Climate Change and Floods

Climate change and floods – findings and adaptation Water Science & Technology strategies for flood protection in Baden-Wu¨ rttemberg W. Hennegriff Federal Institute for Environment, Measurements and Nature Protection Baden-Wu¨rttemberg, Landesanstalt fu¨r Umwelt, Messungen und Naturschutz Baden-Wu¨rttemberg (LUBW), Griesbachstraße 1, D–76185 Karlsruhe (E-mail: [email protected]) Abstract The climatic conditions in Southern Germany have changed noticeably in the 20th century, especially during the last three decades. Both in specific regions and interannually, the trends found Vol 56 No 4 pp 35–44 exceed the natural margins of deviation previously known from long measurement series for some measured quantities. The mean and also the extreme floods are expected to increase significantly, although the results of the model chain global model–regional climate models–water balance models are still uncertain. As a precaution an adaptation strategy has been developed for the field of flood protection which takes into consideration the possible development for the next decades and also takes into account the uncertainties. Keywords Air temperature; climate development modelling; flood protection; flood runoff; precipitation; Q snow cover IWA Publishing 2007 Initial position and cause The numerous extreme floods of the recent past involving extensive damage have pro- voked heated discussions, both in public and among experts, as to whether these flood events must be considered as part of natural climate variation or as a result of climate change already in progress with long-term future effects. According to current prognoses of climate researchers the large-scale climate in the European region will undergo changes, over and above the natural variations, due to anthropogenic influences and in particular due to the increasing CO2-concentration and other increasing greenhouse gas concentrations in the air. Climate researchers currently assume that the mean global air temperature will increase by 1.4 to 5.8 8C in the coming approximately 100 years due to the anthropogeni- cally caused greenhouse effect (Katzenberger, 2004). This global warming will have effects on the water cycle. In general, an increase in temperature leads to an intensification of the water cycle, which may result in increased evaporation, higher cloud formation and higher precipitation. The statements derived from global climate models for future climate change to date refer mainly to large-scale regions such as Europe. Detailed data on the effects on climate and water balance on a regional scale have not up to now been available at federal state level. Knowledge of possible effects of a changing climate on the water cycle, and in particular on a possibly increasing flood hazard, is in the original interest of the federal states, as these are responsible for flood protection. In the cooperation plan KLIWA (climate change and consequences for water management) of the federal states Baden-Wu¨rttemberg and Bavaria and the German Meterologi- cal Service, possible consequences of climate change on the water balance in the individual river areas of the federal states are assessed. The consequences are portrayed and recommendations are developed in terms of a precautionary water management doi: 10.2166/wst.2007.534 35 policy. The investigations, which were started in 1999, focused first on climate conditions up to now and subsequently on future climate conditions. The questions regarding water management take this up. The examinations to date placed special emphasis on a possible flood intensification. Climatic development in the 20th century W. Hennegriff The investigation of the long series of hydrometeorological and hydrological measurements available provides information about the natural variations observed to date and any noticeable changes. These investigations were systematically carried out for Baden- Wu¨rttemberg and Bavaria on the basis of a large data set within the context of KLIWA. Here the long-term behaviour of the flood runoffs (KLIWA, 2002), the mean runoffs, the regional and heavy precipitation, the air temperature (KLIWA, 2005a), the evaporation and the snow cover period (KLIWA, 2005b) were analysed for time periods in the 20th century. Increase in the air temperature The air temperature at ground level is of crucial importance for the water cycle, as it affects the absorbing capacity of an air mass for water vapour as well as the evaporation. The air temperature is a hydrometeorological variable which, in relation to climate change, can be best simulated in the climate models. Knowledge of the development of this variable is therefore of particular interest. After detailed examination of all available time series in the examined period from 1931–2000, 354 weather-stations proved to be suited for a further regionalisation of the air temperature. Time series of the daily mean temperatures in the catchment areas were determined from the regionalised measurement series for 33 investigation catchment areas, which cover Baden-Wu¨rttemberg and Bavaria. The following remarkable results were found: † increase in the annual mean temperature: between 0.5 and 1.2 8C † rise in the monthly mean temperature for August: between 0.7 and 1.7 8C † rise in the monthly mean temperature for December: between 1.8 and 2.7 8C † increases in temperature – although less significant than in August and December were also found in the months January, February, March and October. † region-specific distinctive features: e.g. main focus of the temperature increase in winter at lower altitudes; higher temperature increase in the west of Baden-Wu¨rttemberg. The greatest increase in the monthly mean temperature occurs in most regions in December. The regional values can be seen in Figure 1. Milder winters with less snowfall Long-term examinations of the snow cover conditions should be heeded because global warming will also probably result in changes in the frequency and duration of snow cover. Changes in snow cover regimes and their parameters have effects on water balance, especially on the soil water balance, the groundwater recharge and the regime of the catchment runoff (flood formation). The parameters snow cover period, snow cover time, longest snow cover period (winter cover), start of the maximum snow cover height, constancy of snow cover, conservation of winter cover and maximum water equivalent values are best suited to describe the snow cover conditions and the water reserves stored in the snow cover. All the snow cover parameters mentioned correlate strictly with the elevation of the ground. The winter cover allows conclusions to be drawn about changes in the characteristics of winter periods. The trend towards winters with less snowfall with 36 less lasting snow cover is definitely apparent. W. Hennegriff Figure 1 Increase [8C] in the monthly mean temperature in December in the period 1931 – 2000 in the catchment areas examined in KLIWA Up to moderate altitudes the durations decrease markedly in general. In the observed period, however, some regional distinctive features can be seen (Figure 2). In the western parts of the regions (Upper-Rhine plain and the western declivity of the Black Forest) the duration of snow cover decreases by approximately 50% and more on lower ground and decreases at moderate altitudes to 10 to 20%. In the higher regions mean values under 10% are observed. Here, too, the trend weakens with increasing altitude. However, only in isolated cases are values observed where the trend is reversed. At lower and moderate altitudes the number of days with snow cover has decreased markedly: † approximately 30–50% in lower regions ( , approximately 300 m above sea level), † approximately 10–20% at moderate altitudes (between 300 and 800 m above sea level), † less than 10% on high ground, or in some cases even increasing at higher altitudes ( . approximately 800 m above sea level). Increase in precipitation Extensive and, as far as possible, homogeneous data on precipitation behaviour are funda- mental prerequisites for the better understanding of the interaction of climate and water cycle. For this purpose the long precipitation series on all available stations in Southern Germany were interpolated with a geostatistical method to yield grid point precipitation and daily catchment precipitation heights were calculated. Statistical parameters for the daily values for the catchment area in one month were then analysed as representative partial samples. The series of the monthly values formed in this way were analysed in detail in a time series analysis. Station time series were employed to examine heavy precipitation. The following changes in precipitation behaviour appear to be especially remarkable: † significant decrease in the catchment precipitation in the summer half year, especially in North Wu¨rttemberg 37 W. Hennegriff Figure 2 Relative Trend [%] in mean duration of snow cover, series 1951/52 to 1995/96 † increase, in most cases significant, in the catchment precipitation in the winter half year † regional clear increase in heavy precipitation by 30–35% in the winter half year; however, in summer only small changes † regional foci of heavy precipitation in winter can be found in the Black Forest, in the Northeast of Baden-Wu¨rttemberg (see Figure 3) † winter half year more humid, summer half year drier. In the winter half year, the precipitation-bearing Western weather fronts have increased in Southern Germany. These fronts that are particularly important for the formation of floods which may go some way towards explaining the changes found. This can be seen in Figure 3. Figure 3 Increase [%] in heavy precipitation at individual stations with 24 h-duration in the winter half year 38 in the period from 1931-2000 Long-term behaviour of flood runoffs The investigation of the long-term behaviour included determination of any linear trends present in the time series of the annual and monthly highest runoffs. The annual and monthly highest runoff values at 107 gauges, which have long observation series since at least 1931, formed the basis for the trend investigations.

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