Sankey Diagrams

This article on “Conceptualizing the built environment as a social-ecological system” by Sebastian Moffatt (CONSENSUS Institute) and Niklaus Kohler (University of Karlsruhe) published in Building Research & Information, Volume 36, Issue 3 May 2008 , pages 248-268 has an exciting Sankey diagram in the section ‘Current perspectives, promising methods, missing pieces’ (scroll down about half way).

The authors explain Sankey diagrams as an instrument of Material Flow Analysis (MFA)

“Sankey (directional flow) diagrams are often used to summarize the MFA visually as an entire connected and balanced system. In a Sankey diagram the material flows begin with inputs from nature, then flow into intermediary processes (any infrastructure used for processing, converting, storing, or regulating), and then into the various end use(s). After use, flows may be reconverted by infrastructure systems for reuse or recycling. Ultimately, all flows are directed to a category of output (waste products emitted into air, into water bodies or into landfills; long-term storage; export). The balanced accounting thus tracks every flow from source to sink.”

The Sankey diagram shown in this article is for an resource efficient house, planned or built in New Delhi (India). It shows the water flows through five groups of processes (sources, converters, demands, re-converters, and sinks). The authors call it a “five-partition metabolic profile”, and suggest that it can be done not only for a single house, but “for the built environment at any scale, from parcel to urban region”.

The unit for the quantities given is not indicated, but I presume the water flows are in litres.

When reproducing the Sankey diagram I tried to make it a little more clearer by changing the order of the (invisible) nodes, thus avoiding crossing flows.

A visitor to this blog pointed me to the work and life of system ecologist Howard T. Odum.

Odum “in his early work used a diagramming methodology very similar to the Sankey diagrams used in chemical process engineering. In this model energy and matter flows through an ecosystem”.

In Odum’s ‘Silver Spring Study’, he

…mapped in detail all the flow routes to and from the stream. He measured the energy input of sun and rain, and of all organic matter - even those of the bread the tourists threw to the ducks and fish - and then measured that gradually left the spring. In this way he was able to establish the stream’s energy budget. (Wikipedia)

This diagram shows the energy flows in the ecosystem. The main contributors are sunlight and other biomass imports. Energy “leaves” the system as exports (extraction of animals and biomass) and mainly as decomposed matter. There are no absolute figures in this diagram, but the proportions seem to be represented by the arrow magnitudes.

The diagram has a nice “natural touch” to it, and at first sight one might think that you are looking at the arms of a river delta…

In the Wiki of the CD4WC (Cost Effective Development of Urban Wastewater Systems for Water Directive Compliance) project, I found an interesting Sankey diagram that I wanted to share with you.

This project, funded by the European Commission, deals “with optimising the efficiency of urban wastewater systems with regard to ecological consequences in natural water bodies and with regard to investment and operation costs.”

The waste water treatment process system is shown with a schematic flow diagram. For individual substances that can be found in the waste water, the diagram is then displayed with Sankey flows, that represent the quantity. Thus, Sankey diagrams are a possibility for the “determination of fluxes of substances per unit of time”. This presentation is part of a method is coined Substance Flow Analysis.

This presentation is very advantageous: The nodes in the system (the process blocks) remain at the same position, only the magnitude of the arrows changes, when switching to the substance flows view. Flows with large quantities substances are clearly visible.

It would also be a possibility to introduce as a third view (next to absolute water quantity, and substance quantities) the substance concentrations (impurities per m³ of waste water).

I had bookmarked a number of Sankey diagrams a while ago when visiting the website of Lawrence Livermore National Laboratory. These diagrams on energy, CO2 and freshwater are a great source of information. You can, for example, find the typical energy Sankey diagrams (like the one in my previous post) for the U.S. all the way back to 1950, 1960 and 1970 and then from 1973 to today.

Summer is coming closer, and we can again expect water shortage in some states. So the Sankey diagram I have selected for presentation today (original PDF), is for freshwater withdrawals in the year 2000.

The overall extraction was 345.000 Mgal/day (roughly 1.3 bio litres per day) with approximately 75 % from surface water and 25 % from ground water. Another 62.300 Mgal/day of saline water is withdrawn for thermoelectric use.

The largest portion of the water is for irrigation, livestock and aquaculture, closely followed by the water used in thermoelectric power generation. Domestic self-supply is a comparatively marginal 3,590 Mgal/day (1,04 %), which makes my Mom’s call to “close the tap” sound somewhat ridiculous.