Depletion Index (DI) depends on the amount of resources removed from the environment and not returned. However, to reflect the abundance of certain resources, and scarcity of others, we will also include the known reserves of the resource in the index. The Depletion index can be thus expressed as:
Right now we consider all the resources to be of the same importance, thus the last factor in the DI will be set to 1 for all resources.
Resources can be of any kind here: the most popular are of course minerals, but also timber, plants, animals, even land or habitat. Some of them, like oil or coal, have pretty well know estimates as to the reserves; others we will need to estimate ourselves.