changes in the upper and middle part of the Szigetköz region and decrease of ground water levels along the Danube riverbed are characteristic for the high flow rate conditions.

The monitoring results highlights the necessity of solving the ground water level decrease in the lower part of inundation area on both sides. The necessary ground water level increase in the lower part of the Ásványi river branch system, in the Bagoméri river branch systems and Istragov island on the Slovak side could be solved by extension of the water supply system to these areas or by measures applied in the Danube old river bed upstream of the confluence with the tailrace channel. Such measures can improve the situation in this region.

Increase of ground water levels in the strip along the Danube old riverbed on both side could be ensured only by increasing water level in the Danube by measures realised in the riverbed. The most simple, quickly realisable measure seems to be the solution applied at Dunakiliti at rkm 1843, realised according to the “Agreement”.