Vízügyi Közlemények, 1958 (40. évfolyam)
4. füzet - VII. Kisebb közleménye
(бб) phenomena at the initial pumping stage then those allowed for by the well-known non-equilibrium theories. Main characteristics of the actual percolation phenomenon are as follows : a ) The entire water volume is withdrawn from the depression cone, yet a small proportion of the water stored in the pores of the soil is only released in the first instance, while full depletion ensues but slowly, gradually in the subseguent periods. b) In lowering the groundwater tabic three characteristic phases can be distinguished (Fig. 5) : I. Upon starting the pumps a sudden depression occurs, and only a small proportion of water stored in the pores is released; II. In subsequent periods the rale of depression becomes more gradual (the relationship between s and log t, respectively between s — anc l ' can be r eP rcsent ed by a straight line ; III. Increasing the duration of pumping this gradual drop ceases as the groundwater table approaches a virtual equilibrium condition first in the vicinity, and later in the farther environment of the well. Even in case of a steady discharge the extent of drawdown depends in all three phases upon the distance of the observation point from the well. Proceeding further from the well the drawdown becomes gradually smaller. c ) The discharge of the well (Q) can be resolved info two components, of which the first (Q' j is drawn from the water volume stored in the soil interstices concurrently with lowering the groundwater table. The second portion (Q" ) is the delayed discharge due to water retained in the pores, and is too significant to be neglected (Fig. 6). d) The coefficient of hydraulic conductivity, as a physical characteristic of the soil, is not suited alone to describe the phenomenon, unless the flow is steady. The introduction of the pore volume, or of the storage coefficient ( S ) assumed as a constant physical characteristic of the soil, offers similarly no solution of the problem. The unsteady flow occurring while drawing on reserves stored in the soil is influenced by further physical constants of the soil. These are: 1. A varying parameter characterising the depletion of water, occurring concurrently with lowering the groundwater table. This parameter depends upon the soil structure, the rate and duration of pumping (the discharge), the drawdown, and upon Lhe distance from the well (Eq. 21). 2. The time of delayed percolation (т) of water remaining in the depression cone. Familiar theories bear no account of these influences. Therefore, depending upon the distance from the well and the time of observation at a pumpiny lest, different values are obtained for the unknown factors if computing these from recorded data (Figs. 8 and 11). The correct approach appears to be to carefully separate different approximating methods and to apply each only for the section where the degree of approximation attainable thereby is deemed acceptable. „Virtual values" in fair agreement with actual conditions can hereafter be computed for the unknown factors from observed pumping data. These values can be refined further by considering main characteristics of unsteady flow (Fig. 7) using the degree of'depletion of the depression cone (Eq. 22). The coefficient of hydraulic conductivity can be determined from the Dupuit formula only by applying the so-called е-method introduced by Thiem, yet the intermediate section of the meridian curve showing formal agreement should be selected carefully (Figs. 3 and 9). The non-equilibrium method of Theis and Jacob can be applied only to data observed in the vicinity of the well, and relating to the second phase of the drawdown phenomenon. The virtual coefficient of hydraulic conductivity that can be computed also for these cases is higher than the actual one, since the withdrawn amount of water incorporates a significant portion due to delayed percolation which has been neglected in the computation. Ну increasing the pumping time, computed values tend to approach actual conditions (Fig. 10). The storage coefficient (S ) cannot be considered as a constant physical characteristic of the. soil. Unsteady seepage can be described by the varying parameter (S') expressing instantaneous depletion. The degree of depletion of the entire depression cone, and approximating limit values of the varying parameter are given in Fig. 1'2, respectively in Table 1. and Fig. 13.
