## CHANGES IN THE SOIL MOISTURE REGIME IN THE AERATION ZONE AFTER SETTING THE GABCIKOVO HYDROELECTRIC POWER STRUCTURES INTO OPERATION

#### Zoltan HLAVATY1 - Boris CAMBEL2

1GROUND WATER Consulting Ltd., P.O.Box 6, 84000 Bratislava 4, SLOVAKIA
2Research Institute of Irrigation, Vrakunska 29, 82563 Bratislava, SLOVAKIA

CONCLUSIONS Setting the hydroelectric power structures Gabcikovo into operation directly impacts the regime of surface and ground water. Changes in the ground water regime are transferred to the biosphere through the aeration zone. These changes are manifested in the changes of soil moisture regime in the aeration zone. It may be stated that if there is an increase of the ground water level due to construction of hydroelectric power structures, then there is also an increase in humidity in the aeration zone, while it may occasionally remain unchanged at some locations, in no case does it decrease. The same is valid also reciprocally. If there is a drop of ground water level due to hydroelectric power structures, then there is also a decrease of moisture in the aeration zone, occasionally it may be unchanged at some locations, but in no case does it increase. Therefore, neither increase of moisture due to the decrease of ground water level, nor the opposite can happen. Changes in the soil moisture in the aeration zone for selected localities along the Danube are charted on the graphs for the interval of years 1990-1994. For the comparison the fluctuation of ground water, the date of damming the Danube and the date of the start of river branche water supply, etc., are given.

#### INTRODUCTION

Water monitoring in the aeration zone is one of the the basic components for monitoring environmental impact of the Gabcikovo water engineering structures. The operation of the water engineering structures has a direct impact on the regimes of both surface and ground water. This impact is transferred indirectly to other components of environment. Namely, water in the aeration zone is the component which transfers the impact of the water engineering structures to the biosphere. The aeration zone is delineated by ground surface - the terrain on its upper level, and ground water level, which is its bottom border. The basic processes influencing soil moisture conditions in the aeration zone take place

(i)
  on the surface: precipitation and evaporation,
(ii)
  on the bottom layer: exchange of water between the ground water and the aeration zone, and
(iii)
  directly in the aeration zone: extraction of moisture by plant roots.

The aeration zone is influenced also by some other impacts, e.g. soil cracks, composition of soil horizon, etc.

The goal of soil moisture monitoring in the aeration zone is not just to observe increases or decreases of the moisture due to the construction of hydroelectric power structures. This could be done simply by measuring the changes of ground water level. More important is to quantify these changes and to find out, from different points of view, whether they are positive or negative when compared with previous conditions. Another important task is to define previous long-term trends of any nature. The goal of monitoring the water engineering structures impact is thus to obtain data on retention and dynamic changes of individual characteristics, which define the water regime and water chemistry in the aeration zone, caused by setting the hydropower plant project into operation.

Water in the aeration zone is the subject of monitoring by four expert groups:

- water monitoring in the aeration zone,

- soil monitoring,

- forestry monitoring,

- biota monitoring.

The distribution of the water monitoring sites in the aeration zone for all four expert groups is given on the map enclosed, see Fig. 1. Monitoring is mostly focused on the area along the Danube. For the presentation of results we have selected the following downstream river Danube sites, which are referenced in the contributions of the biota expert group in this Publication (Fig. 2.).

Monitoring sites

Name of locality | ID Number of monitored area | ID Number of soil moisture monitoring place
--- | --- | ---
Dunajska Luzna | 2621 | 2713
Hamuliakovo | 2622 | 2714
Samorin-Cilistov | 2623 | 2715
Dobrohost Dunajske kriviny MP-6 | 2600 | 2703
Bodiky-Bodicka brana MP-9 | 2603 | 2704
Bodiky-Kralovska luka MP-10 | 2604 | 2705
Gabcikovo-Istragov MP-14 | 2608 | 2706
Klucovec-Sporna sihot MP-18 | 2612 | 2707

#### MONITORING EVALUATION

Putting the water engineering structures of Gabcikovo into operation influences the adjacent environment through the changes in the ground water level and its fluctuation in comparison with those that would be observed without the impact of structures. These changes are manifested through the changes of moisture conditions in the aeration zone when compared to that state without the impact of hydroelectric power structures, under unchanged other influences. Easy to say, but it is difficult to identify and measure these changes. The only certainty is that if there is an increase of the ground water level due to building up a water engineering structure, then there is also an increase of humidity in the aeration zone, or occasionally the situation may remain unchanged at some locations, but there is no humidity decrease. The same is valid reciprocally: if there is drop of ground water level due to the construction of structures, then there is also a decrease of humidity in the aeration zone, or the situation is unchanged at some places, but there is in no case an increase of soil moisture. Therefore, neither increase of humidity due to decrease of ground water level, nor the opposite can happen.

Moreover, the increase of ground water level in gravel, which does not change moisture conditions in the soil profile of the aeration zone, is also valuable if the water level reaches root the systems e.g. of forest trees or undergrowth with a deep root system.

To compare results of moisture measurement in the aeration zone, the course of discharge in the Danube (at Bratislava and Gabcikovo), precipitation and temperature (at Gabcikovo), (Fig. 3) are given. For each monitored area a map at a scale 1:10,000 is enclosed, which shows contours of reference levels of ground water for the years 1962, 1992 and 1993. From contours one can read general and long-term changes of ground water levels, and also rates of changes with distance from the Danube. Further, there are shown observing objects and delineated monitoring areas of biota. For observing objects, there is shown the fluctuation of ground water level in the nearest SHMU (Slovak Hydrometeorological Institute) wells and other local wells. Data on soil moisture are charted as a cumulative soil moisture as a function of time for the depth under surface in the ranges 0-100 cm and 100-200 cm.

To improve clarity of time and depth dependencies of the moisture in the aeration zone, the colour pictures were drawn with the depth values on the vertical and the time on the horizontal axis. The humidity levels are distinguished by colour. The shades of brown stand for deficit of moisture and its low accessibility for plant roots, green and blue represent sufficient moisture, and shades of violet represent high soil moisture and soil fully soaked with water (water logging). Exact times of measurement are marked on the top of the picture by ticks. Moreover, ground water level fluctuation is drawn at the same depth scale. When lack of data from the local probe was encountered, the plot of water level was estimated according to the nearest SHMU well. From the pictures the impact of ground water level fluctuation on the moisture conditions in the aeration zone is evident. Besides, it is possible to compare the impact of precipitation, seasons with high evaporation (high temperature), and to deduce general conclusions about the impact of ground water level changes on the moisture conditions. From the pictures it is also evident how, in the measured soil, moisture values reflect the image of the geological profile, granulometric structure of sediments and impact of capillary barrier. "Moisture" measured under the ground water level clearly reflects the structure of gravel formation whilst individual layers are distinguished by porosity and by the ratio of the fine-grain material. The dates of damming the Danube and filling up of the arm system are also given. It is possible to see the changes which took place in the soil moisture conditions as a consequence of damming the Danube or filling the river arm system.

#### MONITORING AREAS

#### Dunajska Luzna

This monitoring area is located at a distance of 450 m from the reservoir, near the wood, in an agricultural area. The upper limit of gravel is at a depth of 2 m. This locality is typical for the area downstream of Bratislava, where during the last 30 years the ground water level decreased by 2 - 3 m, to the depth of about 6 m under the surface. The gravel was nearly dry to the depth of the ground water level.

After the Danube damming, the ground water level increased nearly to a level 2 m under the ground surface, it filled the gravel with water and increased moisture above the gravel, especially when compared with the previous year 1992. The impact of the water constructions on the environment was that the ground water levels returned to the level they were 30 years ago. In the depth zone, down to 1 m, moisture was only negligibly raised, it increased in the 1 to 2 m zone by 5 - 15 %, and the gravel under this zone had been filled by water. This impacts mainly on the plants with roots at a depth of 1 m and more, or with roots reaching deeper into the gravel. Seepage canals at the reservoir permit regulation of the ground water level in this locality, so it can be increased further by approximately 0.5 m or decreased by from 1 to 1.5 m.

Fig. 4 Monitoring area No. 2621 - Dunajska Luzna
a) situation
b) plot of ground water level and plot of integrated soil moisture in intervals 0-100 cm and 100-200 cm
c) plot of soil moisture dependent on time, in vertical cross-section

#### Hamuliakovo

The monitoring area at Hamuliakovo is localisated about 1 km from the reservoir in an agricultural area. During the last 30 years the ground water level had decreased by about 1.5 m, and before the Danube damming it occurred at a depth of about 6 m in gravel. The upper limit of gravel is at a depth of 3.5 m. This gravel was rather dry in the depth range from 3.5 m to the ground water level of about 6.5 m in pre dam conditions. In the depth range from 1.8 to 3.5 m, comparatively low permeable clay sediments occur.

After the Danube damming, the ground water level increased by 2.5 - 3 m, reached low permeable clay sediments, and filled the gravel situated below it. This obviously did not influence the moisture conditions above the low permeable layers ranging from the surface to the depth of 1.8 m. This means that the impact of the engineering water structures does not appear above the depth of 1.8 m. Under this depth humidity increased and gravel under the ground water level, now at a depth of about 3 m, was saturated by water. Drainage canals permit the regulation of ground water level in a range of up to 2 m, however, it will have only a small impact on the moisture conditions of soil horizon and it will have only small a impact on the humidity in sub(base gravel which is now saturated by water.

This monitoring area is typically agricultural, where, in spite of the increase of ground water level, there is no visible influence to the moisture of the agricultural soil conditions. For the plants with long roots, of course, it is an improvement of the previous state.

Fig. 5 Monitoring area No. 2622 - Hamuliakovo
a) situation
b) plot of ground water level and plot of integrated soil moisture in intervals 0-100 cm and 100-200 cm
c) plot of soil moisture dependent on time, in vertical cross-section

#### Samorin - Cilistov

The monitoring area near Samorin is situated in an agricultural area where the ground water level during the last 30 years decreased by about 1 - 1.5 m, and after damming it increased by about 2 m. The ground water level was positioned in gravel both before and after the Danube damming and its increase was not visibly recorded neither in the surface horizon 0.8 m under surface, nor at the depth of 2.5 m. The changes occurred only at the depths from approximately 3 to 5 m, where gravel was saturated by ground water.

This monitoring area of the upper part of Zitny ostrov (Upper Danubian Lowland), similar to the previous, Hamuliakovo, area, is of a typical agricultural nature. The moisture regime of soil at a depth less then 80 cm depends mainly on precipitation and evapo-transpiration. Capillary action in gravel does not exceed 0.5 m. Generally we regard the ground water level increase at the upper part of Zitny ostrov (Danubian Lowland) as a positive impact of the hydroelectric power structures.

Fig. 6 Monitoring area No. 2623 - Samorin - Cilistov
a) situation
b) plot of ground water level and plot of integrated soil moisture in intervals 0-100 cm and 100-200 cm
c) plot of soil moisture dependent on time, in vertical cross-section

#### Dobrohost - Dunajske kriviny

The monitoring area Dunajske kriviny is situated between the beginning of the by-pass canal and Old Danube, upstream of the intake structure at Dobrohost which supplies the branch system with water. This locality was to the greatest extent influenced by the construction works: a Dunajske kriviny river branch dried out and discharge in the old river bed had dropped. The area is characteristic of the decrease of ground water level by about 1.5 m within the last 30 years. In the soil horizon down to approx. 1.5, m there was no change in humidity conditions of the aeration zone, and for the generation of soil moisture, climatic conditions are dominant. After damming, the ground water level decreased by an additional 1 - 2 m, (mainly the high water levels have decreased). This decrease caused the loss of even capillary contact between the ground water and the soil profile. At a capillary height of about 20 cm in gravel, the soil profile is not supplied by the ground water. The impact of the water level decrease in the Danube was reflected mostly in the substantial decrease of humidity at the depth interval of 1.6 - 4.5 m. Things may be turned back to the pre-dam conditions, provided the ground water level will increase by 1.5 m to a depth of about 3.5 m, or even above. Such restoration can be accomplished by submerging underwater weirs in the Danube so as to raise the water level in the river Danube at Dunajske kriviny by 2 m. It will be convenient to create such a submerged weir at rkm 1840, or a bit downstream. A Project already prepared to supply water to the dried old river arm Dunajske kriviny. This will positively influence the situation only in part of this rather small area. Also very important seems to be also occasional flooding of the area and/or substantial increase of ground water level.

Fig. 7 Monitoring area No. 2600 - Dobrohost - Dunajske kriviny
a) situation
b) plot of ground water level and plot of integrated soil moisture in intervals 0-100 cm and 100-200 cm
c) plot of soil moisture dependent on time, in vertical cross-section

#### Bodiky - Bodicka brana

The monitoring place is situated in the inundation area of the Danube near its native bed. During the last 30 years the ground water level decreased by approx. 1 m, and after damming the high and middle water levels additionally decreased. Development of soil humidity to the depth of about 0.7 - 0.8 m reveals that this locality had already been progressively drying before damming. After the Danube damming, due to the decrease of both the ground water level and the fluctuation amplitude, this process is more notable. The impact of climate is visible to the depth of about 0.7 - 0.8 m. This impact is most expressive at coarse-grain gravel-sand positions in the depth from 0.4 to 0.8 m and from 1.3 to 1.5 m, which did not allow capillary transport of water to higher layers at a low water level state, neither before, nor after damming. In the depth range 0.9 - 2.3 m the relative humidity decreased from nearly 100 % to 10 - 25 %. This is the main impact of the water level decrease in the old Danube on the moisture conditions in the aeration zone. Below this depth the moisture did not change. Conditions would remain unchanged provided the water level does not decrease to a depth greater than 3.5 m; in such case additional drying of the horizon would occur - down to a depth of 3 m. Soil moisture conditions can be turned back to the pre-dam state by raising the water level in the Danube river by 1 - 2 m, e.g. by shallow under water weir.

Fig. 8 Monitoring area No. 2603 - Bodiky - Bodicka brana
a) situation
b) plot of ground water level and plot of integrated soil moisture in intervals 0-100 cm and 100-200 cm
c) plot of soil moisture dependent on time, in vertical cross-section

#### Bodiky - Kralovska luka

The monitoring area is situated on the border of the inundation area where the ground water level in the previous period decreased by about 1 m. Fine-grain sediments, mostly fine-grain sands, reach the depth of 4.6 m. The ground water level in these sediments always fluctuated, and it also fluctuates after the damming. In this locality the impact of climate is scarcely visible. Only an extremely dry period, such as the end of the 1992 summer (before damming), influences soil humidity to the depth of approx. 0.5 - 0.9 m. Ground water systematically contributes to the water supply in the soil horizon. In spite of the water level's moderate increase after damming, the humidity conditions of the aeration zone are virtually unchanged, stable.

Fig. 9 Monitoring area No. 2604 - Bodiky - Kralovska luka
a) situation
b) plot of ground water level and plot of integrated soil moisture in intervals 0-100 cm and 100-200 cm
c) plot of soil moisture dependent on time, in vertical cross-section

#### Gabcikovo - Istragov

This monitoring area is situated narrowly at the Danube in an inundation area. In the past, water levels decreased by about 0.6, m and after damming by 1 m more. The higher levels decreased the most. The impact of climate is visible down to a depth of about 0.6 m. This upper horizon is also supplied by ground water, mainly during increased discharges in the Danube. In the depth range 0.6 - 1.0 m, the impact of climate on the soil humidity gradually weakens and the impact of the ground water is strengthened. Since this layer consists of coarse-grain sediments, probably gravel, soil humidity is very sensitive to the ground water level position. In principle, one can definitely state that if the ground water level is situated under this layer the soil moisture decreases to the minimum, i.e. approx. to 5 %. A similar situation occurs also in the depth under 2.2 m, where a continuous layer of gravel and gravel-sand begins. The layer in the depth range 1.0 -2.2 m obviously contains also fine-grained sandy material. Since a layer consisting of gravel occurs under this layer a capillary barrier is formed here. This may be a reason why there is relatively high level of moisture in this layer even in the periods when ground water level is down in the gravel. At the end of the year 1993, and mainly in the year 1994, there was a decrease of soil moisture also in the mentioned sandy layer, in spite of the fact that ground water level had not substantially changed when compared with the period before damming. This is a result of the soil profile lacking a ground water supply during high water levels, as is evident for the years 1991, 1992, and 1993. It would be suitable to raise low ground water levels by approximately 1 m by increasing the water level in the Danube by about 1.0 - 2 m at rkm 1815.

Fig. 10 Monitoring area No. 2608 - Gabcikovo - Istragov
a) situation
b) plot of ground water level and plot of integrated soil moisture in intervals 0-100 cm and 100-200 cm
c) plot of soil moisture dependent on time, in vertical cross-section

#### Klucovec - Sporna sihot

This monitoring area is situated in the inundation area of the Danube downstream of the mouth of the hydropower outlet canal, at a distance of about 300 m from Danube. This monitoring area was thus not directly influenced by water engineering constructions. In the summer of 1994, low discharges in the Danube caused rather less ground water levels which was manifested by the decrease of soil moisture in the zone down to 2 m. The impact of climate is dominant down to the depth of 1.0 m. Moreover, this impact is multiplied by the position of coarse-grain sand in the depth range 0.4 - 1.0 m which is supplied with water only at high ground water levels. It may be stated as a conclusion, that as concerns soil humidity, this locality was not influenced by setting the water engineering structures into operation. The increase of ground water level at this locality is not directly caused by setting the water engineering structures into operation, and due to less capillary transport in the lower part of the soil profile, it has no substantial influence on the course of soil moisture in this locality. Since this area is not influenced by water engineering structures, the soil moisture decrease in the year 1994 should be regarded as a result of an extraordinarly dry and hot summer. If this area is used as a reference area not influenced by water engineering structures, then, considering soil moisture changes in 1994, a decrease of humidity within other localities should be regarded partially as a result of an extremely dry and hot summer.

Fig. 11 Monitoring area No. 2612 - Klucovec - Sporna sihot
a) situation
b) plot of ground water level and plot of integrated soil moisture in intervals 0-100 cm and 100-200 cm
c) plot of soil moisture dependent on time, in vertical cross-section

#### PROJECTED MEASURES

Some arrangements are projected to improve soil moisture regime. First of all, seepage canals exist in the area adjacent to the reservoir. These canals enable regulation of the water level and thus the ground water level in adjacent territory. An increase of levels in this area is generally a positive fact, but it has little impact on the moisture conditions of agricultural soil and the aeration zone at a little depth under the surface. The regulation of ground water level by seepage canals will not have a significant impact on the upper part of aeration zone.

In the monitoring area of Dunajske kriviny, a water supply for the dried out old river arm has already been prepared. This will positively influence situation in that arm surrounding and at some distance from Danube.

The zone of decreased ground water levels along the Danube can be influenced by installing small submerged weirs in the Old Danube. The water level in the Danube should increase by about 2 m for the discharge of 250 m3/s in the reach from Dobrohost (from an already existing submerged weir at rkm 1843) to Gabcikovo-Istragov. Oscillation of discharge in the range 250 - 600 m3/s will cause sufficient oscillation of water level in the Danube (about 1 - 2 m), depending on the Danube cross-section and a shape of submerged weirs. This weir design may be tuned by models.

#### REFERENCES

  [1] Sutor, J., 1991: Design of field project of monitoring, data processing, and interpretation of moisture and moisture potential of soil. Institute of Hydrology and Hydraulics, SAS, Bratislava, (in Slovak)

  [2] Sutor, J., et al., 1993: Monitoring, treatment, and interpretation of water resources and its chemistry in the aeration zone of the covering layer of Danubian Lowland (report 1990-1992), Institute of Hydrology, SAS, Bratislava (in Slovak)

  [3] Sutor, J., et al., 1994: Monitoring of the water resources dynamics and chemistry in the aeration zone after putting the hydroelectric power structures into operation (report 1993). Institute of Hydrology, SAS, Bratislava (in Slovak)

  [4] Sutor, J., et al., 1995: Monitoring, data processing, and interpretation of water resources and its chemistry in the aeration zone of the covering layer of Zitny ostrov island (report 1990-1994), Institute of Hydrology, SAS, Bratislava (in Slovak)

  [5] Lisicky, M., J., et al., 1991: Report on starting state ("zero" state( of the environment at the System of Hydroelectric Power Structures Gabcikovo-Nagymaros, step Gabcikovo, from the point of view of biology and environmental ecology. Institute of Zoology and Ecosozology, SAS, Bratislava (in Slovak)

  [6] Rovny, B., et al., 1992: Monitoring of the environment influenced by the construction and operation of Hydropower Structures Gabcikovo - expert group "biota" (report 1991). Institute of Zoology and Ecosozology, SAS, Bratislava (in Slovak)

  [7] Matecny, I., et al., 1993: Report by the expert group "biota" on the results of monitoring of the environment influenced by the construction of the Hydropower Structures Gabcikovo in 1992. Institute of Zoology and Ecosozology, SAS, Bratislava (in Slovak)

  [8] Matecny, I., et al., 1994: Monitoring of the environment influenced by construction and operation of Hydropower Structures Gabcikovo - expert group "biota" (report 1993). Faculty of Natural Sciences, Comenius University, Bratislava (in Slovak)

  [9] Matecny, I., et al., 1995: Monitoring of the environment influenced by building works and operation of water engineering structures Gabcikovo - expert group "biota" (report 1994). Faculty of Natural Sciences, Comenius University, Bratislava (in Slovak)

  [10] Mucha, I., Rodak, D., Bansky, L., 1994: Evaluation of the monitoring of environmental components in the area influenced by water engineering structures Gabcikovo, according to the decision of OUZP Bratislava-vidiek c. Vod. 449/Am-39/1993-W, Surface and Ground Water, Final Report, Ground Water Consulting Ltd. Bratislava (in Slovak)