**2.1.2 Evaluation of natural conditions**

### **2.1.2.1 Climate**

From a climatic point of view the study area belongs to a warm locality in the scope of the south-eastern part of the Danubian lowland, where it has a warm and dry climatic zone with a mild winter. First, the temperature characteristics and yearly air temperature average shows that the south of Slovakia is the warmest locality of the republic. The average 10.4 oC at Štúrovo is convincing. Uniformity of moisture conditions is clear already from the yearly relative air moisture average, which varies from 74 - 81 % and is the lowest in the bottom most parts of the Danubian lowland (Štúrovo 74 %, Komárno 75 %). In the territory of the West-Slovakia district, which is the most productive agricultural locality, the precipitation has significant importance. The centre of this locality is the Danubian lowland, which is indeed the warmest but also has the driest locality. In a series of long-term observations, the lowest annual precipitation totals vary in terms of 300 – 400 mm and minimum monthly precipitation totals in particular months do not even reach (except for July) 5 mm precipitation, whereupon significant dry periods are more often in summer half-year than in winter half-year. The lowest July precipitation totals do not drop under 10 mm. On the other hand wet (precipitation) periods are lasting here mostly from 18 to 20 days, and their appearance is relatively more rare than the appearance of dry periods and it occurs mostly in spring and autumn periods. The highest annual precipitation totals could reach 900 mm, even in singular cases up to 1000 mm of precipitation.

## **2.1.2.2 Hydrogeology and geology**

56 Studies on Water Management Issues

From a climatic point of view the study area belongs to a warm locality in the scope of the south-eastern part of the Danubian lowland, where it has a warm and dry climatic zone with a mild winter. First, the temperature characteristics and yearly air temperature average shows that the south of Slovakia is the warmest locality of the republic. The average 10.4 oC at Štúrovo is convincing. Uniformity of moisture conditions is clear already from the yearly relative air moisture average, which varies from 74 - 81 % and is the lowest in the bottom most parts of the Danubian lowland (Štúrovo 74 %, Komárno 75 %). In the territory of the West-Slovakia district, which is the most productive agricultural locality, the precipitation has significant importance. The centre of this locality is the Danubian lowland, which is

Fig. 1. Geographical situation of the Čenkov plain

Fig. 2. Water management map of the Čenkov plain

**2.1.2 Evaluation of natural conditions** 

**2.1.2.1 Climate** 

The Danube fluvial plain at observed river sections is built by sediment deposits of the Danube River, where their thickness varies irregularly between 5 - 12 m and the most frequent thicknesses are between 6 - 9 m. Gravel and sand dominate soil layers, which are in the highest part covered by alluvial loams. Gravel–sand fillings of the Danube fluvial plain's bed in this section belong to Würm, and the cover of sandy loam is Holocene.

Fig. 3. Hydrogeological profile 1-1 (400x exceeded). Comments: 1-young Pleistocene blown sands, 2-medium to smooth sands, 3-sandy gravel to rough sands with gravel, 4-downhill loamy sediments along upper terrace step, 5-dusty to loamy sands, eventually dusty – sandy loams, 9- marking of the tertiary base surface, 10-groundwater level on 29 Sept.1954, 11-the highest groundwater level in years 1954 – 1956 (Duba, 1964)

It is possible to observe their partial subtilization in longitudinal profiles of gravel sand alluvia (Fig. 3 and 4) in the Danube direction, although the appearance of heavier gravel layers is possible in the whole profile. The left edge of the Danube's fluvial plain is lined by

Change of Groundwater Flow Characteristics After Construction of the

∂∂

∂∂

have been obtained by analytical solution of equation (1).

**3.1.2 Three-dimensional modular model of groundwater flow" MODFLOW"** 

The finite difference model originally published by McDonald & Harbaugh (1988), in the form of later modifications and addendums, and its modular computer program was utilized by the solution of the mentioned task. The modular structure consists of the "main program" and a series of independent subroutines called "modules". The explanation of physical and mathematical concepts, on which the model is based, and an explanation on how the modules are implemented into the structure of computer program, is listed in detail in the mentioned work. Ground-water flow in hydrogeological ground-water body is

**3.1.1 Mathematical model of groundwater flow** 

∂

∂

**3. Methods and material** 

**3.1 Modelling** 

1988):

Where

Waterworks System Protective Measures on the Danube River – A Case Study in Slovakia 59

Three-dimensional groundwater flow of constant density through porous earth material may be described by a partial differential equation (McDonald, M.G. & Harbaugh A.W.,

> *xx yy zz s h h hh K K K WS x xy yz z t*

*x*, *y*, z are Cartesian coordinates in the direction of main axis of hydraulic conductivity *Kxx*, *Kyy*, *Kzz*, *Kxx*, *Kyy*, *Kzz* are the values of hydraulic conductivity in the direction of the axis of Cartesian coordinates *x*, *y*, *z*, which are assumed that they are parallel with major axis of hydraulic conductivity [L T-1], *h* piezometric pressure head [L], *W* volumetric flux per unit volume, which represents sources and (or) sinks of water [T-1], *Ss* specific storage of the porous material [L-1], and *t* time [T]. In general, *Ss*, *Kxx*, *Kyy*, and *Kzz* may be the functions of space (*Ss = Ss* (*x*, *y*, *z*) a *Kxx* = *Kxx* (*x*, *y*, *z*), etc. and *h* and *W* could be the functions of space and time (*h* = *h* (*x*, *y*, *z*, *t*), *W* = *W*(*x*, *y*, *z*, *t*)) which means that equation (1) describes groundwater flow for unsteady conditions in a heterogeneous and anisotropic medium, provided that the principal axes of hydraulic conductivity are aligned with the coordinate directions. Equation (1), together with the specification of flow and (or) head conditions on aquifer boundaries and specification of initial head conditions, creates a mathematical model of groundwater flow. A solution of equation (1), in an analytical sense, is an algebraic formula which indicates *h* (*x*, *y*, *z*, *t*), so that when the derivatives of *h,* with respect to space and time are substituted into equation (1), the equation and its initial and boundary conditions are satisfied. Besides these very simple systems, it is possible to reach an analytical solution of equation (1) only rarely, so therefore it is necessary to use numerical methods for solution. One of the methods is the finite difference method, where the continuous system of equations (1) is substituted by the finite set of discrete points in the space and time, and the partial derivatives are substituted by terms calculated from the differences in head values at these points. Such an approach leads to the system of linear algebraic differential equations. Values of head in specific points in time are obtained by their solutions. These values represent approximation of the time-variable distribution of piezometric head, which could

 ∂

⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎜ ⎟ + + −= ⎜ ⎟ ⎜ ⎟ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ (1)

 ∂  ∂

> ∂

 ∂∂

 ∂∂

a markedly terraced step with relative height approximately 15 m and base 3 m above the Danube water level. Absolute height of the base is around 110 m a.s.l. it is slowly descending from the Chotin village to the Štúrovo town. Hydrogeological conditions of the terrace were proofed only by a few boreholes, after which hydraulic conductivity of gravel varies from 6.6E-05 m.s-1 (the Chotín village) up to 2.0E-03 m.s-1 (the Štúrovo town – the Nana village). Groundwater recharge happens entirely from precipitation in locations where permeable blown sands or loamy sands and sandy loams are located in hanger. Groundwater from the terrace is drained on its edge to the lower step, partly on contact as it comes up to the surface and it is taken away by the drainage channels. The ground-water level in the alluvia is mainly influenced by the surface stream of the Danube River and then on other side by water seeping down from an adjacent terrace and through precipitation. The ground-water flow direction according to bilateral relation of the Danube water level and ground-water level was either to the aquifer or to the Danube.

Fig. 4. Hydrogeological profile along the Danube bank (400x exceeded), (Duba, 1964)

#### **2.1.2.3 Hydrology**

Through the hydrological characteristics of the study area and the description of surface flows in an objective time it is necessary to concentrate on the Danube River, which has here first- rated importance. Slovak Danube river reach belongs to the upper part of middle part of the river. Danube is keeping its alpine character in Slovak reach, in its upper part it has considerable slope around 0.4%o, it is flowing in its own alluvia and it is creating multiple systems of river arms. Water stages are first of all dependent on the water supply from the Alps. Maximum water stage reaches the Danube in June at the time of alpine snow and glacier melting. From June it comes to permanent decrease and minimum water stages are reached in December and January. The Danube water stage on 29 September 1954 in RK 1742.9 (Radvaň nad Dunajom) was 105.20 m a.s.l. Other surface flows in study area are rather small and short and their discharges are low. The maximum occurs in spring months, and in summer their discharge is considerably decreased. Such streams are Modrianský potok (creek) (from Veľká Dolina), its left-hand side tributary Vojnický potok (creek), and Mužliansky potok (creek). Main channels: the Obidský, the Búčský, the Kraviansky and the Krížny channel belong to the system, as well as the large amount of side drainage channels without any name.
