For assessing water security at global scale, the term 'security' is conceptualized as a function of availability, accessibility, affordability, quality, safety and stability.

Water Resources Vulnerability Assessment provides a macro-level overview of the status of water resources in the region using regional analysis and individual country vulnerability profiles. It is a way to mitigate vulnerability. It is composite of the socio-economic and natural components. It considers solutions to reduce vulnerability by importing virtual water or using available energy resources. The assessment provides an overview of water resources threats in the region considering the capacity of the water management to absorb such threats. 

Vulnerability  and security  are both quantitative  and qualitative. 

In both water security and vulnerability assessment, quantity and quality aspects of water will be considered with equal importance. Water security is to conserve and protect the water resources to meet the needs of the people and the environment on a sustainable manner. Vulnerability is to analyse the probable risks involved towards the quantity and quality of available water and to workout the mitigative measures and also adoptability plans in the changed scenario.  

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You may find these articles at this website of interest: http://water-pire.uci.edu/wp-content/uploads/2013/08/1_Hao_Science-2000-V%C3%B6r%C3%B6smarty-284-8.pdf