Using the connection between water and energy as a case study, we present a model that uses the effects of geospatial and temporal context on embedded energy to approximate resource sustainability for water.

First, the basic steps of calculating the energy intensity for a given location are discussed. Intensity is presented in units of energy per volume of water. In the case of supplying fresh water, energy intensity depends upon the quality of the original resource, its location relative to the end use location, and the type of technology in use to move and treat the water. Pumping, and conveyance, purification, distribution, wastewater treatment, and system inefficiencies (e.g. evaporative losses, leaks) increase the total energy investment, while water recycling decreases the total investment. Lift and purification are typically the greatest contributors to the overall energy intensity of a fresh water supply, but system inefficiencies can have a substantial impact as well.

Over time, growing cities tend to progress from using their least energy intensive water resources (e.g. untreated surface water) to their most energy intensive (e.g. long distance transfers, desalinated water lifted to high elevations) as water demands begin to outstrip supplies. As a function of water availability, we assign each location an intensity value that approaches the intensity of its next “best” (i.e., least energy intensive) source of water. Hence, an area which is depleting its available surface and groundwater may have desalinated surface or groundwater as its next (and last) resort. The area would be characterized as undergoing water stress, and relatively less sustainable than areas which use their local fresh water supplies with no perceivable negative impact.

An operating principle of this research is that with enough energy, it is possible to supply any location with fresh water. Desalinated ocean water, moved over long distances and lifted to great heights represents that upper limit. Working backwards from this extreme scenario, it is possible to not only move away from the paradigm of unitless or vague sustainability indices, but to quantify resource scarcity in a way that is both intuitive and actionable.

The model is also self-correcting: areas may reduce the energy intensity of a sustainable water supply through better management of existing fresh water resources or through technological innovations that produce fresh water from degraded sources in an energy efficient manner.

A major conclusion of this research is that the amount of energy necessary to maintain a reliable supply of fresh water greatly varies by location and technology choice. Further, many areas of the country overuse their local fresh water sources. To create a durable water supply, such areas can 1) reduce their use of local fresh water to sustainable levels and invest in alternative water sources—at a high financial and energy cost, or 2) aggressively pursue water efficiency measures so that they can both reduce their reliance on local fresh water sources and avoid the high costs associated with alternative water supplies.

Additionally, by converting water use to energy consumption as a function of scarcity, it is possible to weigh the relative importance of water use efficiency to conservation in other areas (e.g. electricity, direct heating, waste disposal).

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