Ice Regime Variation in the 20th Century Along the Serbian Sector of the Danube and Assessment of Global Climate Change Impact - page 04

The following conclusions can be drawn:

  • From a qualitative perspective, P1 and P2 exhibit similar characteristics with regard to SNAT variation and alternating average and severe winters.
  • From a quantitative perspective, there are differences between the two periods as exceptionally cold winters (SNAT>180°C) were more frequent in P1. At Veliko Gradište, such winters were registered 18 times in P1 and six times in P2. At Novi Sad, severe winters (SNAT>180°C) were recorded 16 times P1 and nine times in P2. Maximum SNAT values at both stations were considerably higher in P1 than in P2 (505°C vs. 400°C at Novi Sad, and 450°C vs. 365°C at Veliko Gradište). The SNAT histogram for Veliko Gradište shows that the frequency of winters with SNAT>180°C was 18/53 = 34% in P1 and 6/44 = 14% in P2, while at Novi Sad it was 16/43 = 37% at P1 and P2 9/44 = 20% in P2.
  • A comparison of P1 and P2 histograms shows that both periods exhibit cycles of cold and mild winters.
  • A comparison of the difference in SNAT between the stations at Novi Sad and Veliko Gradište shows that there was no change in P1 and P2.

For a more exact comparison of the winter temperature regimes in P1 and P2, a probabilistic analysis of the SNAT time series at Veliko Gradište and Novi Sad was undertaken. The results were interpreted using standard probability plots (Figures 10 and 11). It is apparent that severe winters are less probable to occur in P2 than in P1, as is, consequently, ice formation on the river.

 

Fig10
Figure 10: SNAT probability for the meteorological station at Veliko Gradište.

Fig11
Figure 11: SNAT probability for the meteorological station at Novi Sad.

 

Based on these analyses, it appears that there is a certain variation in meteorological drivers of the winter regime, which can also affect the ice regime of the Danube.

The winter temperature regime variations and the frequency of ice formation on the river are of special importance for assessing the climate change impact on the ice regime in the Serbian sector of the Danube. It is almost certain that the radical decrease in the frequency of severe winters and ice formation in recent times (after 1970) can be associated with global climate change. In this regard, if there is no major variation in the climate change trend, it is likely that the probability of any future ice hazards in the Serbian sector of the Danube will be reduced. However, the SNAT histogram at Veliko Gradište shows that the period 1901-1924 is rather similar to 1971-2006 with regard to the frequency of severe winters. This means that mild winter cycles have occurred in the past. Consequently, regardless of the lower probability of severe winters in recent times, it is not possible to reliably claim that there will be no exceptionally cold winters in the future.

The best argument in favor of the above comes from the ice event that took place in the winter of 2011/12. An exceptionally cold wave hit the Danube River Basin in February 2012, with very cold air temperatures (below -20°C). As a result, considerable ice formations were registered on the Danube and its tributaries, involving both drift ice and static ice in a large part of the Serbian sector of the Danube. However, there were no critical situations in the winter of 2011/12, from an ice defense perspective.

The reason for this was twofold: the period of low air temperatures was relatively short (less than a month) and the hydrological conditions at the time were highly favorable (no significantly high flows of the Danube and its tributaries).

 

Hydrological Parameters of the Ice Regime

The Danube's winter hydrographs exhibit characteristic features. As the air temperature drops at the beginning of winter, the inflow from the river basin decreases and the Danube's discharges drop to low flows. During the period of low air temperatures and ice formation, the discharges generally stagnate. In the spring, the temperature increases rapidly and as snow melts, the inflow from the river basin increases as well. As a result, there are spring flood waves on the Danube, along with ice detachment and drift. Generally speaking, ice formation begins in the falling branch of the autumn wave, and ends in the increasing branch of the spring flood wave.

Table 2 shows the range of discharges of the Danube downstream from the mouth of the Velika Morava, including all three phases of ice formation (autumn ice drift, static ice cover and spring ice drift), for the 20 severest winters in P1. The discharges during the period of autumn ice drift were in the interval from 1400 to 9000 m3/s, during the static ice cover period from 1200 to 9000 m3/s, and in the spring ice drift period from 2200-12000 m3/s. It is apparent that relatively high flows of the Danube are possible in all these phases.

 

Tab02