Ice Regime Variation in the 20th Century Along the Serbian Sector of the Danube and Assessment of Global Climate Change Impact - page 05

As part of the comparative analysis of all winter regime parameters of the Danube in P1 and P2, it was interesting to compare the hydrological conditions during river ice periods. The range of discharges of the Danube by ice phase (for all winters in which there was ice) in P2 downstream from the mouth of the Velika Morava are shown in Table 3.

Winters can be categorized applying the criterion of "favorable" hydrological conditions during the period of ice formation, based on the Danube's discharge of Q=6500 m3/s, which roughly corresponds to average hydrological conditions during the period of ice formation. In P1 (Table 2), the probability of favorable hydrological conditions in severe winters is relatively low (30%). It is much more realistic to count on ice formation coinciding with high flows of the Danube. 

In the most critical period of ice formation (spring ice drift), in 70% of the cases a discharge of Qmax ≥ 6500 m3/s is to be expected, in 50% Qmax ≥ 8000 m3/s and in 15% Qmax ≥ 10000 m3/s. In the autumn drift ice and static ice phases, the probability of elevated discharges of the Danube is much lower.

On the other hand, it follows from the hydrological conditions in P2 (Table 3) that there were no elevated discharges of the Danube in the static ice and final ice drift interval in any of the six severe winters (in which ice and ice cover were formed). Based on a comparison of "favorable" hydrological conditions in P1 and P2, it is apparent that the conditions in P2 were very favorable downstream from the mouth of the Velika Morava. As the combination of meteorological and hydrological conditions along the Danube at any one time was favorable, there were no ice hazards and no threat of ice floods.

 
Tab03

 

Climate change impacts water circulation in nature, including hydrological parameters of the ice regime. However, climate change certainly has a more profound effect on ice formation than on hydrological parameters. The above analysis of the "favorability" of winter hydrological conditions shows that the period since 1970 exhibits a favorable coincidence of meteorological and hydrological conditions along the Danube, such that there were no ice hazards and no threats of ice floods. It is therefore possible to conclude that climate change is having a positive effect on the ice regime because the lower frequency and shorter duration of ice occurrences reduce the probability of coinciding high flows and ice formation on the Danube.

 

Conclusions

  • When ice regime changes in the Serbian sector of the Danube are considered, the complexity of ice formation and its dependence of several important factors need to be addressed. Apart from natural drivers related to climate conditions, anthropogenic factors that affect hydraulic and morphological parameters and the river's thermal regime also play an important role.
  • Damming for the purposes of the Iron Gate 1 HPP and consequent hydraulic and morphological alterations have had the greatest impact on the ice regime. Impoundment has reduced the ice discharge capacity of the river channel and modified the boundary conditions of the ice regime.
  • Changes in the Danube's thermal regime, due to chemical and thermal pollution, are evident. In recent decades, the thermal regime has resulted in considerably slower river cooling.
  • The analysis of meteorological conditions along the Danube in the 20th century shows certain changes in the meteorological drivers of the Danube's winter regime after the year 1970. These changes have had a major impact on the Danube's ice regime. The frequency of ice formation before the year 1970 varied from 70 to 90%, and thereafter from 30 to 60%. With regard to static ice, its frequency before 1970 was 30–60% and after 1970 3–18%.
  • The much lower frequency of severe winters and ice formation in the more recent period (after 1970) can almost certainly be associated with global climate change. In this regard, if there is no major variation in the climate change trend, one can say that the probability of ice hazards is low in the Serbian sector of the Danube. However, it should be kept in mind that mild winter cycles have occurred in the past. As such, regardless of the evidently lower probability of severe winters in recent times, one cannot reliably claim that there will be no exceptionally cold winters in the future.
  • An argument in favor of the above is ice formation in the winter of 2011/12, when the Danube River Basin experienced an extremely cold wave, with very low air temperatures (below -20°C). This resulted on considerable ice on the Danube and its tributaries, for the first time after 27 years. Still, there were no critical situations in terms of ice floods and ice defense, because the period of low temperatures was short and the hydrological conditions favorable at the time.