Projekt A 1.1.4:
Long-term Behaviour of Snow Cover
in Baden-Wuerttemberg and Bavaria
Summary of project's results
(valid as of 1996)
Special attention should be given to long-term studies on snow cover conditions, because as a consequence of changes in climate, changes in the frequency of snow cover are also to be expected. Changes in the snow cover regime and its parameters have lasting effects on the hydrological balance (groundwater recharge) and the discharge regime (flood formation).
As part of the KLIWA project (Climate change and consequences for water management), and in agreement with its co-operation partners, the State Office for Environmental Protection in Baden-Württemberg, and the Bavarian State Office for Water Management, the Hydrometeorology Department at the DWD has been studying the long-term behaviour of various snow cover parameters in southern Germany. After detailed study of all the available time series, the records from 508 climate stations proved to be suitable for a further regionalisation of the snow cover. These records were then regionalised and converted to 2139 grid areas in Baden-Württemberg and Bavaria, so that a detailed presentation according to altitude is available (see annex, figures).
Snow cover parameters studied
The following parameters are suitable for describing the snow cover conditions and the water-reserves stored in the snow cover, including their long-term changes:
- Snow cover duration
- Snow cover time
- Continuance of the longest snow cover period (winter cover)
- Beginning (date) of the maximum snow cover depth
- Constancy of the snow cover
- Retention of the winter cover
- Maximum value of the water equivalent
With the exception of the snow cover time, almost all of the above-mentioned quantities showed a decrease (negative trend) practically everywhere, i.e. area-wide, in the reference period 1951/52 to 1995/96. For some parameters (e.g. snow cover duration, winter cover duration), (fig.1, fig. 3), however, a reduction in the negative trend can be observed with increased height of the terrain, and occasionally even a trend reversal on mountain ridges and peaks. The decrease in the mean snow cover duration (fig. 1) showed some distinctive regional features. In the eastern parts of the area under investigation (eastern Alps, mountain forests) the decrease amounts to 20% to 30% in the lower altitudes. In the western areas (upper Rhine valley and the western slopes of the Black Forest), on the other hand, the decrease amounts to approx. 50% and more and is reduced to 10 to 20% in the middle altitudes. In the higher regions values of under 10% are observed on average.
The snow cover time (first day/last day with snow cover in the six months of winter) (fig. 2), shows distinctive features in the regional distribution of the absolute trend. Large parts of the area under investigation show increases (positive trend); above all in the south-eastern part of the Bavarian Forest and the northern part of the Frankish Alb the snow cover time has increased in the period under investigation by up to 30 days. An explanation for this could be the increase in extreme weather conditions, i.e. snow fall can already occur in early autumn or still in spring (without the snow cover having to last during the whole of the winter, as the snow cover time also includes days without snow cover). A change in the trend dependent on terrain altitude is not discernible.
The winter cover (longest snow cover period) (fig. 3) allows conclusions to be made on changes in the characterization of the winter periods: the trend is doubtless towards winters with less snow and with a shorter duration of snow cover. In the reference period significant decreases are recorded in the continuance values. These extend up to the middle altitudes. In relation to the mean values of the period under investigation, this is a reduction of 20% to 60%.
Due to the very great variability of all snow cover parameters, proof of the statistical significance for the found trend values is possible only in isolated cases. Nevertheless, climatological changes can certainly be concluded from the area-wide concurrent trend behaviour of the quantities studied.
The trend behaviour of the snow cover parameters is, however, not in contradiction to each other, and it concurs at other places in its tendency and magnitude of the determined values with the studied behaviour of various meteorological quantities important for the snow cover regime. The general decrease in the snow cover duration, especially at lower altitudes, can be attributed to the considerable rise in air temperatures in the months of December to March. There is also general concurrence with the findings on the long-term behaviour of the snow cover described in the literature.
As yet no area-oriented statements can be made for the water equivalent of the snow cover. Time series studies of the maximum water equivalent values at certain stations show that the decreases which occurred in the reference period are between 25% and 60%. Further information is absolutely necessary for making concrete regional statements on the water equivalent.
Of the snow cover parameters, the continuance of the snow cover, determined directly from measurements and observations, is the most expressive basic quantity for describing snow cover conditions. For this reason, this quantity is given priority when looking for confirmation of the expected snow-hydrologically relevant trends and their regional differences in the area under investigation. The clear decrease in the snow cover duration trend in the lower and middle altitudes confirms this approach.
The present area-representative investigation for southern Germany also concurs in its tendency and the magnitude of the determined values with the behaviour of various meteorological quantities important for the snow cover regime already investigated in other areas. The widespread decrease in the snow cover duration, particularly at lower altitudes, can be attributed to the considerable increase in air temperatures in the months of December to March. Due to the great variability of the snow cover parameters, the statistical significance of the changes can only be proved in isolated cases. Although the effects of climate changes on the snow cover regime cannot be proved beyond doubt statistically, the existence of real changes can be assumed to be certain from the area-wide concurring trend behaviour of the various quantities. With regard to the different spatial and temporal references of the investigation results described in the literature, there is good qualitative concurrence in the long-term behaviour of the snow cover described here.
Fig. 1: Relative trend of the mean snow cover duration (in days), series 1951/52 to 1995/96.
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Fig. 2: Absolute trend of the mean duration of the snow cover time (in days), series 1951/52 to 1995/96.
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Fig. 3: Relative trend of the duration of the winter cover (in per cent), series 1951/52 to 1995/96.
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