Vízügyi Közlemények, 2003 (85. évfolyam)

4. füzet - Nováky Béla: Éghajlat és víz: bizonyosság és bizonytalanság

542 Nováky Béla both of them depending on the climate. The time series of natural change of water resources, AW„ can be separated into two components: one deterministic component (5) depending on the climate and one stochastic component (6). On this basis, the natural change of water resources can be modelled. The observed and the computed (modelled) variant of the curve continuously integrating the deviances from the average value of annual natural changes of water resources, show a good accordance among each other (Fig. 2). Thus, the model may be serviceable, after the necessary stability studies, for investigating the impacts of potential climate changes or for carrying out climate sensitivity investigations. In the present paper, the 52 years long time series of the annual mean temperatures observed at the station Keszthely and the 82 years long one of the annual precipitations falling onto Lake Balaton were investigated. In these time series, no significant trends could be detected, i.e., chance is playing a determining role in the timely pattern of both annual precipitation and mean temperature (Table Г). The investigation of the probabilty distributions of these two climatic elements, including the computation of confidence intervals, demonstrates that even under the present climatic conditions, due to the high variabilty of our climate, more extreme values than those recorded in the past may occur. The extremes as observed in the last years may be only a part of natural variability, thus they cannot be considered as proofs of any climate change. At the same time, one cannot exclude with full certainty their being already parts of a climate under changing conditions. It is sure that this change, if it exists at all, is slow and can be followed. It is therefore very important for hydrology that continuous investigations (like that presented in this paper) be carried out. Anyhow, the extreme climate values - no matter if they present themselves as parts of the variability of the present climate or as those of an already changing climate - admonish that they may have impacts onto the water household, including runoff regime, which could hamper the basically recreation-orinted utilization of Lake Balaton. The extreme situations occurred in the last years would indicate, even without any climate change, the necessity of a reconsideration of the utilization of Lake Balaton. According to the regional climatic scenario available for a wider surroundings of Lake Balaton

, it is probable that the temperature of the winter and summer half year will increase by 1-2 °C, while an increase by 5-10% of the winter precipitation and a decrease by 10-15% of the summer precipitation cannot be excluded. These were our starting values, when setting up, with monthly time steps, the climatic scenario of our climatic impact study, whose chosen methodology was the chronologic analogy. In accordance with the latter, such years have been selected from the (instrumentally observed) past, whose annual, summer and winter mean temperatures as well as average precipitation values are equal to, or at least fit well, those resulting from the regional climatic scenario. It was also assumed that the monthly distribution pattern of the temperature and precipitation will be the same as in the selected years of the past, i.e. the similarity hypothesis was accepted. According to the first scenario, fO years, and according to the second, 7 years were selected and then the expected changes of precipitation and temperature were calculated, in accordance with these two scenarios (Table IV). Starting from the climatic scenarios, the equilibrium water household and the equilibrium water surface of the lake were investigated. Under the present climatic conditions, the equilibrium water surface of the lake is 1300 km

2, just slightly overpassing the double of its actual value of 600 km

2 (Fig. 1). As a consequence of a decrease by 5-10% of the annual precipitation and an increase by 1-2 °C of the annual mean temperature, the equilibrium water surface of the lake would be reduced to ca. 700-900 km

2, closely approaching the extent of its present surface, maintained by artificial regulatory measures, while also the outflow from the lake would be lower (Fig. 4). Under the postulated new climatic conditions, the present water surface and the present regime of outflows at the sluice of Siófok, cannot be ensured at the same time. Thus, from the point of view of the lake's water household, the following options are available: (1) Maintenance of the present water surface of the lake, with a con-