IWRA Proceedings

AbstractIntroduction Glacial and snowmelt is essential for the well being of all Central Asian states and provides over 90 % of their water requirements. Climate change is causing rapid recession of the glaciers, which helps to meet in short-term the states ambitious water requirements, but in the long term decreased runoff and increased evapotranspiration from higher temperatures will result. Additionally, climate change has an effect on the frequency and intensity of extreme droughts, with the consequence of increased exceptional water deficits as occurred for the lower Amu Darya River during 2000-2002. The development of effective risk-management strategies for securing future water supply under varying conditions of water shortage in semi-arid and arid river basins needs (i) to revise the existing storage capacities, (ii) to improve the forecasting methods, (iii) to associate possible water saving mechanisms and improved crop growth patterns at the downstream areas, and (iv) to adapt the dam operation at upstream and downstream regions accordingly. Therefore, a risk assessment is substantial to investigate the changing conditions and the interaction of processes within the run-off generation upstream, reservoir storage capacity losses and increased water consumption downstream. Objective The focus of the study lies on the assessment of ongoing reservoir storage capacity losses and its effect on the compensation of water deficit volumes during exceptional drought events, in order to provide critical values for an enhanced drought management. The objective is to analyse current capacity losses and the impact of past reservoir operation strategies on the sedimentation processes, to characterise the risk of future reduction in the reservoir storage capacity. The obtained results will be used for the estimation of adapted reservoir operation strategies related to a combined sediment and drought management. Methods The study uses actual reservoir bathymetric data for its comparison with the design capacities, to assess the current storage capacity losses. Furthermore, the three- dimensional water quality model mohid will be used to analyse the effect of past and current reservoir operation on the inside sedimentation processes. The mohid model will use the outcomes of the characterisation of sedimentation processes to analyse sediment management options. The applicability of enhanced strategies, addressing sediment management as well water availability under water deficit conditions, will be assessed by water balance criteria. Results Results are presented for the in-stream Channel Reservoir, which is the largest of four reservoirs of the Tuyamuyun Hydro Complex (7.8 km³) and is located at the lower Amu Darya River, at the border of Turkmenistan and Uzbekistan. The results for investigating the risk of storage capacity losses and therefore the opportunity to compensate water deficits by drought events indicate an increased risk of failure for the THC storage volume. The obtained risk assessment has been used for enhanced reservoir operation schemes, including the estimation of effective and applicable options (e. g. flushing or sluicing), and combining improved sediment management to shorten future capacity losses with adapted drought management plans. Conclusion The study has emphasized that a more precise understanding of reservoir sedimentation processes and resulting storage capacity losses provides necessary background information for assessing management options during drought events and the impact of climate change on water availability during the next 50 years.