This site uses cookies. By continuing to use this site you agree to our use of cookies. To find out more, see our Privacy and Cookies policy.
Skip to the content

[IOP] A community website from IOP Publishing

Tag Archives: footprint

Land use and energy

By Dave Elliott

By their nature, renewable energy flows are diffuse and the technology for capturing energy from the flows has to cover relatively large areas. It is instructive, and sobering, to revisit Professor David MacKay’s calculations about the areas required to match the energy needed per person from renewable sources: http://www.withouthotair.com/.

However, as I noted in an earlier post (on his comparisons between wind/solar and shale gas), some of his analysis is a little limited, and the general conclusions have to be put in perspective. (more…)

Posted in Renew your energy | Tagged , , , , , , , | Leave a comment | Permalink
View all posts by this author  | View this author's profile

Water-footprint measure can be locally and temporally specific, but isn’t used that way

I just attended the conference Understanding, Measuring, and Managing Water Scarcity Risks and Footprints in the Supply Chain this past week. This conference was primarily attended by sustainability managers of corporations along with a few academics and non-governmental organizations. There was much discussion of how to measure water impacts of industry as well as how to act on measured or calculated information. Many of the speakers and attendees were familiar with several methods for measuring water “usage” such as the Water Footprint (www.waterfootprint.org) and the Global Water Tool of the World Business Council on Sustainable Development (www.wbcsd.org/web/watertool.htm). The former presents information on the green water (soil moisture for the most part provided by precipitation) and blue water (stored water in rivers, lakes, and aquifers) consumed in the supply chain of a product. The latter is a mapping program that allows businesses to understand if they have operations in regions of the globe that have water scarcity.

There was general agreement within the community that the Water Footprint is not properly used as Jason Morrison of the Pacific Institute summarized by saying “different interests use the term ‘water’ footprint’ to mean different things” for their own purposes. Technically speaking, the water footprint is in units of water volume per time. By multiplying by the time per product manufactured, one can obtain the water footprint in units of water per product. This last term is the one most commonly presented in such examples as the quantity of water needed for a pair of jeans or a cup of coffee. This water volume per product is a handy unit of measure that consumers and business people can easily grasp. The problem is that it doesn’t seem to be helping either water resource management practitioners or sustainability managers at companies.

The issue stems from culminating into one term the water consumed over a supply chain that occurs in time and in space. If your supply chain for a product occurs in more than one location and/or at more than one time, then by definition you cannot capture all of that information into a single number. Mathematically this is like taking the derivative of a number. Each time you take the derivative, you lose one degree of freedom or information. For example, the volume of a sphere is described as V = 4/3pir^3 and is in units of cubic meters (m^3) to describes a three-dimensional space. Taking the derivative of volume with respect to its radius results in the surface area of the sphere at A = 4pir^2 in units of square meters (m^2) to describe a two-dimensional space. Hence we went from three dimensions to two dimensions. If I show only the final value for the surface area of the sphere, say 1 m^3, I do not know that a sphere is being described. However, if tell you the equation for the sphere’s surface area and tell you it is equal to 1 m^2, then you know how to calculate the volume (or radius) because I have just provided more information that told you about the third dimension.

What does this have to do with water footprinting? Well, similarly to needing to know more than one piece of information about the surface area being described (need two of either equation, radius, and surface area) to know it is for a sphere, you need more than a single value for the water footprint of a product to understand the environmental impact caused by its production. For example, if a shirt requires water during farming of cotton and dyeing of the fibers, then one could present the information in two numbers on a bar chart (among many other means for presenting information). Part of the bar chart would represent the cotton farming, and the other part would represent the dyeing step. By telling people where you source your cotton and where you perform your dyeing, you have now presented more information – information again that cannot be understood using a single value. I have just described four pieces of information: water for cotton, water for dyeing, location for cotton, and location for dyeing. A map with the water consumption value in each location the water is consumed could present all four pieces of this information. I could go on for temporal components. The World Water Tool exists to take the information described in this paragraph to relate to water scarcity around the globe. They of course use a map for this.

This thought exercise is meant to show that people understand that describing environmental impacts is somewhat complex. In describing water flows for human appropriated needs, from a basic standpoint we should focus on avoiding the word “use” to describe water flows. Instead, use “consumption” to describe water that enters the system as a liquid and exits as water vapor or in another chemical form. Use “withdrawal” to describe water entering and exiting the system in a liquid form, and note that consumption is a subset of withdrawal. The water footprint is a consumptive descriptor that for the most part includes evapotranspiration (green water) on top of what the term consumption (the blue water component) takes into account. If we stick to some of these basic rules, we can better understand how human and ecosystem services are subjected risks in water availability.

Posted in Energy the nexus of everything | Tagged , , , , , , | Leave a comment | Permalink
View all posts by this author  | View this author's profile