Vízügyi Közlemények, 1967 (49. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

(4 3) lo the onset of continued frost weather attained the order of 200 mm, and owing to uninterrupted cold weather of long duration (see Fig. 1) the soil froze to a depth of 15-20 cm conserving thereby soil moisture at near field capacity for the spring period. By the end of January the soil was covered by a 40 to SO cm deep snow cover. Melting commenced at the beginning of February. Melting started at a fortunately early time, round one month earlier than usually, and the resulting inundations were less damaging to agriculture. Melting occurred also at a relatively favourable rate. Both volume and distribution of the accompanying precipitation were rather favourable. Indeed, precipitation water that occurred in February and March from snowmelt and rainfall attained a total depth of 80 to 120 mm and was the direct cause of inundations. The combination of the above favourable factors resulted eventually in the development of extreme, though not catastrophic conditions. The greater part of water could be removed by the end of February and even major inundations could be drained in most places, but stagnant pools persisted for considerable periods in depressions and alkaline plots. Interrelations between factors affecting the occurrence of excess surface water Basic data for the plain-land catchments of the country together with data on the excess water in Spring, 1966 have been compiled in Table I. Volumes of excess surface water drained from plain-land catchments of individual district water authorities, as well as changes in the inundated area, expressed as percentage of the total catchment area, are given for successive 10-day periods in Fig. 2. Inundation by excess surface water occurs in plain-land catchments having, as a result of slope conditions, no natural drainage, or where drainage occurs at a very slow rate only, if owing to meteorological factors a volume of water depending on local conditions saturates the upper soil layers and causes surface inundations. These unwanted waters are removed at a rate and to an extent depending on human interference from the area. The factors involved are thus meteorological ones, M, local ones influencing the occurrence of excess surface water, ht, the extent kt of area affected, the time t of removal, the degree of developmen qs of the drainage system, as well as the volume В of excess surface water removed. Once numerical values representing also weight can be assigned to each of them, the relationship may be regarded as the fundamental equation of excess surface water, described symbolically by Eq. (1). This fundamental equation has been used for the analysis of excess surface water. Considering the factors in Eq. (1) in succession, the following conclusions have been arrived at: factor M is substantially a function of the two indirect and two direct meteorological factors mentioned earlier. In the situation under consideration this may be assumed to be of the same order of magnitude for all plainland catchment areas of the country, so that it may be neglected. Consequently, Eq. (2) may be used for further calculations. Local factors ht can no more be regarded as identical, unless the assumption is made that M exceeded the limit below which the rate of infiltration is still influenced by the type of soil, cultivation, or position of the groundwater table, and in consequence, practically the same volume of excess surface water occurs from the incident precipitation. Naturally, this simplifying assumption holds true only in areas with overwhelmingly cohesive soils. Where, on the other hand, considerable movement of water occurs towards groundwater (regardless to what extent the top soil layer is saturated) and water once infiltrated does not appear again as excess surface water, the conclusions arrived at by this assumptions should be accepted with great reserve only. For the simultaneous expression of the magnitude of inundated areas (et, %) and the time t of drainage, the index e has been introduced [defined symbolically in Eq. (5), formally in Eq. (5a)j. The degree of development of the drainage system has been represented by the specific capacity (q, litres/second per sq. km) and specific density (s, km/sq.km) of the canal network, as the index qs obtained as the product of the two, see Eq. (7). The basic equation applied to this particular situation [represented symbolically by Eq. [6)], characterizes thus eventually the relationship between the inundation index e, the degree of development qs and the total depth В (mm) of excess surface water removed.