Water laws are so strict in Colorado that rainwater collection is virtually prohibited. The doctrine is written into the state’s Constitution. All the rain is already spoken for. It belongs to someone, and that someone probably isn’t you. So don’t you touch it.

“The rain barrel is the bong of the Colorado garden,” local columnist Dave Philipps wrote in 2007. “It’s legal to sell one. It’s legal to own one. It’s just not legal to use it for its intended purpose.”

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That might change soon, slightly.

On Monday, Colorado representatives voted to allow people to store up to 110 gallons of the rainwater that flows off their roof. One hundred gallons is on the high end of how much water a person in America uses per day. It’s about three tubs full of water, or four loads of laundry.

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Rain barrel legalization will not save the world, nor even Colorado, where already the law against rainwater collection is rarely enforced. H.B. 1259 might not even pass Colorado’s Senate. But it’s a symbolic step toward a more modern way of thinking about water in America’s dry Western states.

In the West, water belongs to someone

The principle at stake is called prior appropriation, which is legalese for “first come, first served.” This doctrine forms the bedrock of water law in the Western states, where long ago settlers raced to gobble up all the water rights. Prior appropriation helps explain why water-intensive agriculture is still a major industry in a place as arid as the West: Many of the early claimants were farmers seeking to irrigate their crops.

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These days, with drought parching the region, there’s hardly enough water to go around. According to the law, the people who get first dibs are the ones who called it first, which tend to be the agricultural users and not the city dwellers.

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In Colorado, other people’s water rights even extend to the raindrops that fall onto your roof.

Why? Because those raindrops might tumble into the gutter; they might seep into the ground; might, in some other eventual, serpentine fashion, find their way to a river where somebody’s great-great-grandfather once established a claim.

Legal experts have long criticized the Wild West principle of prior appropriation. They say that the tradition of dibsies is incongruent with the way that people demand water in the 21st century.

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“It’s this very rigid, very old system of water rights that hasn’t really changed that much in over a century,” said Reed Benson, a law professor at the University of New Mexico.

“Prior appropriation is so deeply embedded in Western water law,” said Robert Glennon, a law professor at the University of Arizona. “We academics criticize it but it’s not going anywhere.”

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Benson has studied all the different ways that Western states have grappled with an increasingly stale idea like prior appropriation, which allocates water according to seniority instead of need. Bills legalizing rainwater collection are an example of how legislators have sought to carve exceptions into that way of thinking. (California passed a similar law in 2012.)

“There are a lot of good, practical, common-sense arguments in favor of a bill like this,” Benson said. “The fact that it’s controversial, the fact that it’s taken this long, shows you how well-entrenched that old tenacious legal system is.”

Whose idea was this, anyway?

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Some laws are crafted by Congress; but the system of water rights in the West is a lesson in how customs can calcify into legal doctrine.

As the story goes, the dibsies approach to water management dates to the California Gold Rush. Flowing water has long been a gold miner’s best friend: As it cuts through the landscape, it picks up pebbles, dust, and occasionally, specks of something more precious.

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These treasures end up naturally at the bottom of riverbeds, waiting to be sorted out of the sediment. During the 1849 Gold Rush, prospectors schlepped across the country to pan for those rare glints in California’s streams.

Miners of means sought to speed up the process. They set up high-pressure hoses to blast entire cliff sides. The runoff would flow through boxes that caught any gold fragments washed loose.

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Hydraulic mining harmed the environment and placed exorbitant demands on a dry region’s water resources. The practice eventually fell out of favor, but not before it made a lasting impact on water law.

To feed their thirsty operations, miners dug channels that siphoned water from sources that could be miles away. They followed an apportioning rule carried over from mining principles. The first person to dig his canal was entitled to whatever water he carried away.

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Eventually, Western states began to recognize and regulate this practice. They parceled out permits to surface water in the order and quantities that people came to claim it. As settlers arrived to start irrigated farms on parched soil, people continued to treat water as its own and separate property right: Just because you owned the land didn’t mean you owned the water.

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The Eastern states, in contrast, were wet enough that they regarded surface water as a shared, inexhaustible resource (as in the common-law tradition, imported from soggy England). They did not keep tabs on how much water anyone took out of a river. People who owned land next to a river were free to use that river’s water in any reasonable fashion, as long as it didn’t affect their neighbors.

Though it’s maligned today, the system of prior appropriation suited the West’s arid climate. Unlike in the Eastern states, settlers could not rely on what streams, if any, ran through their property. They needed a legal system that allowed them to bring in water from far afield. Furthermore, water was scarce enough that it had to be measured out, which called for an orderly system of permits. These rights were given to those who would use the water productively — to irrigate a field, or to supply a mine.

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In theory, prior appropriation made sure that water wasn’t wasted. People could not simply claim part of a river and divert it onto their property. They had to show that they had plans for the water, and that their plans did not interfere with the designs of the people who came before them.

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Only then would they get rights to the water — and only enough water to serve their needs. As long as they continue putting the water to work, those rights are theirs forever.

Decades later, the unforeseen consequences

Fast forward 150 years to the present, when nearly every river basin has been burdened with claim on top of claim. Cue a record-breaking drought.

Prior appropriation has no provision for shared water conservation; the priority system is strict. During dry times, someone with a senior claim gets to suck down her full allotment. The people down the line might get nothing.

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(In Colorado, she’s even entitled to the rain that falls onto her neighbor’s roofs. That rain, by law, must be allowed to flow unimpeded into the river for her to use.)

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Critics say this system encourages waste. People with senior water rights don’t have any reason to cut back on their water use. (In practice the system is a bit looser, Illinois Institute of Technology law professor Dan Tarlock notes. It’s frowned upon to completely hog the water, even if someone has the legal right to do so. There’s some cooperation among growers.)

Consider the situation in California, where last week the governor imposed mandatory water conservation rules. Residents may not water their lawns more than a couple times a week. Restaurants can no longer serve water unless patrons specifically ask for it. If these measures don’t work, the state will consider punishing people with fines.

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Yet, as the Economist noted last year, agriculture guzzles 80 percent of the water that California pumps, while representing only 2 percent of the state’s economic activity. Cities are responsible for most of the growth in the West, but irrigated farms still account for most of the water used.

California Gov. Jerry Brown (D) has set a goal for homeowners to cut back on water consumption by 20 percent; but even if every suburbanite complies, the reduction would mostly be symbolic.

“There’s a strong push to conserve municipal demand in part to send a message, because that’s where the people are,” said Benson. “But also because that’s viewed as easier to accomplish. Agricultural water conservation is hard to do: in part because it’s expensive, and in part because the law doesn’t incentivize conservation.”

How to make water markets more liquid

It’s a tired refrain, but economic theory says that water, like any scarce resource, should go to the highest bidder.

“The water used to grow California cotton, for example, has much higher value producing silicon chips in San Jose or as drinking water in Los Angeles than it does as irrigation water,” George Mason economists Tyler Cowen and Alex Tabarrok write in their economics textbook.

Already this is happening a bit. Before he became a law professor, Benson was a lawyer in Colorado who specialized in helping cities buy water rights from farmers. “Markets are almost the only thing that gives prior appropriation a chance of making any kind of sense in the 21st century,” he said.

Colorado has a fairly robust system of trading water permits, though the agricultural industry has been reluctant to give up those rights. There’s even a derogatory phrase for those transactions: “Buy and dry.”

“The agricultural industry sees that as eroding their irrigated land base, taking away from their economy and their future,” Benson said.

But farmers who have inherited senior water rights could be sitting on fortunes, especially in drought years when they are the first ones in line for available water. Los Angeles is offering to buy up to $71 million worth of senior water rights, at top dollar. Just last week, rice farmers in the Sacramento Valley announced that they had sold some of their rights for the stunning price of $700 per acre-foot. (An acre-foot is roughly how much water a household uses in a year.)

That price translates into $2,100 per acre of rice that they don’t plant, KQED calculated. The profit on an acre of harvested rice? Maybe half that.

Los Angeles’s expensive water-buying spree illustrates one reason city residents need to conserve water; not because there isn’t enough out there, but because it costs a lot to buy water rights. The farmers who hold those rights tend to sell only as a last resort.

Robert Glennon, the law professor, believes that more farmers would participate in the market if they had more sophisticated ways of trading water rights.

“It’s critical to recognize that the cities and industry don’t need a huge percentage of agricultural water,” Glennon said. “But they do need a low single digit percentage.”

In a report for the Brookings Institution last October, he and co-authors Peter Culp and Gary Libecap suggest a menu of water contracts that farmers and cities could buy and sell.

“The perfect example is a dry year option, whereby a broccoli grower agrees not to grow broccoli in a dry season to let either a thirsty orchard producer or a city use the water,” Glennon said. The broccoli grower gets paid every year, wet or dry, for a steady stream of income. The orchard producer gets insurance that he’ll have enough water for his almond trees, even during a drought.

“It’s a win-win,” Glennon said.

What does this all have to do with rain barrels?

In 2012, California passed a law allowing residents to capture and store the water that runs off their roofs. It used to be illegal in California to harvest the rain, but now cities like Los Angeles are handing out rain barrels for free.

Municipalities like rain barrels because they take pressure off city water systems. People don’t need treated, chlorinated water to quench their thirsty lawns anyway; rain works just fine.

The law used to be the only obstacle; collecting rain was technically illegal in many states because any precipitation was subject to that strict hierarchy of water rights stretching back to the mid-1800s.

But studies estimate that only a fraction of rain actually makes it to a river — less, during a drought. One influential 2007 report from Douglas County, Colo., estimates that only about 3 to 15 percent of rainwater returns to a stream system. Most of it is lost to evaporation, rising into clouds only to fall again later.

On the strength of that report, Colorado began a pilot program in 2009 that allowed people who got their water from wells to apply for rain collection permits. Yesterday, the house voted 45 to 20 to allow all homeowners to store up to 110 gallons of rainwater. HB 1259 now heads to the Senate.

Setting up a rain collection system takes at least a couple hundred dollars, and many households won’t find it cost-effective. But legalizing rain barrels in Colorado sends a twofold message to the state’s farmers.

On one hand it can be interpreted as a gesture of goodwill. Encouraging residents to collect the rain shows that municipalities care about their impact on the state’s water resources.

“One concern from the agricultural community is that farmers are getting a raw deal and that the cities are not doing their part,” Glennon said. “I often hear farmers say, ‘Well, why is a lawn in L.A. better than my alfalfa in Imperial Valley?’ ”

But the bill also signals that as Colorado’s cities grow, and as the political balance shifts, the legal custom of prior appropriation may be slowly renegotiated in favor of the urbanites. At the committee meeting last week, agriculture industry representatives strongly opposed HB 1259.

“It is a small step. And it’ll get bigger, and bigger, and bigger, until you dry up all of agriculture without buying it,” said Jim Yahn, a commercial water manager and farmer.