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Friedrich-Schiller-Universität Jena (2010)

Integrated modelling of ecohydrological processes along ephemeral rivers

Sven Arnold

Titre : Integrated modelling of ecohydrological processes along ephemeral rivers.

Auteur : Sven Arnold

Université de soutenance : Friedrich-Schiller-Universität Jena

Grade : Doctor rerum naturalium (Dr. rer. nat.) 2010

Résumé
Ephemeral rivers are located throughout the world’s arid regions. They are characterised by temporary surface flow that strongly varies between seasons and years. Along the river course often a coupled eco-hydrological vegetation-groundwater system has established, which is referred to as linear oasis, reflecting the ecological and socio-economic importance of ephemeral rivers in otherwise dry areas. The Kuiseb River in Namibia denotes such a linear oasis with eco-hydrological feedbacks between the vegetation and the ground water resource. Temporary floods infiltrate into sediments, which are accumulated in geological pools of impermeable bedrocks. This enables the formation of shallow ground water. The low depth to ground water allows root water uptake by plants and the establishment of a thriving ecosystem. Besides, the river and its environment is diversely used by humans, e.g. by exploiting the ground water for drinking, farming, and mining. Further, it is essential for the survival of the rural Topnaar community and economical important due to its touristic attraction. Therefore, a sustainable resource management is needed, which clearly requires a well developed understanding of the ecohydrologcial processes along ephemeral rivers. The objective of this research was to develop a model framework based on the Kuiseb River that integrates both ecological and hydrological system dynamics. Such a framework helps to increase the mechanistic understanding of driving ecohydrological processes along ephemeral rivers by testing assumptions and generating hypotheses. Further, it can be applied to investigate management strategies in terms of their ability to sustainably exploit the ground water resource while preserving the natural vegetation structure. Uncertainty played a critical role throughout this research due to the scarce information available for both the eco- and the hydrosystem. In particular, the research focused on three types of uncertainty : (1) The parameterisation of the population model, which was challenging as this requires long-term observation of species abundance that is not available. This parameterisation problem was addressed by using pattern-oriented model calibration, in that the species parameters were adjusted such that the resulting parameterisation reproduces the observed three species coexistence pattern along the river course under study. (2) The inherent uncertainty in the occurrence of flood events, which is driven by unpredictable rainfall events. Throughout this study the unpredictability was described with a stochastic process characterised by parameters such as frequency, duration and short/long term memory of flood events. In order to address the parameterisation problem to this type of uncertainty, for each parameter combination the model run 100 times with stochastic identical flood realisations, eventually leading to a quantification of the uncertainty in parameterisation. (3) The uncertainty in parameters describing the (stochastic) flood regimes. This uncertainty arises due to the scarce information about the runoff data along ephemeral rivers because often monitoring systems are rare and the temporary character of the flood events hinders the measurement of large time series. The influence of this particular type of uncertainty on the robustness and significance of integrated management strategies was investigated without neglecting the preceding types of uncertainty

Mots Clés : Ecohydrology — Resource management — Uncertainty — Kuiseb River

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Page publiée le 30 novembre 2012, mise à jour le 10 juillet 2017