PART 3

 

Ground Water Regime

Monitoring of impacts of temporary technical measures and discharges into the Danube and the Mosoni branch of the Danube, realised according to the Agreement, and of impacts of water supply on the ground water regime and ground water levels in the hydrological year 2010 continued. Water levels were monitored by 262 observation wells on the Slovak and the Hungarian territories (136+126), however part of the Hungarian objects (64) were observed to a limited extent due to limited funding. Monitoring objects are situated in the area of Žitný ostrov and in the Szigetköz region. List of observation wells is given in National Annual Reports on environmental monitoring respectively. The situation of the observation networks on both sides is shown in Fig. 3-1.

The evaluation of ground water level data in 2010 in a local scale was done by the Parties themselves and is given in National Annual Reports. The jointly elaborated regional evaluation given in this report was prepared according to the jointly constructed groundwater level equipotential lines. The equipotential lines were constructed for comparison of the groundwater levels in the influenced area in the current year and the groundwater levels before construction of the bottom weir and introducing the water supply into the river branches on Hungarian side.

3.1. Joint evaluation of ground water regime

Groundwater levels in the observed area are primarily influenced by surface water levels in the Danube and reservoir. Similarly to the previous year the regime of the Danube in hydrological year 2010 was typical. Flow rates in the winter period mostly fluctuated below the long-term average daily values. From November 2009 to January 2010 several discharge waves occurred, when flow rate exceeded the long-term daily average, due to higher precipitation amount in the Austrian and German Danube catchment area. At the end of February the flow rate considerably increased due to warming. After a temporary decrease the flow rate at late of March started to increase again. Flow rates in April ranged significantly below the long term daily average values. In May the flow rate increased due to increasing amount of precipitation and at the end of month exceeded 3000 m³.s^-1. At the beginning of June a big flood wave occurred due to huge amount of precipitation in the German and Austrian Danube catchment area. The flood wave culminated on June 5, 2010 at 8071 m³.s^-1. At the end of June the flow rate decreased below 2000 m³.s^-1, however, at the beginning of July it increased over 3000 m³.s^-1 again. More considerable increase of flow rate occurred in the mid of the third decade of July and in the first half of August, when the flow rate culminated over 4500 m³.s^-1 and 4900 m³.s^-1 respectively. Subsequently the flow rate decreased to 2000 m³.s^-1. However, at the end of month another discharge wave occurred due to rich precipitation, which culminated above 5000 m³.s^-1 at the beginning of September. Due to the rich rainfall since mid-July until late September, the flow rates of the Danube ranged mostly are above the long-term daily average values. At the end of the hydrological year the flow rate oscillated around the long-term daily average values and fluctuated between 1200 and 1500 m³.s^-1.

Short discharge waves during the year had only limited influence on groundwater levels. The greatest influence occurred during the flood at the beginning of June 2010. It was expressed mainly in the area along the Danube riverbed, where the groundwater level raised by 2.0-3.5 meters. Another significant influence on groundwater level was observed in the inundation during the discharge wave at the beginning of September, when a great part of flow rate was discharged into the Danube old riverbed due to technical maintenance of the water barrage system. During the discharge wave the groundwater level at some places in inundation area increased up to 2.5 meters.

With respect to the course of flow rates the minimal groundwater levels in most of observed wells occurred at the end of February 2010. Minimal groundwater levels in wells along the Little Danube occurred at the beginning of hydrological year, in November 2009. The maximal yearly levels in most of observation wells, depending on their distance from the Danube, occurred during June, reflecting the passage of the flood wave. With increasing distance of observation well from the Danube the fluctuation appeared with delay or in case of smaller discharge waves did not occurred at all. However, the maximal yearly groundwater levels in the area along the Danube old river in several observation wells close to the Danube occurred during the discharge wave at the beginning of September. During this discharge wave higher flow rate was released into the Danube old riverbed, than during the flood wave in June due to the maintenance works at the barrage system. The groundwater regime at distant observation wells reflected to a great extent other conditions (e.g. climatic conditions, surface or groundwater levels on boundaries) – maximal levels occurred at the beginning of hydrological year (wells along Hanság and Lajta channels), in May or June.

Regarding the yearly groundwater level fluctuation similar values were reached as in the previous year. In the upper part of the observed territory and in the inland area the ground water level fluctuation ranged from 0.3 to 1.0 m, in the vicinity of the reservoir maximally up to 0.7 m. In the lower part of inundation area the fluctuation reached 3-4.0 m. At the end of hydrological year the position of ground water levels mostly reached higher levels as at its beginning.

For computing the ground water level differences three hydrological situations were chosen in the period before and after introducing the water supply. The selected hydrological situations characterise the low, average and high flow rate conditions in the Danube, corresponding to flow rates approximately 1000, 2000 and 3000 m³s^-1.

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

Table 3-1:  The selected dates and the corresponding flow rates in the Danube at Bratislava-Devín gauging station

Low flow rate period was chosen in the winter period, in the mid of February 2010. The meteorological and climatic conditions slightly differed in air temperature and precipitation amount compared to the conditions in 1993, when the low flow rate period was selected in the first half of March . The average and high flow rate periods were chosen in similar climatic conditions, but the hydrological situations were not fully comparable. Moreover, the Hungarian Party did not monitored 64 observation wells in the period from April 1 to October 31, 2010 due to insufficient funding. This means that for average and high flow rate conditions approximately only a half of the whole set of observation objects on the Hungarian side were used for equipotential lines computing.

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 was computed by linear interpolation. In 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 constructing equipotential lines computed surface water level data in the Danube was used as well. This level was computed by a calibrated model, using river morphology data and measured water levels data on a given stretch. The other surface water levels were not used for constructing the equipotential lines. The constructed equipotential lines represent general ground water levels and flow direction, and do not show the local influences of the channels or river branch systems.

Differences between ground water levels for selected hydrological situations in years 1993 and 2010 are expressed in Figs. 3-7 to 3-9.

The evaluation is mainly focused on 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 rate conditions

Comparing the hydrological situation at low flow rate (approximately 1000 m³.s^-1) in the period before implementing technical measures and releasing discharges according to the Agreement and the low flow rate period in the year 2010 (2010 versus 1993), it can be stated that no change or slight decrease can be seen in the upper part of the Szigetköz area, both on the flood-protected area and the inundation area as well (Fig. 3-7). The ground water level increase in the upper part of the Szigetköz area between Rajka and Dunasziget is reduced by ground water level decrease in the area around the reservoir caused by decrease of permeability of the reservoir bottom between the compared periods. Based on the results from years 2009 and 2010 it can be stated that probably there was an adverse change in sediment transport regime in the Danube, which may be related to measures in the riverbed taken on the Austrian stretch of the Danube upstream of Bratislava. In the middle part of the Szigetköz area between Dunasziget and Dunaremete, higher ground water levels were created at low flow rate conditions as a response to the water supply of the river branch system and inland channels. An increase in the ground water level also occurred along the Mosoni Danube. In the lower part of the Szigetköz, downstream from Ásványráró and around the Bagoméri river branch system the ground water levels were slightly lower than in 1993, because of not completed water supply system. Lower ground water levels were recorded also on the left side of the reservoir, in the upper part of the Žitný ostrov, due to the big difference in the permeability of the reservoir bottom. However, the ground water levels at present are much higher than before constructing the dam. Significant decrease in 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. On a large part of the Slovak side there were no changes in ground water levels observed. Slight increase along the Little Danube and around the channels in the central part of Žitný Ostrov area is related to surface water regime in the channel system and hydrological regime on the area boundaries.

In general the increase of average ground water levels vary between +0.2 and +0.7 m in comparison to ground water levels in 1993. This occurs mainly in the area along the Danube old riverbed. However, the results in the year 2010 were significantly influenced by different climatic conditions and adverse change in sediment transport regime in the Danube, probably related to measures realized on the Austrian stretch of the Danube upstream of Bratislava, thus the changes varied between –0.5 and +0.5 m. There is an 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 measures according to the Agreement. The ground water flow direction in the upper part of the river up to Dunakiliti shows infiltration from the river and the reservoir into the surrounding area. Along the Danube from Dunakiliti up to the confluence with the Mosoni Danube the groundwater flows into the riverbed and the river is draining the adjacent area, but the ground water in the inland area is flowing parallelly with the Danube.

Average flow rate conditions

Comparing the ground water levels in the period before realising technical measures, 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 m³.s^-1), the actual results are better than at low water conditions. A significant increase in the ground water levels occurred in mainly in the middle part of the Szigetköz area and along the Mosoni Danube. The groundwater level increase in the upper part of the Szigetköz area is reduced by ground water level decrease in the area around the reservoir caused by decrease of permeability of the reservoir bottom. Even at the average flow rate conditions can be felt the adverse changes in sediment transport regime in the Danube, that are probably related to measures realized on the Austrian stretch of the Danube upstream of Bratislava. The average ground water level changes vary between -0.3 and +0.5 m (Fig. 3-8). The ground water level increase in the middle part of the Szigetköz area (including inundation) reaches 0.7-1.5 m. Compared to 1993, the ground water levels along the Danube in the lower part of the Szigetköz (at the mouth of Bagoméri river branch system) were lower. The decrease of ground water levels is due to not completed water supply system and due to riverbed erosion in this Danube stretch. On the Slovak territory no impact from the technical measures according to the Agreement appears. Higher ground water levels in the left side inundation area reflect the different water supply regime in the river branch system in 1993 and 2010. A decrease in the ground water levels can be observed around the left side of the reservoir. This results from a lower water level in the reservoir than in 1993 and a decrease in the 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 an increase in ground water levels observed, which is related to different boundary conditions and higher water levels in the channel system in the middle and lower part of Žitný ostrov. The ground water flow direction in the upper part of the river up to Dunakiliti shows infiltration from the river and the reservoir into the surrounding area. Along the Danube from Dunakiliti up to the Gabčíkovo the ground water flows into the riverbed and the river is draining the adjacent area, but the ground water outside the inundation is flowing into the inland area. In the lower part of Žitný ostrov area the Danube and Váh rivers supplied the adjacent area.

High flow rate conditions

When comparing the high flow rate conditions in the Danube (approximately 3000 m³.s^-1) for the period before (1993) and after (2010) realising the measures according to the Agreement, a decrease in the ground water levels can be seen along the Danube. In the evaluated year and in the compared period the ground water level is slightly lower than in the period in 1993 (Fig. 3-9), however the preceding hydrological situation is not fully comparable and adversely influences the result. The adverse changes in sediment transport regime in the Danube, that are probably related to measures realized on the Austrian stretch of the Danube upstream of Bratislava, can participate on the groundwater level decrease as well. The decrease in the upper and lower part of the inundation area mainly reaches 0.25-0.75 m. This difference is partially caused by the difference in the discharges released to the Danube old riverbed in 1993 (1020 m³.s^-1) and in 2010 (476.8 m³.s^-1). On a great part of the flood-protected area in Szigetköz there is an increase of the ground water levels, which is caused by the water supply into the Mosoni Danube, the inland channel system and by higher ground water levels on the examined area boundaries. The decrease in the ground water levels on the left side of the reservoir are related to lower water level in the reservoir comparing to the 1993 and decrease in the permeability of the reservoir bottom. In the upper part of the Žitný ostrov area no change is documented. The increase in the middle and lower half of Žitný ostrov area is connected with the surface water regime in the channel system and the Little Danube. The ground water flow directions along the Danube, except the upper and middle part of the inundation, show water supply from the Danube into the adjacent area.

3.2. Conclusions

Based on the evaluation of ground water regime 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 measures realised according to the intergovernmental Agreement, a significant increase in the ground water levels occurred for low and particularly for average flow rate conditions in the Danube. The increase was reduced in the upper part of the Szigetköz region and around the reservoir in last two years due to adverse changes in sediment transport regime in the Danube, that are probably related to measures realized on the Austrian stretch of the Danube upstream of Bratislava. A decrease in the ground water levels along the Danube riverbed can be registered for high flow rate conditions, however in certain distance from the Danube old riverbed no change was observed in inundation area and increase of ground water levels in the inland area of Szigetköz. In general the overall situation in last two years is slightly worse compared to the previous period Generally, the overall situation in last two years has deteriorated slightly in comparison with the previous period, and probably it will be needed to take some action in the reservoir.

The monitoring results also highlights the necessity to solve the water supply in the lower part of the inundation area on both sides. The ground water level increase in the lower part of Ásványi river branch system and in the Bagoméri river branch systems and on the Istragov island on Slovak side could be solved by measures applied in the Danube old riverbed upstream of confluence with the tail race channel. Such measures can improve the overall situation in this region.

An increase of ground water levels in the strip along Danube old riverbed on both sides could be ensured only by increase of water level in the Danube by some measures realised in the riverbed.