PART 3
Ground Water RegimeMonitoring of the impact of temporary technical measures and discharges into the Danube and the 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 continued in the hydrological year 2009 without significant changes. The water levels were monitored by 262 observation wells on the Slovak and the Hungarian territories (136+126). These monitoring objects are situated in the area of ®itný 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 2009 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 regimeGroundwater levels in the observed area are primarily influenced by surface water levels in the Danube and reservoir. Contrary to several previous years the regime of the Danube in hydrological year 2009 was typical. Flow rates in the winter period mostly fluctuated below the long-term average daily values. In November and December 2008 however, the average daily values 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 and precipitation, which lasted till the end of the second decade of March. After a temporary decrease the flow rate started to increase again and during the first half of April exceeded 4000 m3.s-1. Subsequently, till the end of April the flow rates continuously decreased. The flow rates in May fluctuated around or above the long-term average daily values, but in the first half of June they significantly decreased. From the middle of June the flow rates steeply rose up due to increased precipitation amount. Due to lasting rainfall in the Austrian Danube catchment area high flood wave occurred in the third decade of June, which culminated on June 26, 2009 at 8288 m3.s-1. In July and August another two considerable increases of flow rate occurred due to heavy rains. The flow rate exceeded, 4000 m3.s-1. At the end of August and in September the flow rates mostly ranged below long-term average daily values. The flow rates decreased at the beginning of October as well, but to the end of the hydrological year they slightly increased. The short discharge waves during the year had only limited influence on groundwater levels. The greatest influence occurred during the flood at the end of June 2009. It was expressed mainly in the area along the Danube riverbed, where the groundwater level raised by 2.5-3 meters. Another significant influence on groundwater level was observed in the inundation during the artificial flooding, which was realized from the end of April till the end of first decade of June 2009. During the artificial flooding 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 in January and February 2009. Minimal groundwater levels in wells along the Little Danube occurred at the end of hydrological year, in October. The maximal yearly levels in most of observation wells, depending on their distance from the Danube, occurred at the end of June or during July, 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. 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 in August, March or April. 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 3-4.5 m. At the end of hydrological year the position of ground water levels reached similar 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 (Tab. 3-1, Fig. 3-2, Fig. 3-3a, Fig. 3-3b): Table 3-1
Low flow rate period was chosen in the middle of winter period, at the end of January 2009. The meteorological and climatic conditions were comparable. The average and high flow rate periods were chosen in similar climatic conditions, but the hydrological situations were not fully comparable, especially in case of high flow rates. Maps of equipotential lines were jointly constructed for the selected dates using the measured ground water levels (Fig. 3-4, Fig. 3-5, 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 2009 are expressed in Fig. 3-7, Fig. 3-8, Fig. 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 low water hydrological situations (approximately 1000 m3.s-1) in periods before and after realising the technical measures and releasing discharges according to the Agreement (2009 versus 1993), it can be stated that, in consequence of water supply into the right side river branch system and into the Mosoni Danube and partially the increased water level in the Danube old riverbed, increase of ground water levels occurred on large part of the influenced 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. 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 ®itný 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 lower part of ®itný 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 is about 0.2-0.75 m in comparison to ground water levels in 1993. This occurs mainly in the area affected by water supply realised on the Hungarian side. 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. In the lower part of ®itný ostrov area the Danube, together with the Váh river are draining the area. Average flow rate conditions Comparing the ground water levels in the period before realising the 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 result is similar to that at low water conditions or even better. A significant increase in the ground water levels occurred in both the upper and middle part of the Szigetköz area and along the Mosoni Danube. The average ground water level increased about 0.3-1.5 m (Fig. 3-8). The most significant ground water level increase occurred in the middle part of the Szigetköz area (including inundation), where it 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 2009. 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 no changes in ground water levels observed. Slight decrease in the lower part of ®itný ostrov is related to water levels kept in the channel system. 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 ®itný ostrov area the Danube supplies the adjacent area and the Váh drains the ground water. 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 (2009) 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 in the section between Dunasziget and Dunaremete show negligible changes against the period in 1993 (Fig. 3-9). The decrease in the upper and lower part of the inundation area mainly reaches 0.1-0.75 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 2009 (479.3 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 middle ®itný ostrov area no change is documented. The increase in the lower half of ®itný 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. ConclusionsBased 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. 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 is similar as in the previous year and no significant changes can be observed. The monitoring results 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 also 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. An overall evaluation of impacts in the influenced area based on the comparison of periods before constructing the dam and the present time is recommended.
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