This paper focuses on the measurement of the relative sustainability of renewable water resource systems. Being able to quantify sustainability makes it possible to compare alternative plans and policies, and to include sustainability as one of the multiple objectives to be considered when making decisions regarding the design and operation of these systems. Commonly used measures of reliability, resilience and vulnerability, based on subjective judgements concerning what is acceptable or unacceptable with respect to multiple system performance indicators, are combined into an index and used as a measure of changes in relative system sustainability over time.