Vízügyi Közlemények, 1958 (40. évfolyam)
4. füzet - VII. Kisebb közleménye
Ш (79) the direct evaluation of long-term records. Every recorded rainfall value is multiplied by the runoff coefficient applying to the same day (a). Data thus obtained — which represent the surface water due to précipitation that does not infiltrate into the soil — are ordered according to functions given by Eqs. (5) and (6) respectively. The runoff coefficient is used to allow for irregular effects influencing the occurrence of surface waters, е. д., soil freezing, precedent precipitation, etc. No similarly detailed evaluation is deemed necessary in the summer period, inasmuch as moisture conditions of the soil in irrigated areas can be estimated readily for this period on basis of the irrigation schedule. A general form of the function is given in Eq. (5), while precipitation conditions prevailing in Hungary are already taken into consideration in Eq. (6), respectively in data compiled in Table IV. In connection with surface waters due to precipitation, investigations have been extended to cover the size, shape and micro-topography of the catchment, effects of vegetation as well as of irrigation methods. Effects of size have been studied by methods of mathematical statistics using daily precipitation averages first of two, while later of 3, 4, 5 and 6 observing stations. Investigating rainfalls of duration t and height of cover h attained, or exceeded on the average once every m years, a relationship according to Eq. (4) has been obtained between the quantity of precipitation and the size F of the catchment for the area considered (Fig. 1. and Table III ). The effect due to size on the depth of precipitation is the more pronounced, the shorter the duration and the rarer the occurrence of the rainfall. Having established functions (4), the intensity relations of form (2) required for the investigation can be determined for catchment areas of any given extension. Intensity values attained in Hungary once every 5, 10 and 25 years over a plain catchment area of F = 1000 sq.km are illustrated by functions (8) to (10). As regards the role of vegetation attention is drawn to data given in Fig. 4 and Table V which permit the selection of rainfall periods to be investigated from the agricultural point of view. Under conditions prevailing in Hungary the draining of rice-fields prior to harvesting is of particular importance ás far as the occurrence of surface waters due to irrigation is concerned, inasmuch as a water cover of about 10 cm depth is to be removed during a time T = 1 to 15 days. This volume is increased by precipitation (h T) occurring during the corresponding period. Having gained sufficient information about the origin, the distribution in space and time of excess surface waters, the determination of runoff volumes (g, lit/sec hectare) required for the dimensioning of drainage system can be proceeded with. The determination is founded on the simultaneous consideration of the time of concentration and of the waterbalance. Concentration is taken into account by adopting the assumption of t = r on basis of relation (11), i. е., a rainfall having a duration equal to Ihe time of concentration. Water retained temporarily at the surface up to the z'-lh clay (V, , millimetres) is allowed for by Eq. (12) derived by computing of the waterbalance. In Eq. (12) <?< = the runoff on the i-th clay, and q is the runoff used for design purposes, lit/sec. ha. The two basic relations are applied concurrently. E. д., in case of drainage systems designed for a copious conveyance capacity (r/j in Fig. 6a), surface waters spill but rarely over the canal embankments, i. е., no inundation occurs ( Fig. 6c) and waters are stored exclusively within the canal system. In such cases the consideration of flow conditions is regarded as sufficient. On the other hand, if the capacity of the conveying system is small (q 2 in Fig. 6b) the canals are unable to cope with the excess water volume, inundations ensue and large areas act as reservoirs (Fig. 6d ). Under such conditions the second relation becomes predominant. In case of canal systems designed for intermediate capacities the two relations should be coordinated by careful judgement and should be applied simultaneously. In order to apply the two relations simultaneously the concept of the time of tolerance (d, in days) is introduced. The latter is undersiood as the inundation period varying during the growing season which the plants are capable of withstanding without significant agricultural damage. In early spring the time of tolerance extends to a few days, to one or two weeks at best, while to but one day, or 1,5 to 2 days in summer. With this knowledge and using relationships (13) and (14) the design flow can be computed, which, used as a basis in developing the canal system ensures for
