Optimizing long-term water allocation in the Amudarya River delta: a water management model for ecological impact assessment

In the semi-arid Amudarya delta region (Aral Sea Basin) the human-controlled hydrological regime is a major factor influencing ecosystem dynamics. Alterations to the flow of the Amudarya River, mainly to serve the needs of irrigated agriculture during Soviet times, have caused severe environmental degradation. Since independence, the former Soviet Union states of the basin are searching for new, ecologically sound, water management strategies to mitigate the damages to economy, human populations and ecosystems. To assist in the evaluation of tradeoffs in water allocation and the determination of restoration goals, we created a simple water management model for the Amudarya River and its delta region with the modeling system EPIC (originally developed by the USAID project ‘Environmental Policies and Institutions for Central Asia’). The water management model determines optimal water allocation in the irrigation network by multi-objective optimization in monthly time steps. Water management alternatives can be developed for a time period of up to 15 years based on changing requirements of the water users (e.g. as a result of increased water use efficiency in agriculture), inflow to the delta (e.g. increase in water use upstream), priorities of the optimization criteria (e.g. reflecting policy decisions) or introducing minimum flow requirements to selected canals. Historic salt dynamics of the Tyuyamuyun reservoir system at the entrance to the Amudarya delta were investigated and EPIC was extended to treat such multi-body reservoir systems. The model was calibrated and tested using a high water (1994) and a low water (1997) year. Modeled water allocation takes place in accordance with observational data. The model reacts well to changes in allocation priorities given by the user. Application of the model to a 14-year characteristic time period was successful. The model constitutes a main module of an integrated GIS-based simulation tool that facilitates the evaluation of the ecological effects of alternative water management strategies in the Northern Amudarya delta.