Irrigating From a Well in Montana: Legal Limits and System Design

How much water can a Montana exempt well use?

DNRC's guidance is that a groundwater development of 35 gallons per minute or less, not exceeding 10 acre-feet per year, is exempt from the water use permit process under section 85-2-306 MCA. Above either number, a permitted water right is needed. For scale, the U.S. Geological Survey defines an acre-foot as 325,851 gallons, so the annual cap is a little over 3.2 million gallons.

That sounds like a lot until you remember it is a combined limit in many situations. DNRC says a permit is required for a combined appropriation from the same source by two or more wells or developed springs exceeding 10 acre-feet, and that wells drawing from the same source may be treated as a single exempt use sharing one 10 acre-foot exception. DNRC applies a definition in its rules (ARM 36.12.101(12)) and weighs several factors to decide whether separate developments could have been one appropriation.

What changed for exempt wells on January 1, 2026?

The filing process. DNRC's Exempt Well Updates page says House Bill 681 passed in the 2025 Legislature and took effect January 1, 2026, and that it requires a Notice of Intent (Form 602I) before anyone uses water from an exempt well. DNRC says it reviews a correct and complete Notice of Intent within 10 business days, and that an authorized notice gives the user five years to complete the project, put the water to use and file a Notice of Completion, with one five-year extension available. There is a filing fee; check DNRC's current form for the amount.

DNRC's permit exceptions page adds that the Notice of Intent must be filed before the water is put to use, though it can be submitted before or after the well is drilled. Then the Notice of Completion of Groundwater Development (Form 602) is due within 60 days after the development is put to use, and the priority date of the water right is the date DNRC receives the completed Notice of Completion. DNRC also says it can no longer process a Form 602 without an authorized Notice of Intent unless the 602 was pending before January 1, 2026, and that this applies even to wells already drilled or in use.

Is a well log the same as a water right?

No. DNRC's permit exceptions page explains that for a new well, the driller completes a well log (Form 603) and sends it to the Montana Bureau of Mines and Geology within 60 days, with a copy to the owner, and states plainly that a well log is not a water right. The water right comes from the DNRC filings. If you bought a property with an existing well, look it up in DNRC's Water Rights Query System and confirm that the use you plan, including lawn and garden irrigation, matches what is on record.

What about subdivisions and shared wells?

This area changed recently. DNRC reports that on February 14, 2024, a district court ruled that all phases of a multi-phase subdivision must be treated as one combined appropriation, so a four-phase subdivision cannot claim 10 acre-feet per phase, and that all lots in a multi-phase subdivision now share one 10 acre-foot per year limit. DNRC also notes it may propose further policy changes for 2027. If your lot is in a subdivision served by individual wells, the covenants and the subdivision's water approvals may limit irrigated area, and that is worth knowing before you design a large lawn.

This is general information, not legal advice. Water law turns on the documents and history of a specific property, so confirm anything that matters with DNRC or a Montana attorney who practices water law. DNRC's Missoula Regional Office, which serves Missoula and Ravalli counties, is the place to ask about a specific well.

How do you find out what your well can deliver?

Start with the well log and the pump. Rain Bird's homeowner guidance on how many sprinklers a zone can run says that for well or pump systems, the pressure and flow capacity should come from the well and pump dealer or the pump's owner's manual rather than a bucket test at a faucet. The well log records what the driller measured when the well was finished; the pump's curve tells you what it delivers at a given pressure. The lower of what the well can sustain and what the pump produces is your budget.

We design zones so that each one draws comfortably less than that budget, leaving headroom for the house. A zone that asks for more than the well yields will pull the water level down, drop the pressure at the heads and leave dry arcs in the lawn. Our low water pressure guide covers how that shows up.

Why do well systems need more zones and lower-flow heads?

Because well flow is usually modest compared with a municipal service, and every zone has to fit inside it. That pushes design toward smaller zones, heads with lower flow and lower precipitation rates, and more drip. Colorado State University Extension's sprinkler guide notes that rotors apply water at roughly a quarter to three quarters of an inch per hour while pop-up sprays apply 1 to 2 1/2 inches per hour, so rotors and multi-stream rotary nozzles cover more ground per gallon per minute, at the cost of longer run times. Our sprinkler head types guide explains the trade-offs and the zone design guide shows how zones are drawn.

Drip is especially well suited to wells. CSU's drip guide puts drip efficiency above 90 percent, compared with 50 to 70 percent for sprinklers, and drip zones are measured in gallons per hour rather than gallons per minute. Beds, shrubs and trees on drip leave more of the well's capacity for lawn. See our drip irrigation guide.

Does well water need filtration?

Often, especially for drip. CSU says drip filters need cleaning more often with well or pond water than with municipal water, and that in-line emitter tubing calls for fine 200 mesh filtration. Utah State University Extension's guide to drip irrigation for trees says the water must be clean and filtration is often needed. Sand, sediment and minerals that do not bother a toilet will plug emitters and the small passages in rotary nozzles, so we put a filter on any well-fed zone that needs one and show the owner how to clean it.

Do well-fed irrigation systems need backflow protection?

Protecting the well matters as much as protecting a city main. An irrigation system that sits in fertilized soil should never be able to siphon back into the water you drink. Inside Missoula Water's service area, the utility's backflow program requires backflow protection on all properties where auxiliary water supplies exist, which covers a property that has both city water and a private well. Elsewhere, ask your local water provider or building department what applies. Our backflow testing guide explains the assembly types.

What changes in fall and spring for a well system?

The pump. CSU's winterization guide advises that pumps be drained and stored indoors, or drained and insulated where that is not possible, and that compressed air, not drain valves, is what reliably clears the lines. We winterize well-fed systems the same way as city systems, with extra attention to the pump and pressure tank connection; see our sprinkler blowout and winterization service. In spring the system is recharged slowly, as covered in our spring startup guide.

How can we help with a well-fed system?

We are not well drillers, pump installers or water-rights consultants. We design and install the irrigation side of a well-fed property: zones sized to the yield your pump dealer gives us, drip where it makes sense, filtration and controls, and the seasonal service that keeps it running. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide, and our irrigation systems page explains how we work. Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate. We work across Missoula and Ravalli counties, from Missoula and Lolo to Florence, Stevensville, Corvallis, Victor, Hamilton and Darby.

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