Conventional knowledge suggests that water reclamation and reuse is implemented as a response to scarcity of natural water resources. As water demand increases natural water resources (surface waters or groundwater) become insufficient and individual communities or larger regions turn to water reuse to meet an increasing fraction of their needs. However, the analysis of data on water consumption and water reuse collected by the US Geological Survey indicate that water scarcity is perhaps only one factor behind water reuse. This work suggests that relative variability, measured as the coefficient of variantion combined with the Hurst exponent may explain to a large extent differences in water reuse in different parts of the U.S.

Sustainable development gradually becomes an important concept embedded in many societal activities including economy, politics and perhaps even regulations. Sustainability is now a growing concern of businesses, governments, civic groups and individuals. These concerns are often linked to energy efficiency, reduction of environmentally harmful emissions, ecosystem preservation and other "save the Earth" efforts. They are becoming a part of a "triple bottom line" for business accounting: financial, social and environmental. Despite its increasing importance, current definitions of "sustainability" are somewhat vacuous. This paper presents a possiblity to characterize sustainability in terms of system theory and chaotic dynamics with strong links to to moral and ethical issues.

The meaning of sustainability in the context of water resources management has changed through the time. Initially meeting water demand was the dominant concern. While later quality issues became more important followed by wider water reuse, today sustainability must include a whole range of aspects (e.g., energy, pollution, persistent chemicals), spatial and time scales. New approaches to define sustainability metrics are needed. A possible approach is to use fundamentallybased entropy and energy flows.