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In anthropogenically heavily impacted river catchments, such as the Lusatian river catchments Spree and Schwarze Elster in Germany, the robust assessment of potential impacts of climate change on the regional water resources is of high relevance for water resources management. Large uncertainties inherent in future scenarios may, however, reduce the willingness of regional stakeholders to develop and implement suitable adaptation strategies to climate change.
This thesis proposes the use of an integrated framework consisting of i) an ensemble based modelling approach and ii) the incorporation of measured and simulated meteorological and hydrological trends to consider uncertainties in climate change impact assessments. In addition, land use, as the most responsive catchment characteristic to buffer potential climate change impacts, is considered as one suitable trigger for climate change adaptation.
The ensemble based modelling approach consists of the meteorological output of four climate downscaling approaches (DAs): two dynamical and two statistical. These DAs drive different model configurations of the two conceptually different hydrological models WaSiM ETH and HBV light. The objective of incorporating measured meteorological trends into the analysis was twofold: trends in measured time series can i) be regarded as harbinger for future change and ii) serve as a mean to validate the results of the DAs. In order to evaluate the nature of the trends, both gradual (Mann Kendall test) and step changes (Pettitt test) are considered as well as temporal and spatial correlations in the data. The suitability of land use change as an adaptation strategy to climate change is evaluated in the form of different land use change scenarios: i) extreme scenarios where the entire catchment is parameterised as coniferous forest and uncultivated land and ii) scenarios of changes in crop cultivation and ii) a combination of a change in crop cultivation and forest conversion. As study areas serve three almost natural subcatchments of the Spree and Schwarze Elster (Germany).
The results of the ensemble based climate change impact analysis show that depending on the type (dynamical or statistical) of DA used, opposing trends in precipitation, actual evapotranspiration and discharge are simulated in the scenario period (2031 2060). While the statistical DAs simulate a decrease in future long term annual precipitation, the dynamical DAs simulate a tendency towards increasing precipitation. The trend analysis suggests that measured precipitation has not changed significantly during the period 1961 2006. Therefore, the strong decrease in precipitation simulated by the statistical DAs should be interpreted as a rather dry future scenario. The dynamical DAs, on the other hand, are too wet in the reference period and needed to be statistically bias corrected which destroys the physical consistency between the parameters. Concerning temperature, measured and simulated trends agree on a positive trend. The uncertainty related to the hydrological model within the climate change modelling chain is comparably low when long term averages are considered but increases during low flow events. The proposed framework of combining an ensemble based modelling approach with trend analysis on measurements is a promising approach to gain more confidence into the final results of climate change impact assessments and to obtain an increased process understanding of the interrelation between climate and water resources.
In terms of climate change adaptation, land use alternatives can have a considerable impact on the water balance components as the analysis of the extreme scenarios revealed. The scenarios of changes in crop cultivation in combination with forest conversion show, however, that the impact on the long term annual water balance is comparably low. An intra annual shift in the water balance components can be triggered which makes these scenarios suitable to reduce low flow risks during the summer. Overall, land use change can serve as one part of an integrated climate change adaptation strategy. Such as strategy needs, depending on the severity of the climate change impact, to include other, especially technical measures of water resources management, such as additional water storage, different strategies to manage the existing and new reservoirs. It may also consider additional water transfers from neighbouring, more water rich, river catchments. Regional adaptation planning needs also to consider problems related to water quality which are a consequence of the long term mining activities in the Lusatian river catchments. Last but not least, adaptation strategies should not only consider climate but also other aspects of global change.