Vízügyi Közlemények, 1959 (41. évfolyam)

4. füzet - V. Kisebb közlemények-Ismertetések

(50) DISCHARGE OF PERCOLATING GROUNDWATER DETERMINED BY WATERHOUSEHOLD INVESTIGATIONS By Gy. Kovács Candidate of Technical Sciences (For Figures see the Hungarian text, pp. 332—354) A problem often encountered in the design work of hydraulic installations is the determination of the quantity of percolating groundwater. In many cases the strictly hydraulic solution is unsatisfactory owing to the uncertainty of boundary conditions and to the wide-range variations that are likely to occur in the permeability coefficient. All these may lead to wide discrepancies between computed and actual results. The determination of the critical percolating groundwater volumes required for the planning of the drainage system for the Bodrogzug area in the North-Eastern part of Hungary at the junction of the Tisza and Maros Rivers, presented a similar problem. Investigations into the groundwater household were found to yield satisfactory results in similar cases. It has been established by previous studies that the main factor responsible for groundwater recharge is the direct infiltration of precipitation, while water is removed therefrom by evaporation. (Groundwater under agriculturally cultivated areas and unaffected by forests, irrigation practices or surface waters are considered as normal.) The rate of both evaporation and infiltration varies with the depth of the groundwater table below the terrain surface, however the trend of variation is different in the two processes. Consequently, balanced conditions between the two phenomena may ensue at a certain, well defined depth only. Whenever the groundwater table rises to above this level, evaporation is in excess of infiltration and a deficient waterhousehold balance ensues. Conversely, excess water results from groundwater tables located below this level. Similar differences between adjacent areas are equalised by the ensuing horizontal percolation. The characteristic groundwater-household curve can be constructed once the variation of infiltration and evaporation with the average depth of the groundwater table has been determined. This curve supplies the necessary information on the difference between infiltration and evaporation, as a function of depth, which difference is equal to the horizontal supply to, or withdrawal from, groundwater situated at given depth. The curve of infiltration (Fig. 2 and Equation 2) can be established in the following steps: a

) From the long-term average precipitation during the winter period (from the 1st November to the 30th April) the long-term average evaporation — computed by Eq. (1) for the same period — and the local run-off —• determined by the method shown in Fig. 1 — is deducted. The water quantity filtrating from the surface, i. e. one point of the relationship curve, has been established. b

) Using data given in columns 4 and 5 of Table 1 the water quantity retained over the first and second metres, respectively, is determined. This will naturally depend upon the composition of the soil. Deducting this from the infiltration value obtained for the surface, the average precipitation reaching the groundwater at a depth of 2 m is determined. c

) Once the B

0 value pertaining to the depth m

0 = 2 m is known from point b

) variations in infiltration can be computed according to Eq. (2). The relationship can be assumed as valid, for depths greater than 1,0 m irrespective of stratigraphical conditions. . I d) If the variation in storage capacity over the first metre depth is assumed as linear, the relationship can be described by a parabola of second degree over the same distance. Changes in evaporation have been determined in two steps, in a manner similar to that followed in the case of infiltration. For describing the relationship in depths greater than 1,0 m, Eq. (4) is suggested, while the relation of L. A. Ramdes is adopted closc to the surface, Eq. (3). The relationship thus arrived at shows a remarkably good agreement with measured values (Fig. 3). It is proposed that basic data for the computation should be assumed after a thorough study of the relationship between groundwater household and ground-