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Unformatted text preview: d the imbalance has
continued to grow since that time.
In a sustainable system supply exceeds or equals demand indefinitely. We can maintain a sustainable
state only if supply ≥ demand and the time rate of change of supply ≥ that for demand. Usually the rate of
replenishment of renewable resources is fixed, so the time rate of change of supply equals zero, and therefore in
a sustainable state the time rate of change of demand must equal zero.
Let’s look at water in a lake as an example (Figure 5). The stock of water is the amount of water stored
in the lake. Inflow is a measure of supply, i.e., the rate of water replenishment. The outflow is a measure of
demand, i.e., the rate of water withdrawal or consumption. The stock of water reflected by the level of water in
the lake will fall if inflow < outflow, rise if inflow > outflow, and remain steady if inflow = outflow. Water use is
sustainable if the rate of withdrawal is ≤ rate of replenishment. When inflow equals outflow the rate of input
equals the rate of output, and we have a st...
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- Fall '14