An intensifying but unseen force is stealing precious water from rivers in the arid West, but it’s hardly a thief in the night.

The midday sun is one of the more aggressive guzzlers of the Colorado River. Between its high Rockies headwaters and its Sonoran Desert delta, 1 to 2 million acre-feet of water evaporates each year in the Colorado River Basin. That’s a big gulp in a watershed where seven thirsty U.S. states and northern Mexico skirmish for their share of an overallocated, shrinking water supply. And the evaporation will only increase as the Southwest grows hotter and drier.

To cut their losses, a growing number of Western water managers want to install solar-paneled canopies over canals and even flotillas of solar panels on reservoirs to turn the sun’s rays into electricity before they hit the water.

“We believe this is the next step in the future of generating renewable energy,” said David DeJong, director of Arizona’s Pima-Maricopa Irrigation Project, which has partnered with the Gila River Indian Community to shade a half mile of the Casa Blanca Canal on tribal land. The pilot project, set for operation by mid-October, could reduce evaporation 50 to 70 percent, he said.

Pilot Projects Leading the Way

Similar projects are advancing in the driest parts of the West. From central Oregon to Utah, Arizona and California’s San Joaquin Valley, solar canal installations are advancing with the help of tens of millions of dollars in state and federal funding. The technology is also catching on globally. India shaded a canal with solar panels a decade ago, and nations including New Zealand and several members of the European Union are reportedly studying the feasibility of developing similar systems.

Advocates of the technology cite the multiple benefits of saving water, generating clean energy and helping keep farmland in production. But there are also great costs and engineering challenges associated with covering waterways with solar panels. The wider the canal, and the more distant it is from transmission lines, the higher the price tag and the narrower the benefit margin. In fact, some water supply experts doubt significant gains are to be had.

“The reason you don’t see [solar-shaded canals] everywhere is that it’s just not usually worth it,” said Jeffrey Kightlinger, a water and energy consultant and the former general manager of the Metropolitan Water District of Southern California.

Others think the pros clearly outweigh the cons, especially when less obvious benefits are considered. For example, shading or covering canals and reservoirs with solar panels slows aquatic photosynthesis and the growth of algae, which can foul water quality. Moreover, having cool water flowing beneath solar panels maintains their efficiency, which wanes in extreme heat.

“Some people say this is a win-win, but I think it’s a win-win-win,” said Brandi McKuin, a project scientist at the University of California, Merced who is collaborating with a firm called Solar AquaGrid and the Turlock Irrigation District in central California on two pilot projects funded by a $20 million California Department of Water Resources grant. The projects are expected to be operational sometime next year.

UC Merced is also working with the San Luis & Delta-Mendota Water Authority to test floating solar rafts on the Delta-Mendota Canal. The Interior Department awarded $15 million for that project in April along with about $4 million for solar canal projects in Oregon and Utah.

McKuin and several colleagues — including UC Merced engineering professor Roger Bales, who has been studying this technology for a decade — touted the cost-effectiveness of solar canals in a 2021 study published in the journal Nature Sustainability. The researchers assessed the potential payoffs from shading all 4,000 miles of California’s canals with photovoltaic panels and found it a cost-effective way to save 200,000 acre-feet of water each year — enough to irrigate nearly 50,000 acres of almond trees. It would also generate 13 gigawatts, enough to power millions of homes in the U.S.  

The 2,600-paneled solar system under construction on the Gila River Indian Community near Phoenix is on track to be the first solar array over a canal in the Western Hemisphere. Funded by a $5.65 million grant from President Biden’s Investing in America Agenda, the project is expected to keep about eight acre-feet of water in the system each year that otherwise would evaporate, DeJong said. That’s enough to irrigate two acres of farmland or support 15 to 20 households a year.

Eventually, solar canopies could be installed on 16 miles of the Casa Blanca Canal, saving 360 acre-feet of water annually from evaporation and generating about 26 megawatts of power. The project, DeJong said, could make the Gila River Indian Irrigation and Drainage District the world’s first modern carbon-neutral irrigation project.

More local efforts are in the works, too. DeJong and his partners have their eyes on a 3.5-mile section of the San Tan Flood Water Canal, also on the Gila tribe’s land, and they’re working with the Army Corps of Engineers on another project that would cover 1,000 feet of exposed aqueduct connecting a delivery pipeline. Those projects, together, could produce almost 10 megawatts of electricity.  

“We get probably 330 days of sun a year,” DeJong said. “What better way to get energy than solar?”

Central Arizona Project Managers Intrigued

While the benefits of solar-shaded canals are clear in principle, some experts aren’t so sure the investment is worth the time and money. The U.S. Bureau of Reclamation arrived at this conclusion in a 2016 analysis of the Central Arizona Project’s (CAP) 336-mile aqueduct.

The agency studied the feasibility of shading 975 feet of the canal with solar panels. They calculated this would cost $4.4 million while saving just 6 acre-feet of water annually—worth $1,020 at the time — and generating 1 megawatt of power. At this rate, covering the full length of the canal would save about 10,000 acre-feet of water from evaporation, enough to irrigate only one or two large farms.

“CAP remains very interested and engaged in the application of solar over canals.”
~Darrin Francom, a manager with the Central Arizona Project

“[T]he disadvantages outweigh the benefits of placing solar over the CAP canal,” the bureau concluded in a summary of the study.

Reclamation nonetheless is funding four pilot solar projects on smaller canals in the West, including the one on the Gila River tribe’s land, “to better understand the impacts,” agency spokesperson Peter Soeth said. 

Solar canopies also remain a possibility for the Central Arizona Project, said Darrin Francom, who helps manage the aqueduct system. Costs of solar installation, he said, will likely decline as the technology becomes more streamlined. He said project managers will be watching the outcomes of the pilot projects in Arizona and California.

“CAP remains very interested and engaged in the application of solar over canals,” Francom said.  

Other water supply experts are barely lukewarm to the idea. The Metropolitan Water District of Southern California has explored shading its Colorado River Aqueduct with solar panels at least three times since the 1990s, according to Kightlinger, who retired from the agency in 2021 and now works as a consultant on water and power.

“It sounds good, but it usually doesn’t pencil out,” he said.

Each time the issue came to the table, he recalled, the conclusion was the same: Returns on investment would not arrive for at least 20 years.

“And that was just on the power side of the equation,” Kightlinger said.

Evaporative losses, he added, were estimated to be so small—just 1 or 2 percent of the water flowing through Metropolitan’s aqueduct – that taking measures to reduce it was hardly worth discussing.

Still, officials with the Imperial Irrigation District – the single largest user of Colorado River water – are intrigued enough by the technology that they’re planning a field trip later this year to Arizona for a closeup look, district spokesperson Robert Schettler said.

The Salt River Project, which provides some 2 million Arizonans with water and power, is also studying the potential benefits of shading part of its canal system with solar awnings, said Patty Garcia-Likens, a project spokesperson.

Solar Panels on Reservoirs, Too

Though the evaporative loss from Arizona’s canals is about 4 percent larger than those in California, Arizona State University’s Sarah Porter, said it still doesn’t justify the cost of installing overhead solar panels. Reservoirs would be a better fit because relatively motionless water evaporates faster than moving water in a canal, said Porter, who heads the university’s Kyl Center for Water Policy.

In fact, floating solar arrays on reservoirs are emerging internationally. The solar flotillas have been built on reservoirs in Ghana, South Korea, China, and India. The United States’ largest is in New Jersey, and a few are planned or already installed in California.

Several California solar flotilla projects are in the hands of the Fresno-based firm White Pine Renewables, which four years ago installed a 15-acre array on a wastewater treatment pond that is tapped to irrigate nearby vineyards in Healdsburg, Sonoma County. The company plans to deploy a similar system in nearby Petaluma.

The company is building a 15- to 20-acre photovoltaic flotilla on a pond in the San Joaquin Valley’s Corcoran Irrigation District. It has already built solar canopies over 150 acres of groundwater recharge basins in the valley to reduce evaporation and power well pumps, White Pine cofounder Evan Riley said.

Energy production, not evaporative savings, is by far the leading incentive for projects of this type. The water saved from panels shading small ponds is worth about a penny for every dollar’s worth of electricity generated, assuming a market rate of $800 for an acre-foot of water, Riley said.

He explained that his firm’s preliminary analysis indicates that deploying solar flotillas and canal canopies in Colorado and Arizona wouldn’t pan out, largely because of competition from conventional utility-scale solar arrays.

“Maybe over time, as the cost of solar and batteries goes down and water prices go up, what we do will start to make sense in the Colorado River Basin,” he said.

Energy production, not evaporative savings, is by far the leading incentive.
~Evan Riley, cofounder of White Pine Renewables

At larger scales, floating solar panels on reservoirs may save significantly more water than those over canals. At Arizona State University, civil engineer Upmanu Lall estimated that covering 10 percent of lakes Mead and Powell could cut evaporative loss by about 10 percent, saving 150,000 acre-feet a year and generating four times the electricity that the reservoirs’ hydropower plants currently produce.

Even farmland could be a candidate for solar canopies. New research from the University of Arizona found that partially shading fields of vegetable crops relieved plants of heat stress, allowing them to produce greater yields with as little as half the water applied to adjacent plots in the full sun. Advocates for solar canals readily point out that costs slide up and down depending on multiple variables.

“Not every canal mile is created equal,” UC Merced’s McKuin said.

Will Canopies Over Major Canals Pencil Out?

Great lengths of the American Southwest’s water conveyance system may be unfit for solar canopies. The greatest engineering limitation relates to the canal width, The wider the span, the more difficult and expensive to shade.

The California Aqueduct, the main artery of the State Water Project, is more than 300 feet wide at its northern end and about 150 feet in width for most of its length. A canal in India that was shaded by solar panels a decade ago, by comparison, is 33 feet wide.

Turlock Irrigation District canals range from 20 to more than 100 feet wide — still a cumbersome gap to bridge, said Josh Weimer, the district’s spokesperson.

“How do you span such a wide canal?” Weimer said.

That’s one of the questions he and his UC Merced partners hope to answer through their state-funded pilot project, which includes a section of canal 110 feet wide and another just 20 feet across.

Narrower spans, McKuin said, could potentially be fitted with the type of solar canopies used over parking lots. Moreover, the availability of such structures, produced on commercial scales, makes their installation on small canals a relatively straightforward and cost-effective task.

Shading wider spans will require suspension bridge engineering involving cables and sturdy mounts on the opposing banks. Support pillars in the middle of a canal are generally not an option, according to experts. That’s because such features could slow the flow of water and, more significantly, pierce the concrete lining of the canal itself and cause leakage, displacing any benefits of muted evaporation.

The new and planned pilot systems should provide better insight into what works and what won’t.

Bales, at UC Merced, is confident there is a pathway forward. He believes negative assessments of technology from a decade and longer ago are now outdated.

“We’ve moved on,” he said. “The attitude of water suppliers before was that we don’t want anything near canals that might impede maintenance and day-to-day operations.

“Now,” he added, “there’s been a change in attitude for any infrastructure investment these days – let’s see how many benefits we can get out of a single project.”

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For inquiries about this story, email Vik Jolly, News & Publications Director at vjolly@watereducation.org

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