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In light of seasonal climatic and anthropogenic changes, is the Vaud canton (Switzerland) vulnerable to water stress by the medium-term?

Observed changes in hydrological processes during the past 20 years in Switzerland are particularly preoccupying as they directly affect water use. In 2003 and 2011, local water shortage episodes occurred. Water withdrawals and supplies had to be restricted, notably in the canton of Vaud (Western Switzerland). These droughts highlighted increasing competition among water users and new water management issues arose. This study explores how hydro-climatic conditions and water needs could evolve by the 2060 horizon and assesses the vulnerability of the canton to water stress under climatic and anthropogenic changes. A daily semi-distributed hydrological model was used to simulate flows. Future changes were derived from Swiss climate scenarios relying on ten regional climate models. Regarding water needs, a population growth scenario was provided by the canton whereas a business-as-usual scenario was considered for irrigation and breeding trends. Currently, catchments in the canton experience moderate water stress from June to August, except in alpine areas. By the medium-term, water needs could reach more than 80% of rivers' total runoff in July and August. This should be due to higher temperatures and a higher ratio of liquid-to-solid precipitation causing more severe low flows. In addition, water needs should significantly increase from April to July, due to higher irrigation (+25%) and urban (+40%) water needs. This study gives a first overview of where and when water tensions are most likely to occur in the canton of Vaud. Highlighting these regional differences supports the development of strategies to cope with water stress that are currently being discussed with the cantonal authorities.