Sustainability+Revolution+Chpt+1

Usually the rate of replenishment of renewable

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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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