Yes, there needs to be common terminology. How about "depletion rate" and "decline rate"? "Depletion" has become accepted for both definitions (in my view, incorrectly) but why not use "depletion" for percentage of proven reserves produced, and "decline" for year over year drop in production? There is no reason to be confused and use "depletion" for both. In fact, this makes sense when you look at the dictionary definition of "depletion":

de·ple·tion
n.
The act or process of depleting.
The state of being depleted; exhaustion.
The use or consumption of a resource, especially a natural resource, faster than it is replenished.

The definition of "depletion" refers to the verb.

de·plete
tr.v. de·plet·ed, de·plet·ing, de·pletes
To decrease the fullness of; use up or empty out.

The act of pulling oil out of the ground fits this definition, but not a decline in rate of doing so. Therefore, I have always found it odd to hear that a country will be "going into depletion", since depletion happens from day one!