Among published global water resource assessments, regions where annual total water withdrawal exceeds 20% and 40% of the availability, or where per capita water availability falls below 1,700 and 500 m3 person-1 year-1, are categorized as being moderately and highly water stressed, respectively, but the rationale for using these thresholds has not been fully explained. In this presentation we show that these thresholds represent the sufficiency of local and renewable water resources, which are estimated daily by a state-of-the-art global hydrological model. We found that water abstraction from local and renewable sources is insufficient (i.e. more than 1% of the annual total water requirement becomes unmet) in most regions where annual total water withdrawal exceeds 20% of the availability and/or per capita water availability falls below 1,700 person-1 year-1 (moderate stress). In regions where the corresponding values are 40% and/or below 500 m3 person-1 year-1 (high water stress), more than 20% and 60% of the annual total water requirement must be supplied by non-local and non-renewable sources, respectively. These results indicate that conventional indicators and thresholds can be used as a surrogate for measuring daily variations in the sufficiency of local and renewable water sources at the global scale. At the same time, the substitutability varies by region, which can be primarily attributed to the intensity of the seasonal variation in river flow and irrigated area per capita. Based on the analyses, new sets of thresholds for WTA and APC are proposed. These findings are of significance for users of the two most frequently applied water scarcity indexes.
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