Water quantity and quality dynamics of the THC--Tuyamuyun Hydroengineering Complex--and implications for reservoir operation.

Background: In the Aral Sea basin, safe water resources are scarce and steadily becoming scarcer. Particularly high quality water is going to become a rare good. The object of the study was the Tuyamuyun Hydroengineering Complex (THC), a complex of artificial water reservoirs located in the lower Amu Darya River, which provides water for irrigation, industry, and drinking for the lower Amu Darya region. The focus was on operation of one of its four reservoirs, the Kaparas, which is mainly used for drinking water supply. The objective includes the investigation of impacts of conventional operation schemes on the reservoir water quality for improving drinking water quality (salinity). Basic operation rules for Kaparas, which can be considered as representative for conventional dam operation under dry year conditions, had to be identified and improved operation schemes derived.

Methods: Existing data archives were analysed, and further data were acquired from field surveys, data processing and modelling studies. Historical data were identified, which are appropriate to determine representative schemes for the conventional operation. For the simulation of time-dependent and depth-dependent changes of reservoir salinisation, the reservoir water quality model Lac was used and linked with the THC model.

Results And Discussion: Modelling results for the simulation of temperature dynamics and density stratification showed a sufficient congruence with the measured temperature profiles. The conformity of measured and calculated salt concentration is basically ensured. The reservoir, which fill with higher saline water at the end of the summer, aggravates the entrainment of high saline water in the entire water column.

Conclusions: The current conventional operation regime mainly leads to filling the Kaparas reservoir with high saline water during the winter months. Even in the event of starting with comparable low salinity levels, the simulation demonstrates the rapid deterioration of the reservoir water quality. Under dry year conditions, the WHO standards for drinking water will be exceeded by 30% after two years, so that the impact of dry years in the context of water stress becomes visible.

Recommendations And Outlook: Processed data and results are now available to identify enhanced reservoir operation strategies for salinity reduction by changing the period of reservoir filling and release, as well as to initiate a detailed analysis of how water deficits in dry years may be reduced by improved operation regimes. Using adapted and enhanced operation rules for THC reservoirs, the local population within the lower Aral Sea basin might be supplied with more potable water of higher quality in future.