Lecture 12: System Cycles

Reservoirs - Physical boundaries or holding tanks (the ocean is a reservoir)

  • A reservoir is also thought of by the mass of material in it, (the ozone, the fish in the ocean)
  • You must be the one that defines a reservoir when doing your study, (the pesticides in a hawk is a reservoir)


Cycles Can Be Portrayed Visually, Graphically, or Mathematically

  • When we construct a portrayal of the characteristics and functioning of a cycle or any characteristics of a cycle or any other environmental process, it's called a model
  • Models of natural cycles and other processes can be:
  • Physical models
  • Landscape drawings
  • Box models
  • Mathematical models

Physical Model

  • Landscape drawing
  • Basic water cycle drawing

Box Model

  • Reservoirs are portrayed as boxes connected with arrows
  • Even simpler cycle drawing
  • Has units and %s along with the process on each arrow (Ocean\rightarrow  evaporation 86%\rightarrow  Atmosphere)
  • Reservoirs -Boxes
  • Contents - Numbers in that box
  • Transfer processes - Arrows
  • Fluxes - Numbers on the arrows
  • Box models are the first step in developing math and computer models
  • Each process is described by a math equation


The Content of a Reservoir is a Function of Both Concentration and Overall Size

Content (or burden) of a reservoir = total mass of substance in reservoir = concentration \ast mass of physical unit

E.G. Content of Sodium in Sea Water

== 10.78g/kg10.78 g/kg (NaNa concentration of seawater) \ast 1.4×1021kg1.4 \times 10^{21} kg (total mass of ocean)

== 15.1×1021kg15.1 \times 10^{21} kg (burden of Na in seawater)


Am I Going to Sample Every Fish in Lake Ontario? Once I Get a Fish, How Do I Find Mercury Content? (Put It In a Blender)

  • There are not really any boundaries that exist on their own, we as researchers point out and make boundaries.


Fluxes are Flows of Matter Into and Out of Reservoirs

Flux - Amount of material transferred, described in terms of mass or volume per unit of time

  • Evaporation of water from the ocean surface to atmosphere ~ 3831018g/yrH2O~383*10^{18}g/yrH_2{O}
  • Fluxes are controlled by the rates of transfer processes and capacities of reservoirs

Source - Where the flux is coming from; a source gives out more than it takes in

Sink - Where the flux is going; a sink takes in more than it gives out

  • If source - sinks \rightarrow steady state


Can a Source Also Be a Sink?

  • Yes the atmosphere! (it evaporates water, and rain water)
  • If the source and sink are running at the same rate it will stay the same
  • How long will it take to use up a reservoir is a calculation?
  • When source is way more than sink, it is called "burden is increasing"?

Turn over time - The time it takes to empty or fill a reservoir.


What if a Material of Interest Enters a Reservoir?

  • Multiple possibilities
  • Can go from source to sink (the amount of time it goes from source to sink is called residence time)


Feedback Loops

Positive Feedback - Self reinforcing, self perpetuating "vicious" cycle

Negative Feedback - Self regulating, homeostatic



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