Water Stress over the Year: Quantitative Analysis of Seasonality and Severity on a Global Scale
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Blue water (i.e. river discharge) and green water (i.e. soil water) are vital water resources to our society. Yet, assessments of water scarcity or water stress tend to focus on blue water only. This study assessed the amount of pressure put on both blue and green water resources by using the water scarcity index. Monthly water demand for the year 2000 was estimated as a bench mark year and contrasted against 44 years (duration: 1958-2001) of a long-term climate based on ERA-40 and CRU TS 2.1 meteorological data sets. The results indicate that 1.5 to 2.3 billion people (approximately one-third of the world's population) are currently experiencing moderate to high water stress on a global scale. The results vary significantly depending on a temporal resolution (i.e. yearly or monthly time scale). Emerging high water stress regions where green water demand is exceeding green water availability are identified for the first time. In addition, this study reveals new dimension of water stress by using a finer temporal scale than previous studies (i.e. month). Our results indicate that the degrees of blue water stress are underestimated by existing annual assessments in large regions (e.g. Sub-Sahara, the eastern part of Brazil, the western part of Australia, Central Asia and India). The regions vulnerable to the spatiotemporal variability of blue water stress are identified. Also, the monthly time steps enable to assess the seasonality and the severity of global water stress. The severity is quantified by using statistics of persistence, recurrence, average intensity as well as the dynamic water stress developed by Porporato et al. (2001). The results show that blue water stress estimated by this study generally corresponds well to the observation of past water shortage (i.e. extreme events) in the Netherlands and Japan. Even though estimated water demand of the year 2000 stays constant for 44 years, water stress (thus water shortage) was well reproduced by a long-term climate variability.