Vízügyi Közlemények, 1987 (69. évfolyam)
4. füzet - Pálfai Imre: Aszályos évek Magyarországon
Aszályos évek Magyarországon 525 рисункам видно, что территориальное распределение засушливости из года в год изменяется, но чаще всего появляется на Большой венгерской низменности. По рис. 10 можно выделить районы с различной засушливостью по 10% обеспеченности индекса засушливости. Самым засушливым районом страны является Хортобадь (РА1 10 % = 7,50). На основании полученных данных автором сделана попытка прогнозирования засушливости. Приведены примеры при различных начальных условиях для определения вероятности последующей засухи. На рис. 11 приведена зависимость обеспеченности засухи по стране (PAI = 5,2) от индекса предшествующего года, осенних температур воздуха и суммы осадков. Считается возможным использовать цикличность среднего за пятилетный период индекса засушливости (рис. 12) для долгосрочного прогноза, учитывая что наиболее вероятная длина цикла составляет 14 лет (рис. 2). * * * Dry years in Hungary by Dr. I. PÁLFAI, С. E. Hungary is located in the semi-arid climatic zone. The weather is mostly favourable for agricultural production but in some years there are dry periods, and in some others there is ample precipitation. Both may hinder agricultural production and may cause severe damages. The author was engaged in investigating dry years over a period of about one thousand years. Dry spells during the 11th to 19th centuries were characterized by use of historical sources and by collected comprehensive literature (Table I). In order to qualify the severity of droughts, a so-called drought-index (PAI) was introduced shown in formula (1) that is the ratio of the mean temperature of April-August and of the weighted precipitation height of October-August. This index can be corrected by two factors: k { is the ratio between the lenght of the dry spell in summer and of their multiannual average value, k 2 is the ratio between the mean annual groundwater level and of their multiannual average value (3). The drought index of Spring (4) and the drought index of Autumn (5) are supplementary parameters with an identical structure like index (1). Weighting factors for monthly precipitation values are shown in Table II. National average values of the drought-indices were determined by data taken from 67 meteorological stations (Fig. I.). The 86 year-long runs of average temperature (r IV VII [), of the weighed precipitation sums (P x VII I). and of the drought-indices are presented in Fig. 3. On the national level, the drought-index was larger by 10 percent than the multiannual average (4.73), e.g. larger than PAI § 5.2, the year was qualified as dry. The main data for such years are accumulated in Table III. The severest drought during the period between 1901 and 1986 was in 1952, followed by 1904 and 1935. Discharges in the rivers - in these dry years - was minimum. This could be proved by the time-series for low flow at the cross-section of Szeged on the Tisza River (Fig. 4.). Probability of occurrence of dry years was determined also by aid of the drought-index. Some characteristic dry years are presented in Figs. 6 to 9. It is visible that the areal distribution of droughts is different from year to year but they struck mainly the area of the Tisza- Valley, and of the Great Plains, in general. Fig. 10. is suitable to delineate the dry years in different degrees where also the areal distribution of the drought-indices having a probability of 10 percent can be seen. The driest area in Hungary is the Puszta of Hortobágy (PAI !0 % > 7.50). Finally, an attempt is made to forecast droughts. Examples are presented to verify the probabilités of national droughts attached to certain preconditions. In Fig. 11. the probability of occurrence of a nationwide drought (PAI^i 5.2) is presented as a function of antecedent droughtindices, average temperature of the air in autumn, and the height of precipitation in autumn.
