Short answer. No. Article 505 provides that every concession for the use of waters is understood to be without prejudice to third persons, so your government grant does not override or diminish water rights others already hold — if using your concession fully would harm those existing rights, the concession itself does not authorize that harm.
What the law says
Every concession for the use of waters is understood to be without prejudice to third persons.
Civil Code, Article 505 — Concessions Without Prejudice To Third Persons. Read the full provision →
A concession does not create rights against third persons
Article 505 states plainly that every concession for the use of waters is understood to be without prejudice to third persons. A concession from the government is a grant of permission to use a public resource; it is not, in itself, a determination that no one else has a competing claim to that same water. Anyone who already held a right to use the same water before your concession was issued keeps that right, and your concession does not silently wipe it out.
What this means when your use conflicts with someone else's
Because the concession is granted subject to existing third-party rights, exercising it in a way that actually interferes with a right someone else already holds is not protected simply because a government body issued the concession. If your use, taken to its full extent, would deprive another water user of what he was already entitled to draw or divert, Article 505 places that risk on the concession, not on the third person whose prior right the article was written to protect.
The concession settles your relationship with the government, not with everyone else
A useful way to read this rule is that the concession only settles what the government is authorizing you to take from the public waters; it does not settle any dispute you may have with a neighbor or another user over whose claim to that water actually comes first. If two claims conflict, the concession's existence is not the answer to that separate question, because Article 505 expressly reserves whatever rights third persons already had before the concession was granted.
Practical implication for holders of a concession
In practice, holding a concession does not relieve you of the need to use the water in a way that respects rights that predate or coexist with your grant. If a third person can show an existing right to the same water that your use would impair, Article 505 puts you in no better position than if no concession existed at all with respect to that particular conflict — the government's permission runs alongside, not over, whatever the third person was already entitled to before you were granted your concession.