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Rainwater harvesting, which dates back to the earliest days of agriculture and is still practiced by some Native American tribes, involves capturing and using rainwater where it falls. And it's easy to do, requiring nothing more than a barrel or cistern. As the city of Tucson's water harvesting Web site notes, "All you need for a water harvesting system is rain, and a place to put it."

The most common capture method is to place a rain barrel outfitted with a debris screen below a drain spout to collect water running off the roof, although it can also entail creating depressions such as swales and berms to funnel rainwater into landscaped areas.

It can also involve harvesting wastewater, also known as graywater, from household drains, including showers, bathtubs, bathroom sinks and washing machines. A typical system directs graywater to the yard through modified pipes.

Brad Lancaster, a Tucson, Ariz.-based water expert and consultant who has written two books on rainwater harvesting, said the practice holds tremendous potential for helping communities stretch limited water supplies. Between 30 and 50 percent of potable water consumed by the average single-family home is used to water landscaping, but almost all of that demand can be met through rainwater harvesting and graywater capture, he said. And every drop culled from the sky means less water siphoned from reservoir or groundwater supplies, which in many drought-strapped areas are overtapped, Lancaster added.

"It's ridiculous that we're using drinking water to water the dirt," Lancaster said.

Harvesting vs. water rights

But while Arizona, Hawaii, Texas and a half-dozen other states and communities have embraced the concept of rainwater harvesting, either through financial incentives or ordinances, the practice is outlawed in other areas.

The state of Colorado has become ground zero in the debate over rainwater harvesting. There, water officials and the state water court have determined that the practice is illegal: Currently, capturing rainwater is punishable by a $500 fine.

As in many other Western states, Colorado's water law divvies up surface water through a "first in time, first in right" allocation system, in which users with the oldest water rights get first dibs. Prevailing logic in Colorado holds that all precipitation eventually contributes to stream flows, and capturing water before it enters waterways is potentially "stealing" water from downstream users who hold legal water rights. Even if the practice were not considered illegal, a major disincentive to rainwater harvesting in Colorado is a requirement that 100 percent of any rainwater captured be replaced.

"It was meant for use as a vested water right," said Marta Ahrens, public information officer for the Colorado Division of Water Resources. Throughout much of Colorado, surface water is either fully or over-appropriated, and demand is outstripping supplies, she said.

But water experts and some elected officials are taking a closer look at state water law to determine if there is a way to allow rainwater harvesting while preserving existing water rights. They cite studies that suggest only a small percentage -- about 3 percent, according to an analysis done by Douglas County -- of precipitation makes it into streams, which are the source of water rights divvied up under state water law. That means not all of the water that falls from the sky is already claimed, they point out.

"There's a question of wait, are you really causing any decrease? Because in some cases, you're really intercepting water that may not have gotten to the stream anyway," said Doug Kemper, executive director of the Colorado Water Congress, a group of local and state officials and other water interests that works on solutions to the state's water challenges. The group has not yet taken an official position on rainwater harvesting but is open to studying the idea, he added.

In the last legislative session, which ended in May, state lawmakers considered a bill that would have set up a pilot project to test and study the effects of rainwater harvesting. While that effort ultimately failed, momentum in the state appears to be growing, and some form of rainwater harvesting legislation is likely to be introduced again next year, Kemper said.

Part of what is driving the new interest in legalizing rainwater harvesting in the state lies in its potential for reducing strain on conventional water sources. A study last year commissioned by Gov. Bill Ritter (D) found that rainwater harvesting has the potential to reduce outdoor water demand by between 65 and 88 percent. When used by well owners, it can also decrease groundwater withdrawals, helping to slow the drawdown of aquifers, which has become a problem in many areas.

But the report also recognized the potential harm to other water users. "Intercepting precipitation that would have otherwise migrated [to] groundwater or surface water might interfere with the full allocation of existing water rights," the report said.

Mandates and incentives for rainwater harvesting

In neighboring New Mexico, state water law is also based on a "first in time, first in right" approach to water appropriation, yet three communities there -- Santa Fe County, Bernalillo County and the city of Albuquerque -- actually require rainwater harvesting. Their ordinances mandate that new homes capture rainwater from the roof and use that water for landscaping needs instead of potable water. Residences that are 2,500 square feet or less are required to use rain barrels or catchment basins; homes larger than 2,500 square feet must install underground cisterns connected to drip irrigation systems.

Laurie Trevizo, a water resource specialist with the Santa Fe County Land Use Department, said the high desert county treats rainwater and surface water differently. "We consider our roofs an artificial surface, whereas water that goes downstream for water rights is on a natural surface," she said. While the county has not studied whether capturing runoff affects stream flows, indications are that it has not impaired water rights, she added. "Using rainwater doesn't seem to have any negative effect on the downstream users," she said.

In Arizona, Tucson's City Council is developing an ordinance that would mandate rainwater harvesting on commercial properties. Fernando Molina, water conservation manager for the city, said it would require that 75 percent of landscaping water demand be met with captured rainwater. The rest could be met with potable water.

Other governmental entities are using economic incentives rather than legal mandates to encourage rainwater harvesting. The state of Arizona, for example, offers taxpayers who install a rainwater or graywater system a one-time tax credit of 25 percent of the cost of the system, up to $1,000. And builders are eligible for an income tax credit of up to $200 per residence constructed with a water conservation system installed.

Similarly, the state of Texas exempts water harvesting systems from sales tax and property tax. The city of Austin goes one step further, offering rain barrels to homeowners at a discount and rebates for cistern systems.

Meanwhile, the federal government has begun providing incentives for rainwater harvesting. For instance, the U.S. Department of Agriculture's Forest Stewardship Incentive Program will match a landowner's expenses for erosion control and water-harvesting earthworks on private land of 1,000 acres or less, up to $10,000 per landowner per fiscal year. And through Dec. 31 of this year, the federal government is offering a tax credit of up to $500 for homeowners who install metal roofs.

But rainwater harvesting is only useful if Mother Nature cooperates, Ahrens pointed out. In the Denver metro area, which has seen a mere 3.5 inches of rain so far this year, "we're experiencing one of our driest years ever," she said.

"Even if we were to allow rainwater harvesting, we don't even have the rain to catch," Ahrens added.

April Reese writes from Santa Fe, N.M.

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