Stocks and Flows
Water ReservoirOil Reservoir

One of the essential elements of H. T. Odum's symbolism is:

Storage (Stock)

This symbol can be succintly defined as storage (alternatively: stock).  Storage represents the concentration of a quantity (of matter, energy, structure or information).  Such concentrations can serve as energy sources able to drive opportunistic pathways (i.e., they have the potential to make things happen).  Examples of resource stocks are:

  • Water reservoir 
  • Oil reservoir

Storage in a water reservoir can be replenished, but oil exemplifies a stock of non-renewable resource. 

Consider (again) Paul Harrision's characterization of resource drawdown (Chapter 17,  The Third Revolution):
  1. In the beginning is abundance. Resources are vast in comparison with human numbers. Population density is low. There is plenty for everyone: all people need to do is to collect what is there. This is the gathering phase.

  2. Gradually numbers multiply and exert increasing pressure on resources. Yet at first people continue with the technologies and attitudes of the gathering phase. The resource is depleted. This is the mining phase. In some cases, when fuelled by windfall profits, as in tropical forests, it reaches a paroxysm in pillaging.

  3. What follows is the crisis phase. Potentially renewable living resources are not renewed, but depleted below the level at which they can renew themselves. Scarcities develop - hunted species disappear one by one, soil yields fall, firewood and water have to be sought further and further away. Depletion reaches the level at which it becomes visible, even painful

Also consider the chart represented below (in modified form) that Harrison utilizes to support his narrative:







Gathering - Abundant Communal Gathering Hunting Fishing Gathering
Fallow - Plentiful Communal Fallowing burn, hoe Pastoral, resting Fishing, closed seasons From fallow zone
Mining - Depleting Mixed Declining fallow Over-grazing Over-fishing Deforestation
[Crisis] Mixed Falling yields, hard soil, weeds. Desertification Stock decline Wood shortage

The item of immediate relevance in Harrison's chart is the designated sequence of (declining) "resource state" specified in the left column (PRODUCTION SYSTEM & RESOURCE STATE):

  • Abundant
  • Plentiful
  • Depleting
  • Crisis

Can we extrapolate from Harrison's sequence to H. T. Odum's symbolism?  Again, with analogy to a traffic light (in this instance inverted - rotated 180°), consider the following:

Storage drawdown: abundance to crisis  

The color gradient superimposed on the storage symbol in this graphic representation (green, yellow, red) signifies a change of resource stock level from abundant to crisis as drawdown proceeds.

Solar Radiation InfluxGeophyical Energy Flux In addition to the storage symbol, another essential element of H. T. Odum's symbolism is:

Energy source

This symbol is defined as: energy (constant flow) source.  The Earth's two major energy sources are:

  1. constant infux of solar radiation
  2. endemic geophysical heat flux.

Large-scale fluid dynamics within the atmosphere and oceans are driven by solar influx.  The slow, viscous movement of the Earth's rocky continental plates is driven by the convective flow of the more fluid molten layers beneath.  Additionally, energy flow is integral to processes such as the hydrologic cycle and the flow of materials within ecosystems associated with the overall process of maintaining life.

Hydrologic cycle
Terrestrial ecosystemMarine ecosystem

How is energy flow related to energy stocks?   Göran Wall graphically represented the relationship as follows:

After Figure 11, Göran Wall, "Introduction to Life Support Systems and Sustainable Development"

Wall's distinction between stock funds and deposits belies the differing temporal magnitudes required for the geophysical formation of funds (such as forests) and deposits (such as the metal ores); Wall's division of stocks into "funds" (forest, water dam, etc.) and "deposits" (minerals, etc.) parallels the designation by Tom Abel of three resource types:



Renewable Water, air, grass Water
Slow-Renewable Soil, trees Soil
Non-Renewable Metal ores, fossil fuels Oil

Associations of stocks and flows in Odum’s simulation model with Wall’s schematic representation can be rendered as follows:

Understanding in Time