Short answer. Before you can run a water channel across a neighbour's land, Article 643 of the Civil Code requires you to prove three things: that you can dispose of the water and that it is enough for its intended use; that your proposed route is the most convenient and least burdensome to others; and that you will indemnify the servient owner.
What the law says
To prove that he can dispose of the water and that it is sufficient for the use for which it is intended; (2) To show that the proposed right of way is the most convenient and the least onerous to third persons; (3) To indemnify the owner of the servient estate in the manner determined by the laws and regulations.
Civil Code, Article 643 — Conditions for the Easement of Aqueduct. Read the full provision →
What this easement is
An easement of aqueduct is the legal right to carry water through someone else's land, by a channel, pipe, or ditch, so that water can reach the land or the use it is meant for. The neighbour's land that must accept the channel is called the servient estate. Because this forces a burden onto another owner's property, the law does not grant it for the asking. Article 643 of the Civil Code sets three conditions the person who wants the easement must satisfy first. Meeting all three is what turns a mere desire for a water passage into an enforceable right.
Prove your right to the water
The first requirement looks at the water itself. The applicant is obliged To prove that he can dispose of the water and that it is sufficient for the use for which it is intended. This has two parts. He must show a lawful entitlement to the water he wants to move, that it is his to use, not water he has no right to divert. And he must show the quantity is enough for the purpose, whether irrigation, household supply, or industry. The law will not burden a neighbour's land to carry water the applicant cannot lawfully claim, or a trickle too small to serve the stated need.
Choose the least harmful route
The second requirement protects everyone whose land lies in the path. The applicant must show that the proposed right of way is the most convenient and the least onerous to third persons. He cannot simply pick the route that is cheapest or easiest for himself. Among the possible paths, he must select the one that does the least damage and imposes the smallest burden on other people's property. If a less harmful route exists, the neighbour can resist the one chosen. This keeps the easement no larger and no more intrusive than the water need truly requires.
Pay for the burden you impose
The third requirement is payment. The applicant must indemnify the owner of the servient estate in the manner determined by the laws and regulations. The easement is not free: the neighbour who must give up part of the use of his land is entitled to compensation, fixed according to the applicable rules. In short, the right to run water across another's property is real but conditional. Only when the applicant has proven his right to sufficient water, chosen the least burdensome route, and paid the required indemnity does the neighbour have to accept the channel.