Abstract

Changes in climate and land use, the development of human activities, water management and regulations, constitute and contribute to deep uncertainties that define the level of water security. These uncertainties increase in coastal basins where the hydrological cycle interacts with biophysical and social processes such as saline intrusion or tourism floating population. Robust Decision Making (RDM) has been used in the identification of strategies that can perform well under deep uncertainties, over several possible futures. A strength of RDM is its capacity to help make informed decisions without having to rely on precise modeling predictions about the future state of the system, based more on "robustness" rather than "optimality". In the Ligua, Petorca and Quilimarí basins, located in the coastal semi-arid region of Chile, agriculture is the main water user and its supply is primarily groundwater. Increasing water use for tourism contrasts with small communities with shallow boreholes that have become brackish due to saline intrusion. In addition, coastal wetlands under development pressure, have lost biodiversity and ecological services. This study presents the first steps of a RDM implementation at these basins to evaluate how socio-ecological stress and uncertainties affect different stakeholders' demands. We refined a coupled WEAP-MABIA-MODFLOW-SWI2 model that connects hydrological processes with saline intrusion and fog condensation as water input. The participatory process allowed the identification of key elements of concern by stakeholders and governmental agencies, uncertainties that might affect them in near future and actions to improve their performance. Along with information of more than 2500 extraction wells, detailed irrigation schemes and water potable water supply by different sources and costs, the tool allows the representation of key trade-offs related to water resources objectives as agriculture production and job creation versus potable water for rural communities and environmental indicators, showing a complex scenario associated with potential reductions in precipitation and increase in water demand for household and touristic use. Next steps will evaluate local opinion on different trade-offs and strategies for the future of water in semi-arid coastal regions.