PART 3

 

Ground Water Regime

 

The impact of 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 is monitored by 253 observation wells situated in the area of Žitný ostrov and in the Szigetköz region. The network of observation objects on both sides is shown in Fig. 3-1.

The evaluation of ground water level data in 2000 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 equipotential lines. The equipotential lines were constructed for comparison of the ground water levels in the influenced area before and after 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

 The ground water level in the area influenced by the water supply depends mainly on the water level fluctuation in the Danube, water level in the reservoir and water levels in the river branches. For ground water level differences computing comparable hydrological situations were chosen in the period before and after introducing the water supply. Three different hydrologic situations were selected to characterise the low, average and high discharge conditions in the Danube, corresponding to discharges approximately 1000, 2000 and 3000 m³.s^-1.

The selected dates and the corresponding discharges 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

 The low discharge period was chosen at the end of January due to comparable climatic conditions, even if it did not meet the discharge 1000 m³s^-1. The average discharge period was chosen in a comparable hydrological situation as the period in 1993. Because of comparable climatic conditions, the high discharge period was chosen in August 2000, however the preceding hydrological situation was not equivalent to that in 1993. Maps of equipotential lines were jointly constructed for the selected dates using the measured ground water levels (Fig. 3-4, Fig. 3-5 and Fig. 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. The altitudes of the ground water levels are given for each observation object used for the equipotential line construction. For the equipotential lines construction computed surface water level data in the Danube riverbed 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 the local influences of the channels or river branch systems.

The differences between ground water levels for the selected comparable hydrological situations are given in Fig. 3-7, Fig. 3-8 and Fig 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. The influenced area on the Hungarian side is in this sense represented by the inundation and the flood-protected area, and partly by the inundation area on the Slovak side.

Low discharge conditions

Comparing the hydrological situations in the period before and after realising the technical measures and discharges according to the Agreement (2000 versus 1993) it can be stated that as a consequence of increased water level in the Danube old river bed and water supply to the right side river branch system and to the Mosoni Danube on a major part of influenced area increase of ground water level occurred, both on the flood-protected area and inundation area as well (Fig. 3-7). Higher ground water levels at low discharge conditions were created in the upper and middle part of the Szigetköz area comparing to the year 1993, before constructing the bottom weir. Increase of ground water level occurred also along the Mosoni Danube and in the lower part of Szigetköz area. In the area around Bagoméri river branch system, which is not supplied by water, the ground water levels were similar. This region is influenced by the drainage of the tailrace canal.

The increase of ground water level is about 0.5-1.0 m. On the left side there is a decrease observed around the reservoir. This is connected with decrease of permeability of the reservoir bottom. However the ground water level at present are much higher than before constructing the dam. Slight decrease of ground water levels upstream of the confluence of the Danube old river bed and the tail-race canal is caused by partial deepening of the tailrace canal and erosion of the Danube riverbed downstream of the confluence with a tailrace canal. Increase in the Slovak inundation area along the river branches is due to the fact, that there was no water supply into the river branch system in March 1993.

 Average discharge conditions

Comparing the ground water levels in the period before realising the technical measures and increasing the discharges according to the Agreement and introducing the water supply on the Hungarian side at average discharge conditions in the Danube (approximately 2000 m³.s^-1) the overall situation is similar to that at low water conditions - increase of ground water level in the upper and middle part of the Szigetköz area by 0.5-1.0 m (Fig. 3-8). The ground water levels in the lower part of the Ásványi and Bagoméri river branch system have not changed in comparison to the state in 1993. On the Slovak territory no impact of the technical measures according to the Agreement appears. Decrease of ground water levels around the reservoir is connected to the decrease of permeability of the reservoir bottom, but in general the ground water level is higher than before damming the Danube. Slight decrease of ground water levels can be observed downstream of the confluence of the Danube old riverbed and the tailrace canal, which is probably caused by riverbed erosion in this Danube stretch. On a great part of the Slovak side there were no change in ground water levels observed.

 High discharge conditions

When comparing the high discharge conditions in the Danube (approximately 3000 m³.s^-1), for the period before and after realising measures according to the Agreement, generally no significant impact on ground water levels in the Szigetköz region can be presented (Fig. 3-9). On most of the area no change is documented, locally in the inundation an increase or decrease by 0.5 m occur. However, in the Ásványráró and Bagoméri river branch system and downstream of the confluence of the tailrace canal and the Danube old river bed decrease occurred, which is probably connected with river bed erosion and related surface water level decrease. Changes in the area around the reservoir on the Slovak side are mostly related to the decrease of permeability of the reservoir bottom. The slight decrease in the middle part, between Šamorín and Gabčíkovo, relates to lower water supply regime in the river branch system during the summer 2000.

 

3.2. Conclusions

 In general, it can be stated that the water supply plays an important role in creating ground water levels over the Szigetköz region. As a result of the measures realised according to the intergovernmental Agreement an increase of ground water levels for low and average discharge conditions in the Danube. However, no significant increase in the upper and middle part of the Szigetköz region and decrease of ground water levels in the lower part is characteristic for the high discharge condition. The ground water level decrease in this region (area around the Ásványráró and Bagoméri river branch systems) could be solved by extension of the water supply system to these areas. This can help to increase the ground water level and that way to 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 seems to be the solution applied at Dunakiliti at rkm 1843, realised according to the “Agreement”.