TY - GEN A1 - Aich, Valentin A1 - Liersch, Stefan A1 - Vetter, Tobias A1 - Andersson, Jafet C. M. A1 - Müller, Eva Nora A1 - Hattermann, Fred Fokko T1 - Climate or land use? BT - Attribution of changes in river flooding in the Sahel Zone N2 - This study intends to contribute to the ongoing discussion on whether land use and land cover changes (LULC) or climate trends have the major influence on the observed increase of flood magnitudes in the Sahel. A simulation-based approach is used for attributing the observed trends to the postulated drivers. For this purpose, the ecohydrological model SWIM (Soil and Water Integrated Model) with a new, dynamic LULC module was set up for the Sahelian part of the Niger River until Niamey, including the main tributaries Sirba and Goroul. The model was driven with observed, reanalyzed climate and LULC data for the years 1950–2009. In order to quantify the shares of influence, one simulation was carried out with constant land cover as of 1950, and one including LULC. As quantitative measure, the gradients of the simulated trends were compared to the observed trend. The modeling studies showed that for the Sirba River only the simulation which included LULC was able to reproduce the observed trend. The simulation without LULC showed a positive trend for flood magnitudes, but underestimated the trend significantly. For the Goroul River and the local flood of the Niger River at Niamey, the simulations were only partly able to reproduce the observed trend. In conclusion, the new LULC module enabled some first quantitative insights into the relative influence of LULC and climatic changes. For the Sirba catchment, the results imply that LULC and climatic changes contribute in roughly equal shares to the observed increase in flooding. For the other parts of the subcatchment, the results are less clear but show, that climatic changes and LULC are drivers for the flood increase; however their shares cannot be quantified. Based on these modeling results, we argue for a two-pillar adaptation strategy to reduce current and future flood risk: Flood mitigation for reducing LULC-induced flood increase, and flood adaptation for a general reduction of flood vulnerability. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 345 KW - simulation-based attribution KW - Sahel KW - Niger River KW - climate variability KW - hydrological modeling KW - flood mitigation KW - flood adaptation Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-400115 ER - TY - JOUR A1 - Aich, Valentin A1 - Liersch, Stefan A1 - Vetter, Tobias A1 - Andersson, Jafet C. M. A1 - Müller, Eva Nora A1 - Hattermann, Fred Fokko T1 - Climate or Land Use? BT - Attribution of Changes in River Flooding in the Sahel Zone JF - Water N2 - This study intends to contribute to the ongoing discussion on whether land use and land cover changes (LULC) or climate trends have the major influence on the observed increase of flood magnitudes in the Sahel. A simulation-based approach is used for attributing the observed trends to the postulated drivers. For this purpose, the ecohydrological model SWIM (Soil and Water Integrated Model) with a new, dynamic LULC module was set up for the Sahelian part of the Niger River until Niamey, including the main tributaries Sirba and Goroul. The model was driven with observed, reanalyzed climate and LULC data for the years 1950-2009. In order to quantify the shares of influence, one simulation was carried out with constant land cover as of 1950, and one including LULC. As quantitative measure, the gradients of the simulated trends were compared to the observed trend. The modeling studies showed that for the Sirba River only the simulation which included LULC was able to reproduce the observed trend. The simulation without LULC showed a positive trend for flood magnitudes, but underestimated the trend significantly. For the Goroul River and the local flood of the Niger River at Niamey, the simulations were only partly able to reproduce the observed trend. In conclusion, the new LULC module enabled some first quantitative insights into the relative influence of LULC and climatic changes. For the Sirba catchment, the results imply that LULC and climatic changes contribute in roughly equal shares to the observed increase in flooding. For the other parts of the subcatchment, the results are less clear but show, that climatic changes and LULC are drivers for the flood increase; however their shares cannot be quantified. Based on these modeling results, we argue for a two-pillar adaptation strategy to reduce current and future flood risk: Flood mitigation for reducing LULC-induced flood increase, and flood adaptation for a general reduction of flood vulnerability. Y1 - 2015 U6 - https://doi.org/10.3390/w7062796 SN - 2073-4441 VL - 7 IS - 6 SP - 2796 EP - 2820 PB - MDPI CY - Basel ER - TY - THES A1 - Aich, Valentin T1 - Floods in the Niger River Basin in the face of global change T1 - Hochwasser im Niger Einzugsgebiet im Kontext des Globalen Wandels BT - analysis, attribution and projections BT - Analyse, Zuschreibung und Projektionen N2 - In the last decade, the number and dimensions of catastrophic flooding events in the Niger River Basin (NRB) have markedly increased. Despite the devastating impact of the floods on the population and the mainly agriculturally based economy of the riverine nations, awareness of the hazards in policy and science is still low. The urgency of this topic and the existing research deficits are the motivation for the present dissertation. The thesis is an initial detailed assessment of the increasing flood risk in the NRB. The research strategy is based on four questions regarding (1) features of the change in flood risk, (2) reasons for the change in the flood regime, (3) expected changes of the flood regime given climate and land use changes, and (4) recommendations from previous analysis for reducing the flood risk in the NRB. The question examining the features of change in the flood regime is answered by means of statistical analysis. Trend, correlation, changepoint, and variance analyses show that, in addition to the factors exposure and vulnerability, the hazard itself has also increased significantly in the NRB, in accordance with the decadal climate pattern of West Africa. The northern arid and semi-arid parts of the NRB are those most affected by the changes. As potential reasons for the increase in flood magnitudes, climate and land use changes are attributed by means of a hypothesis-testing framework. Two different approaches, based on either data analysis or simulation, lead to similar results, showing that the influence of climatic changes is generally larger compared to that of land use changes. Only in the dry areas of the NRB is the influence of land use changes comparable to that of climatic alterations. Future changes of the flood regime are evaluated using modelling results. First ensembles of statistically and dynamically downscaled climate models based on different emission scenarios are analyzed. The models agree with a distinct increase in temperature. The precipitation signal, however, is not coherent. The climate scenarios are used to drive an eco-hydrological model. The influence of climatic changes on the flood regime is uncertain due to the unclear precipitation signal. Still, in general, higher flood peaks are expected. In a next step, effects of land use changes are integrated into the model. Different scenarios show that regreening might help to reduce flood peaks. In contrast, an expansion of agriculture might enhance the flood peaks in the NRB. Similarly to the analysis of observed changes in the flood regime, the impacts of climate- and land use changes for the future scenarios are also most severe in the dry areas of the NRB. In order to answer the final research question, the results of the above analysis are integrated into a range of recommendations for science and policy on how to reduce flood risk in the NRB. The main recommendations include a stronger consideration of the enormous natural climate variability in the NRB and a focus on so called “no-regret” adaptation strategies which account for high uncertainty, as well as a stronger consideration of regional differences. Regarding the prevention and mitigation of catastrophic flooding, the most vulnerable and sensitive areas in the basin, the arid and semi-arid Sahelian and Sudano-Sahelian regions, should be prioritized. Eventually, an active, science-based and science-guided flood policy is recommended. The enormous population growth in the NRB in connection with the expected deterioration of environmental and climatic conditions is likely to enhance the region´s vulnerability to flooding. A smart and sustainable flood policy can help mitigate these negative impacts of flooding on the development of riverine societies in West Africa. N2 - Während des vergangenen Jahrzehnts nahmen die Anzahl und die Ausmaße von katastrophalen Hochwassern im Einzugsgebiet des Nigerflussess (NEZG) deutlich zu. Trotz der verheerenden Auswirkungen der Hochwasserkatastrophen auf die Menschen und die hauptsächlich auf Landwirtschaft basierende Wirtschaft der Anrainerstaaten wird das Thema von Politik und Wissenschaft noch kaum beachtet. Die vorliegende Dissertation ist die erste ausführliche Analyse des steigenden Hochwasserrisikos im NEZG. Die Forschungsstrategie basiert auf vier Fragen nach (1) der Art der Veränderungen des Hochwasserrisikos, (2) den Ursachen der Veränderungen im Hochwasserregime, (3) den zukünftigen Entwicklungen im Hochwasserregime hinsichtlich der erwartenden Klima- und Landnutzungswandel und (4) den aus den Untersuchungen abgeleiteten Empfehlungen zur Reduzierung des Hochwasserrisikos im NEZG. Die Frage nach den Merkmalen der Veränderungen im Hochwasserrisiko wurde mithilfe von statistischen Untersuchungen beantwortet. Die Analysen zeigen, dass neben den Risikofaktoren Exponiertheit und Verwundbarkeit auch die Hochwasserstände selbst im NEZG in den letzten Jahrzehnten signifikant und entsprechend der typischen dekadischen Klimamuster Westafrikas angestiegen sind. Als potentielle Ursachen des Hochwasseranstiegs werden Klima- und Landnutzungswandel untersucht. Zwei verschiedene Ansätze, basierend auf Daten sowie auf Simulationen, führen zu ähnlichen Ergebnissen und zeigen, dass der Einfluss der Klimaveränderungen im Allgemeinen größer als der des Landnutzungswandels ist. Das zukünftige Hochwasserrisiko wird anhand des öko-hydrologisches Modells SWIM abgeschätzt. Der Einfluss des Klimawandels auf das Hochwasserregime ist auf Grund des problematischen Niederschlagssignals unsicher. Tendenziell werden aber höhere Maximalabflüsse erwartet. Der Effekt der Landnutzungsänderung beeinflusst das Hochwasserverhalten ebenfalls stark, besonders in den trockenen Gebieten. Verschiedene Szenarien zeigen, dass Renaturierung hülfe, Hochwasserspitzen zu kappen. Eine Ausweitung der Agrarflächen dagegen würde die Hochwässer im NEZG weiter verstärken Zentrale Empfehlungen sind eine stärkere Einbeziehung der enorm starken natürlichen Klimavariabilität im NEZG und eine Fokussierung auf sogenannte „no-regret“ Anpassungsstrategien. Dabei sollte den verwundbarsten Regionen des Einzugsgebiets, den ariden und semi-ariden Regionen, Priorität eingeräumt werden. Die enorme Bevölkerungszunahme im NEZG verbunden mit der zu erwartenden Verschlechterung der Umwelt- und Klimabedingungen wird mit hoher Wahrscheinlichkeit auch die Verwundbarkeit bezüglich Hochwässer weiter ansteigen lassen. Eine vernünftige und nachhaltige Hochwasserpolitik kann helfen, die negativen Folgen auf die Entwicklung der Anrainerstaaten des Nigerflusses abzumindern. KW - flood KW - Niger KW - climate change KW - land use change KW - Hochwasser KW - Niger KW - Klimawandel KW - Landnutzungswandel Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-91577 ER - TY - JOUR A1 - Aich, Valentin A1 - Liersch, Stefan A1 - Vetter, T. A1 - Huang, S. A1 - Tecklenburg, J. A1 - Hoffmann, P. A1 - Koch, H. A1 - Fournet, S. A1 - Krysanova, Valentina A1 - Mueller, N. A1 - Hattermann, Fred Fokko T1 - Comparing impacts of climate change on streamflow in four large African river basins JF - Hydrology and earth system sciences : HESS N2 - This study aims to compare impacts of climate change on streamflow in four large representative African river basins: the Niger, the Upper Blue Nile, the Oubangui and the Limpopo. We set up the eco-hydrological model SWIM (Soil and Water Integrated Model) for all four basins individually. The validation of the models for four basins shows results from adequate to very good, depending on the quality and availability of input and calibration data. For the climate impact assessment, we drive the model with outputs of five bias corrected Earth system models of Coupled Model Intercomparison Project Phase 5 (CMIP5) for the representative concentration pathways (RCPs) 2.6 and 8.5. This climate input is put into the context of climate trends of the whole African continent and compared to a CMIP5 ensemble of 19 models in order to test their representativeness. Subsequently, we compare the trends in mean discharges, seasonality and hydrological extremes in the 21st century. The uncertainty of results for all basins is high. Still, climate change impact is clearly visible for mean discharges but also for extremes in high and low flows. The uncertainty of the projections is the lowest in the Upper Blue Nile, where an increase in streamflow is most likely. In the Niger and the Limpopo basins, the magnitude of trends in both directions is high and has a wide range of uncertainty. In the Oubangui, impacts are the least significant. Our results confirm partly the findings of previous continental impact analyses for Africa. However, contradictory to these studies we find a tendency for increased streamflows in three of the four basins (not for the Oubangui). Guided by these results, we argue for attention to the possible risks of increasing high flows in the face of the dominant water scarcity in Africa. In conclusion, the study shows that impact intercomparisons have added value to the adaptation discussion and may be used for setting up adaptation plans in the context of a holistic approach. Y1 - 2014 U6 - https://doi.org/10.5194/hess-18-1305-2014 SN - 1027-5606 SN - 1607-7938 VL - 18 IS - 4 SP - 1305 EP - 1321 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Aich, Valentin A1 - Zimmermann, Alexander A1 - Elsenbeer, Helmut T1 - Quantification and interpretation of suspended-sediment discharge hysteresis patterns: How much data do we need? JF - Catena : an interdisciplinary journal of soil science, hydrology, geomorphology focusing on geoecology and landscape evolution N2 - Sediment-discharge hysteresis loops are frequently analyzed to facilitate the understanding of sediment transport processes. Hysteresis patterns, however, are often complex and their interpretation can be complicated. Particularly, quantifying hysteresis patterns remains a problematic issue. Moreover, it is currently unknown how much data is required for analyzing sediment-discharge hysteresis loops in a given area. These open questions and challenges motivated us to develop a new method for quantifying suspended-sediment hysteresis. Subsequently, we applied the new hysteresis index to three suspended-sediment and discharge datasets from a small tropical rainforest catchment. The datasets comprised a different number of events and sampling sites. Our analyses show three main findings: (1) datasets restricted to only few events, which is typical for rapid assessment surveys, were always sufficient to identify the dominating hysteresis pattern in our research area. Furthermore, some of these small datasets contained multiple-peak events that allowed identifying intra-event exhaustion effects and hence, limitations in sediment supply. (2) Datasets comprising complete hydrological years were particularly useful for analyzing seasonal dynamics of hysteresis. These analyses revealed an exhaustion of hysteresis on the inter-event scale which also points to a limited sediment supply. (3) Datasets comprising measurements from two consecutive gauges installed at the catchment outlet and on a slope within that catchment allowed analyzing the change of hysteresis patterns along the flowpath. On the slope, multiple-peak events showed a stronger intra-event exhaustion of hysteresis than at the catchment outlet. Furthermore, exhaustion of hysteresis on the inter-event scale was not evident on the slope but occurred at the catchment outlet. Our results indicate that even small sediment datasets can provide valuable insights into sediment transport processes of small catchments. Furthermore, our results may serve as a first guideline on what to expect from an analysis of hysteresis patterns for datasets of varying quality and quantity. (c) 2014 Elsevier B.V. All rights reserved. KW - Suspended sediment KW - Hysteresis index KW - Sediment monitoring KW - Overland flow KW - Tropical forest Y1 - 2014 U6 - https://doi.org/10.1016/j.catena.2014.06.020 SN - 0341-8162 SN - 1872-6887 VL - 122 SP - 120 EP - 129 PB - Elsevier CY - Amsterdam ER -