Vízügyi Közlemények, 1959 (41. évfolyam)

4. füzet - V. Kisebb közlemények-Ismertetések

(36) for the period considered the sum of surface runoff, moisture storage in the top soit and evaporation, related to the total rainfall during the same period. The part remaining thus unaccounted for is the precipitation volume that gives rise to periodical groundwater storage. The slope of the equalizing line is equal to the approximate value of the free pore volume (n„) in the groundwater bearing soil layer [71, 73, 74]. The groundwater regime is greatly influenced by the depth of the groundwater table below terrain Level. The deeper the table, the later the annual maximum and minimum values occur, the groundwater regime is the more uniform, and the smaller the range of annual fluctuations (Fig. 5). Under climatic conditions prevailing in Hungary groundwater fluctuations due to infiltering precipitation is only possible where the depth of the groundwater table is less than 7 to 10 m. At greater depths the so-called undisturbed-type of groundwater can be encountered (Fig. 8), above which any infiltering amount of precipitation is used to increase the moisture content of the top-soil. In the vicinity of water courses groundwater stages are influenced, besides the abovelisted factors, also by watersurface, fluctuations in the river. At times of rising river stages water percolating into the soil accumulates in the form of groundwater first along the river, and later, owing to the steeper gradient, at greater distances as well. On the other hand, at times of low water, groundwater < draining from adjacent areas increases the flow in the river. A fairly close relation can be established for soils close to the river bank, between river stage and groundwater level, whereas with increasing distance the relationship becomes less and less pronounced (Fig. 9). The affected range depesds on local conditions and is governed mainly by the physical properties of the permeable layer. In case of a watercourse in an elevated bed, and over gravelly subsoil (Danube), the influence of river stage on groundwater level has been observed to extend to a maximum distance of 10 km. In finely graded permeable layers this range is only 1 to 2 km, or sometimes but a few hundred metres. Summarizing the above considerations it can be stated, that, as revealed by results of practical hydrology, the most significant influence on groundwater is that due to vertical moisture movement through the covering soil layer. 2. Theoretical investigations are expediently based on the waterhousehold equation. Considering a separate soil prism, no change in water volume resulting in a change in groundwater level can occur, unless either lateral inflow or out flow, or vertical moisture movement through the soil cover is possible. Problems related to water movement in the saturated, two-phase layer below the groundwater table, to horizontal groundwater flow, have been investigated first Natural groundwater conditions in the plain territories of Hungary are characterized by flat groundwater gradients, and consequently by low percolation velocities. Maximum representative velocity values vary between v = 0,8 and 2,3 cm/day, i. e. 2,9 and 8,4 m/year. General flow extending to large areas is under similar conditions out of question. Basins and alluvial plains filled with coarse grained layers (the so-called Kisalföld, between river stations 1790 and 1850 kilometres on the Danube), as well as locations, where a significant drawdown has artificially been created, are exceptions to this. However, as regards the quantity of percolating water, higher rates of seepage occur, even in these cases, most probably in the upper part only of the permeable layer. Latest hydraulic investigations point to the fact, that the active (effective) cross section of the soil, through which percolation actually takes place, varies with water pressure and thus rate of percolation decreases with depth. At the lower limit of the velocity range a limit gradient occurs, below which no percolation takes place [29]. On the other hand, variations in groundwater volume ensue much more easily by water exchange through the three-phase layer above the groundwater table. An important characteristic of three-phase soils is that even in those, wich become impermeable when saturated, lively water movement can develop at lower moisture contents. Water can move in such soils either in liquid, or in vaporized form. Movement is caused by gravity, capillary potential and by the gradient in soil temperature (Fig. 15). Considerable vapour-movement mag develop under the effect of