Learn Italian: Meno acqua c'è, più la resa diminuisce. - The less water there is, the more the yield decreases.

In Italian correlative clauses, you drop the definite article. You just use più or meno directly. Adding an article (like il meno or il più) turns them into superlatives ('the least' or 'the most'), which would completely change the meaning.

Since acqua is a feminine noun, why doesn't the ending of meno change to match it?

Meno and più are invariable, meaning their endings never change to match gender or number. This applies whether they are acting as adverbs modifying a verb (like più modifying diminuisce) or acting as adjectives modifying a noun (like meno modifying acqua).

Does the meno... più... structure ever trigger the subjunctive mood?

No, proportional clauses like this one deal with objective facts and direct cause-and-effect relationships, so they take the indicative mood. That is why we use the standard present tense forms c'è and diminuisce here rather than the subjunctive.

What kind of 'yield' does la resa refer to in this context?