Reducing vs. Declining

Petroleum engineers understand decline curves. However, some are a bit fuzzy about the difference between “declining” and “reducing.” To explain these differences, we will have to understand transitive and intransitive verbs. A transitive verb takes an object, whereas an intransitive verb does not.
Examples:

Transitive: He fits the flange onto the pipe. (object)

Intransitive: The flange fits nicely. (no object)

The same verb can transfer the action to an object, or not. Now let’s look at our verbs du jour.

Reduce (transitive) means to make the size or quantity less or to diminish the extent.

Examples: reduce a sauce to concentrate it, reduce a fraction or equation to simplify it, reduce Fe+3 (ferric ion) to Fe+2 (ferrous ion), reduce prices

Reduce (intransitive) means to become diminished in size.

Examples: She is reducing (dieting) after eating all that Valentine candy. The sauce is reducing on the stove. Fe+3 ion reduces in the presence of this additive.

Decline (intransitive) means to slope downward, descend, or droop.

Examples: oil production declined, the road declined toward the lake, her health declined

Decline (transitive) means to refuse or turn down an opportunity.

Examples: She declined his invitation to dinner. She was offered a promotion, but she declined it.

The transitive form of “decline” is not used to denote a decrease.

Bad Example: The supervisor declined his assistant’s salary.

This sentence means the assistant offered her meager salary to the supervisor, but he turned down her generous offer. If you mean that the supervisor had to decrease the assistant’s salary because of the department’s 5% budget cut, then use the word “reduced” rather than “declined.”