**4. The hydrological classification of groundwater regimes**

The basic regime specificities of the fluctuation of groundwater level in the alluvial plains and terraces of rivers in dependence upon the fluctuation of the level in a watercourse may be observed only if the natural conditions create the possibility of the hydrodynamic continuity between the level in the surface watercourse and groundwater. If such a relation is possible, then three zones may be earmarked according to the regime symptoms of the groundwater level fluctuation on the riverine territory:


direction to the river in the Čenkov locality. The second drainage area is bounded from the south from the Obid canal river mouth to the Danube up to the Štúrovo town and from the north by the "Pod kopanicami" drainage canal. The feeding effect of the river is decreased by the drainage of lower parts of the Mužliansky creek and the Obid canal. The remaining part of the study area is drained by the local system (the Obid, Krížny and Búčsky canal). On Fig. 8 there is a 3D visualization of the whole groundwater body, with velocity vectors in the second layer of the groundwater body where there are the highest filtration velocities along the Obid canal and Mužliansky creek and at their river mouths, and also along the bank of the Danube (drainage) around RK 1742. Similarly in the third layer, there are the highest filtration velocities along open streams and along the Danube bank from RK 1744 to RK 1742 and around RK 1729. The range of filtration velocity values in the whole groundwater body varies from 1E-10 m.s-1 to 1.19E-03 m.s-1. Particle tracking is used for the tracing of the groundwater flow directions, which means creating flow lines by carrying out the tracing of infinitely small imaginary elements movement situated in the flow field. In the reach from the Moča village to the Čenkov settlement groundwater flow into the Danube except the northern part in which groundwater flows to the drainage canals. From the Obid canal mouth to the Danube River (RK 1727) up to the Štúrovo town groundwater flow direction is again into the Danube. The volume water budget for the whole model at the end of the simulation is calculated in order to control the results. It indicates acceptation of the numerical solution. Continuity has to be preserved also for the total model inflow and outflow or sub region of the layer. The difference in the water budget of the study area is 0.01 % the difference should be in ideal case smaller than 0.1%, what is fulfilled. In general

an error up to 1% is accepted (Konikow & Bredehoeft, 1978).

groundwater level fluctuation on the riverine territory:

the groundwater level and the next zone is entered.

**4. The hydrological classification of groundwater regimes** 

The basic regime specificities of the fluctuation of groundwater level in the alluvial plains and terraces of rivers in dependence upon the fluctuation of the level in a watercourse may be observed only if the natural conditions create the possibility of the hydrodynamic continuity between the level in the surface watercourse and groundwater. If such a relation is possible, then three zones may be earmarked according to the regime symptoms of the



directly influence the groundwater level behind this line via bank filtration.


It is clear the boundary between narrower and wider riverine zone, similarly as between wider and external riverine zone is conventional to the certain degree. It must be understood to the intent that its determination pursuant to the stated principles is based upon a certain length of observation time, during which all somehow extreme situations need not to occur. The most distant demarcation of the external riverine zone shall be the edge of the terraced step of the bottom land or other its demarcation at the contact with other hydrogeological units.

If the natural conditions are anthropogenically influenced and create a limited possibility of a hydrodynamic link between the level of the surface stream and groundwater, as it is in the case of the constructed underground non-permeable seating wall (the "NSW") between the Danube River and hydrogeological collector with omitted sections in the NSW, so called "windows", the demarcation of the boundaries between the zones is more difficult. The coefficients of determination for the individual boreholes are considered, while they are very important for the assessment of the degree of dependence between the level of the Danube River and the groundwater level. The boundary between narrower and wider riverine zone shall be determined using two-dimensional models of groundwater flow, displaying the isolines of piezometric groundwater heads and the vectors of filtration speed. The boundary is changed in dependence upon the level condition in the watercourse. The average width of narrower riverine zone of the Danube River at the left side of the lower Váh River in the proximity of an "window" is approximately 2500 m and it is approximately 2300 m on the Čenkov plain. Wider riverine zone is earmarked by the boundary of hydrogeological region Q 057 in both cases.
