Impact of Climate Variability and Change on Hydrological Extremes of the Upper Blue Nile Basin (Impact van klimaatvariabiliteit en -verandering op hydrologische extremen in het boven-rivierbekken van de Blauwe Nijl)
Impact of Climate Variability and Change on Hydrological Extremes of the Upper Blue Nile Basin
Over the recent decades, the world has witnessed frequent and intense hydro-climatic extremes in the form of floods, droughts, heat waves, cyclones and other phenomena in various geographical locations. The scientific community offered substantial attention for the alteration in extremes and possible relations with climate variability and/or change since climate is a key driver to various natural and managed systems. Hydrological cycle and the climate system have close interactions, subsequently local and regional hydrology will bear the immediate consequences of climate variability/change. Changes in hydrological extremes have considerable implications on the planning and management of water resources of a given basin. Accordingly, quantification of possible hydrological impacts due to climate alteration and incorporation of the outcomes in engineering designs and policy formulations is crucial. Recent changes in climate observed as global warming are commonly attributed to anthropogenic activities such as the burning of fossil fuels that increase the concentration of greenhouse gases in the atmosphere. Nevertheless, changes in climate may be due to natural internal processes or external forcings in addition to persistent anthropogenic changes in the composition of the atmosphere or in land use. The most advanced tools such as Global Climate Models (GCMs) are being used to project the future climate and provide information for impact investigators in various aspects of the environment and society. Quantifying the impact of climate change on the future hydrological extremes is undoubtedly important. Nonetheless, prior to such estimations it is wise to retrospectively investigate the historical variability of hydro-climatic extremes to better understand their patterns, causes and effects. This research was aimed at studying the observed climate variability and the impact of future climate change on hydrological extremes of one of the most important river basins in Africa, the upper Blue Nile basin. The importance of this study is guided by the location of the study area which is highly vulnerable to climate variability and change as well as its large discharge contribution to the entire Nile basin system. This study was supported by the readily available data from climate models as well as from selected long-term observations. The study commenced with addressing its first research questions which required assessment of historical patterns and trends in hydro-climatic extremes. This analysis aimed at seeking evidence for a certain pattern in historical records and unravelling whether the recent changes are statistically significant. This analysis provides information on whether certain periods are wet or dry compared to the long-term natural variability. Furthermore, it provides the possibility of separating natural variability patterns from long-term trends in extremes that could be attributed to anthropogenic causes. Long-term records of rainfall extremes, high and low flows from the basin were considered for this analysis. The outcome of this study was that there is an evidence of (multi-) decadal oscillation on hydro-climatic extremes of the basin and a statistically significant negative anomaly period during the 1980s. However, the recent years did not show consistent increasing or decreasing trend. Hence, it is intricate to conclude the recent changes in the extremes on anthropogenic climate change. Alternatively, as a possible cause for the historical variability pattern in the hydro-climatic extremes, large scale atmospheric variables from Pacific and Atlantic Oceans were found to have strong correlations. Identification of the primary causes of the observed variability patterns in the hydro-climatic extremes was conducted making use of lumped conceptual hydrological models and statistical analysis on the simulated results. The hydrological models were used to simulate a long-term time series with the assumption that if changes in catchment behaviour had significant influence on the temporal dynamics of the basins extreme flows, a clear difference would be detected between the simulated and observed variability results. The outcome of the analysis revealed that there is no discernible trend of the difference between the variability pattern of the simulations and that of the observations over time. Additionally there is no perceptible change in the catchment response behaviour between different periods. Hence, the temporal dynamics of extreme flows of the Blue Nile River are much more influenced by (multi-) decadal climate variability rather than the changes in land use or other catchment characteristics. Considering that the basin is one of the data scarce regions and the presence of a multi-decadal oscillation pattern, it was important to assess the effect of the variability patterns on the derivation of flow-duration-frequency (QDF) predictions when short-term data is used. The study showed that the QDF predictions are dependent on the period used for the analysis. For instance, high flow QDF statistics estimated using the 1980s data required bias correction of around 15% to match with the long-term period estimation. A strong correlation was found between temporal variability of high flow extremes and that of large scale atmospheric variables. This relation can be used as an indicator for correcting the bias in the estimation of high flow quantiles that utilized short term data. The final research objective sought to analyse the impact of anthropogenic climate change on the future extremes to determine the potential direction of change by also investigating the influence of different downscaling methods. The results indicated that the choice of downscaling method was an important factor to be considered and that the results based on one downscaling method may not entirely give the full picture. For the upper Blue Nile basin, the chosen downscaling methods agree in projecting mostly decreasing flow for the main rainy season. The reason isboth decreasing rainfall and increasing evapotranspiration at seasonal scale. It is worth mentioning that the uncertainty range obtained in this study can get wider if more GCMs, downscaling methods and hydrological models were used. However, despite this uncertainty, water managers are recommended to take actions that are based on no-regret strategies.