CONCLUSIONS For the quantification of the influence of ground water level changes and fluctuation on water dynamics in the aeration zone the three following characteristics have been chosen:

• the course of cumulative water content in the aeration zone, monitored during the period 1990 to 1994
• quarterly averages of water content in the aeration zone in the period 1990 to 1994
• the quantification of the participation of individual soil horizons on cumulative water content in the aeration zone in given time intervals for selected typical monitored locations.

INTRODUCTION.

MONITORING METHODOLOGY

• Monitoring stands situated in the floodplain forests of the Danube inundation area:

No. 17 - Dobrohost; No. 18 - Bodiky; No. 19 - Kralovska Luka; No. 20 - Dekan.

• Monitoring stands on agricultural soils:

No. 7 - Klucovec; No. 5 - Zlatna na Ostrove; No. 9 - Trstena na Ostrove; No. 12 - Bac; No. 13 - Mliecno; No. 14 - Kalinovska horaren; MP2 - Hamuliakovo; MP3 - Cilistov; MP5 - Sulany; MP8 - Baka; MP10 - Gabcikovo; MP12 - Cilizska Radvan

• Monitoring stands on irrigated soils with the possibilities for penetration of irrigation water down to the GWL:

N0 - N10 at Lehnice. expected water penetration through the AZ to the GWL.

STARTING-POINT FOR THE EVALUATION OF CHANGES IN THE AERATION ZONE

As mentioned above, the Gabcikovo dam affects the water dynamics in the AZ by the changing level and fluctuation of ground water. It is possible to divide the monitoring into following time intervals:

• original condition, that includes water monitoring in the AZ before putting the Gabcikovo structures into opperation, i.e. before October 1992;

• transitional conditions, between the Danube damming and the opening of the intake object for the water supply of the river branch system at Dobrohost;

• non-stationary stay until operational conditions are not fully stabilised

• optimised operating conditions, including the determined regime of water distribution to the old river-bed, inundation, water level regulation, etc.

EVALUATION OF IMPACT OF WATER CONTENT IN THE AERATION ZONE

1. course of cumulative water content in the aeration zone, monitored during the period 1990 to 1994

2. quarterly averages of water content in the aeration zone in the period 1990 to 1994

3. quantification of the participation of individual soil stratum on the cumulative water content in the aeration zone in given time intervals for selected typical monitored locations.

The complex study of all observation stations and intervening years is rather wide and was published in more detail in the author's reports [14-21].

Course of cumulative water content in the aeration zone, monitored during the period 1990 to 1994

Negative changes as predicted by some authors, to the ground water level after the Gabcikovo structures were put into operation, did not occur during the monitoring period (two years before starting the operation, one year in the transitional period, and two years after damming) and were not negatively reflected in the changes of the water content in the zone of aeration.

Monitored courses of water content in the AZ responded sensitively to changes in the GWL which further confirms the important role of the GWL on the water balance in the covering layer and also the possibility of its regulation by the optimised operation of the Gabcikovo water engineering structures.

Quarterly averages of water content in the aeration zone in the period 1990 to 1994

The autumn period is characterised by a depletion of water resources during the vegetation period and its recharge by precipitation at the end of year. For this reason data on water quantity in the zone of aeration taken from the five year period, 1990-1994, is presented by individual quarters of a year (see Figs. 6, 7, 8, 9). Plots of water content in the zone of aeration (data axis) vs. locality (category axis) are shown. Also attached to each graphical data representation from individual locations are averages for a quarter and the wilting point of the water amount (relevant to the type of soil at the given locality).

From the measured data it can be summarised that after putting the Gabcikovo structures into operation the average water content in the zone of aeration did not reach wilting point in any quarter of any year and the observed integral water contents in the upper part of the Danubian Lowland, in its central part and downstream from Gabcikovo are higher then former average levels. This also applies for the Kralovska luka location in the inundation territory.

Quantification of participation of individual soil horizons on cumulative water content

The monitoring of water content in the zone of aeration is based on the evaluation of soil moisture at 10 cm intervals from the soil surface to the required depth. As can be seen from Figs. 6, 7, 8, 9 this depth ranged at different monitoring stands from 100 to 200 cm. It is thus possible to use the analysis of individual levels of participation on water content on the whole monitored profile. In Figs 10 and 11 this type of analysis is documented for the localities of Cilistov and Kralovska luka.

Plots of water content in the zone of aeration (data axis) vs. day of count (category axis) are shown. Water content data in individual layers are gradually cumulated from the first (bottom) layer (at a depth of 200 cm at Kralovska luka locality) up to the soil surface. The additional data from the last layer data gives the cumulative water content of all layers. T observed water content in a given layer is expressed in mm of water. In such a way individually analysed quantities directly document the retention properties of individual layers in space and time. Analysis of the individual participation of layers on cumulative water content at Kralovska luka for the year 1993 documents the whole year's water supply from the GWL (Fig. 11). For the Cilistov location (Fig. 10) it is characteristic that during the extreme year 1993 water quantity gradually decreased until the Dobrohost intake structure had been opened. Thereafter a fast response in the form of water content increase was observed.

The analyses described above document:

• the sensitivity of water resources of the zone of aeration to the ground water level changes,

• inertia to sudden changes

• possibility of the optimisation of the water regime in cover layers in the inundation territory by changing the GWL and/or flooding the territory in the frame of operating the Gabcikovo structures

The three characteristics described above, i.e., (a) the course of cumulative water content in the aeration zone, (b) averages of water content in the aeration zone, and (c) the quantification of the participation of individual soil horizons on cumulative water content in the aeration zone of the cover layer of the Danubian Lowland environment, provide an unprejudiced basis for the evaluation of the impact of the Gabcikovo Hydro-electric power structures on the water content in the zone of aeration which serves as a water supply for the biosphere in the investigated territory.

REFERENCES.

[1] Chalupka, J., Popelkova, Z., 1994: Influence of Gabcikovo water engineering structure on hydrology regime of ground water. In: Works and studies, 48, Monitoring of Gabcikovo water engineering structure influence on hydrologic regime of involved territory. SHMU Bratislava, pp.34-55. (In Slovak)

[2] Mucha, I., et al., 1992: Optimisation of the finishing of Hydroelectric power structures Gabcikovo in the area of CSFR from the point of impact on the ground water. Ground Water Consulting, Faculty of Natural Sciences, Comenius University, Bratislava, March 1992, p.120. (In Slovak)

[3] Mucha, I. et al., 1992: Danubian Lowland-Ground Water. PHARE Project. No.PHARE/EC/WAT/1. Workshop in Bratislava. June 10-13th 1992 b, p.91.

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[14] Sutor, J., 1991: Monitoring, analysis and interpretation of soil soil moisture and soil moisture potential. Project of research group Water in Aeration Zone. Co-ordination council of monitoring at SHMU Bratislava, April 1991 a, p.58. (In Slovak)

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[17] Sutor, J.: Cover layer thickness and its relief boundaries parameters in the Danube inundation territory affected by the Danube water engineering structure. In: Proceedings of international conference "Gabcikovo-Nagymaros Water Engineering Scheme, Intentions and Reality", Bratislava, September 7.-9. 1993, pp.333-338. (In Slovak)

[18] Sutor, J.,1993: Monitoring of the water retention in aeration zone of the Danubian Lowland affected by the Gabcikovo-Nagymaros water engineering scheme. In: Proceedings of international conference "Gabcikovo-Nagymaros Water Engineering Scheme, Intentions and Reality", Bratislava, September 7.-9. 1993, pp.455-460. (In Slovak)

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