Multi-objective trade-off analysis of conflicting water demands in the Chilean Laja River basin

Due to population growth, urbanization, industrial and agricultural development, water demands have increased especially during the recent decades. On the other hand, shortage of the available water resources is a considerable challenge for water allocation and conflict resolution particularly in the semi-arid regions. The Laja Lake is a natural reservoir located in the Bío-Bío Region of Chile. It provides water for different stakeholders including energy and agriculture. This lake releases water to the downstream river by natural seepage through the volcanic barrier and by a controlled outlet via a transmission line. Two hydropower plants have been constructed on the mentioned river and transmission line and four other hydropower plants have been built in a series downstream. However, agricultural stakeholders have rights to supply their irrigation water demands from the lake. Accordingly, the control of the upstream hydropower plant effects all other water users. In this research, optimal trade-offs between energy generation by the hydropower plants (transmission line and river) and supply of agricultural irrigation water have been determined by applying NSGA-II (non-dominant sorting genetic algorithm). A water balance simulation model has been coupled with NSGA-II to be applied for monthly time series of water availability and demand. Using TOPSIS (technique for order preference by similarity to ideal solution), a multi-criteria analysis method, the non-dominated solutions from ten runs have been reduced to a few solutions. They were used to perform a trade-off analysis among stakeholders to achieve an acceptable optimal operation. The results have been compared to those of the SOP (standard operating policy) and the actual reservoir operation. The application of trade-off analysis based on simulation-optimization results allowed finding a better compromise between different stakeholder utilities. Furthermore, the system’s analytic tool can be applied for different hydro-meteorological inputs and thus be used for predictive development of operation policies under changed climate.

Keywords: multi-objective optimization, conflict resolution, water-energy trade-offs