Legalizing household rain barrels in Colorado is pitting conservation-minded Democrats against Republicans determined to defend water rights. The two-session standoff, however, has a handful of legal experts wondering why there’s a fight.

None of them could point to a statute that specifically says rain barrels are illegal. Arguments on both sides depend on a broad legal interpretation that says you can’t store a drop without a water right, even if you put it back in the ground to water a garden a few feet away.

Republicans want to make sure rain barrels don’t put a crack in state water law and ensure that those with the oldest and most expensive water rights get their fair share before those with no rights get a drop.

Rep. Jessie Danielson, a Democrat from Arvada, said the House bill she’s sponsoring is about clarifying the law, which might encourage more people to use rain barrels. The measure has passed the state House and is headed to the Senate.

“Even if we can conserve the smallest amount of water, it is less treated drinking water being poured out onto our lawns,” Danielson said.

So why all the fuss?

Water law is so important here that it is embedded in the state’s original constitution, protected by a special state court and managed by a state agency. Farmers, ranchers and drinking-water providers depend on getting what they paid for. And in dry years, those with newer water rights could get left out.

Proving that use of a rain-barrel is causing harm to a water right in court could be a tough job, said Bill Paddock, one of Colorado’s top water lawyers.

To start, someone would have to figure out how many rain barrels are being used in a watershed that feeds downstream water rights. Since there’s no requirement to register them, even in the proposed legislation, analyzing any harm done to holders of water claims would mean checking downspouts door to door to get a count.

Then there would need to be a measurement of how much water each barrel stored. Then the analysis would have to show how much, if any, of that water would have made it back to the river, Paddock said.

“It’s really a question of whether the water that runs off your roof and into a rain barrel and onto a garden is injuring someone else’s water right,” he said.

Colorado already exempts wells that draw from aquifers that are proved not to harm the water rights of others.

In fact, rain barrels are a boost to the water rights of others, said Bart Miller, director of the Healthy Rivers Program for the nonprofit Western Resource Advocates in Boulder.

“They help residents become better and more knowledgeable customers,” he said. “In other states that added rain barrels to the conservation tool box, they’ve seen rain-barrel users learn that water is a limited resource that needs to be carefully managed. … All this, of course, leads to lower per-capita water use and lowering the need to pull water out of agriculture and rivers.”

The question is how many people who aren’t already using them would start.

The water might be free, but the barrels aren’t cheap. A basic 55-gallon plastic drum starts around $70, not including the fittings, and reaches into the hundreds of dollars.

State Sen. Jerry Sonnenberg, a Republican from Sterling, bottled up the rain-barrel legislation last year, but this year he is tentatively on board. That is, if the bill that passed the House on Feb. 29 adequately preserves prior-appropriation rights — the supply guaranteed to those with older rights — and has adequate oversight from the state engineer.

The Colorado Farm Bureau was an opponent of the bill until those amendments were added.

“As the language currently stands, the state’s water court system is fully considered and will allow the state engineer to address injuries to other water rights should they occur in the future,” said Don Shawcroft, president of the Colorado Farm Bureau.

Conservation Colorado, a left-leaning driving force behind the legislation, said rain barrels are a clear symbol of conservation that help drown out the inconsistent chatter around Colorado’s complicated water law.

“It makes water policy approachable, not something just for lawyers and insiders,” said Pete May smith, executive director of Conservation Colorado.

“But legalizing rain barrels is also important because it demonstrates that elected officials can actually be responsive to the public and its priorities.”

Joey Bunch: 303-954-1174