Shade balls are small, hollow plastic spheres that float on the surface of open-air reservoirs to block sunlight. By forming a physical barrier between the water and the sun, they accomplish two things simultaneously: they slow the formation of potentially harmful chemical byproducts in treated drinking water, and they reduce evaporation. The idea sounds almost absurdly simple, but it became globally famous in 2015 when the Los Angeles Department of Water and Power (LADWP) released 96 million of them into a reservoir during California’s drought. The story behind why they exist and whether they actually deliver on their promise is more interesting than the viral images suggested.
Why Sunlight Is a Problem for Drinking Water Reservoirs
Most people assume shade balls are primarily about saving water from evaporation. The original motivation was actually about water chemistry. When treated drinking water sits in open-air reservoirs, sunlight can trigger reactions between the disinfectants added to keep the water safe and naturally occurring substances already dissolved in it. One reaction in particular worried Los Angeles water officials: the formation of bromate.
Bromate forms when bromide, a naturally occurring ion found in many water sources, reacts with chlorine in the presence of sunlight. Bromate is a regulated contaminant because of concerns about long-term health effects at elevated levels. The LADWP found that several of its open-air posttreatment reservoirs had elevated bromate concentrations, meaning the water had already been treated and disinfected but was then sitting exposed to the sun before reaching customers. Covering these reservoirs with shade balls prevented bromate from forming by simply blocking the sunlight that drives the reaction.1Opflow. Made in the: Shade
The alternative to shade balls would have been building physical covers over the reservoirs or constructing entirely new enclosed storage facilities. For a city with multiple large open-air reservoirs, that kind of infrastructure project would cost hundreds of millions of dollars and take years. Shade balls offered a way to address a compliance problem quickly and at a fraction of the cost. LADWP covered three reservoirs with millions of four-inch black high-density polyethylene (HDPE) balls, solving the bromate issue almost immediately.1Opflow. Made in the: Shade
How Shade Balls Physically Work
Each shade ball is a hollow sphere, typically about four inches (10 centimeters) in diameter, made of HDPE. They are filled with enough water to sit low in the surface but remain buoyant, so wind does not blow them off the reservoir. When released in large enough numbers, they self-arrange into a tightly packed floating layer that covers the water surface the way ball-pit balls fill a bin. No anchoring or mechanical attachment is needed.
The balls are black for a reason that surprises many people. Black absorbs sunlight rather than allowing it to pass through to the water below. A clear or white ball would let UV radiation penetrate, defeating the purpose. The black color ensures that the vast majority of incoming solar energy is absorbed by the plastic shell rather than transmitted into the water column. Carbon black, the pigment used in the HDPE, also provides UV stabilization, helping the balls resist degradation from years of sun exposure.
The working principle is straightforward: if you physically block sunlight from reaching the water surface, you interrupt the photochemical reactions that create bromate and other disinfection byproducts. You also reduce the energy hitting the water surface that would otherwise drive evaporation. There is no chemical treatment involved. The balls do not release anything into the water or alter its composition. They are essentially a floating shade structure broken into millions of tiny units.
Reducing Evaporation in Arid Climates
While bromate prevention was the original driver in Los Angeles, evaporation control became the more widely discussed benefit, especially during California’s severe drought years. Open reservoirs in hot, arid regions lose enormous volumes of water to evaporation. In a place where every gallon matters, slowing that loss has real value.
Research on floating covers for water bodies has shown that floating elements assembled to provide maximum coverage on the water surface can effectively reduce evaporation.2Agricultural Water Management. Effect of floating balls on evaporation inhibition, surface energy balance and biological water quality parameters at different coverage fractions The mechanism works on two levels. First, the balls block direct solar radiation from heating the water surface, which reduces the thermal energy available to drive evaporation. Second, the physical barrier reduces wind interaction with the water surface. Wind moving across open water accelerates evaporation by carrying away the thin layer of humid air that sits just above the surface. With a layer of balls in the way, that wind-driven moisture removal slows down.
Los Angeles officials estimated that shade balls conserved up to 300 million gallons per year through evaporation suppression.3Nature Sustainability. The water footprint of water conservation using shade balls in California That sounds dramatic, and in absolute terms it is a substantial volume. But it is worth putting into perspective: Los Angeles uses roughly 500 million gallons of water per day. So the annual evaporation savings from shade balls represent about half a day’s citywide consumption. That is still meaningful, particularly for a system that carefully manages every reservoir’s inventory, but it makes clear that shade balls are not a silver bullet for urban water scarcity.
Coverage Fraction Matters
One finding from evaporation research that often gets lost in the headlines is that the effectiveness of floating balls depends heavily on how completely they cover the water surface. Partial coverage produces partial results, and the relationship is not perfectly linear. Studies examining different coverage fractions have found that evaporation reduction, surface energy balance, and even biological water quality parameters all shift as coverage increases.2Agricultural Water Management. Effect of floating balls on evaporation inhibition, surface energy balance and biological water quality parameters at different coverage fractions
In practical terms, this means that deploying shade balls is not a matter of throwing a few thousand onto a reservoir and calling it done. The reservoir needs enough balls to maintain near-complete surface coverage even as water levels fluctuate, wind pushes balls around, and the surface area of the reservoir changes with inflow and outflow. LADWP’s decision to use 96 million balls on the Los Angeles Reservoir was driven by this need for density. Gaps in coverage allow sunlight through, which undermines both the chemical and evaporation benefits.
This also means that shade balls work best on reservoirs with relatively stable surface areas and contained shapes. A long, narrow canal or an irregularly shaped lake with many inlets would be harder to cover effectively because wind and currents would push balls into uneven distributions, leaving open patches.
The Water Footprint Paradox
One of the more thought-provoking critiques of shade balls came from researchers who asked a question that seems obvious in hindsight: how much water does it take to make 96 million plastic balls? The balls are made from HDPE, a petroleum-based plastic. Producing HDPE requires crude oil, natural gas, and electricity. Extracting oil and natural gas is water-intensive, and so is generating the electricity needed for manufacturing.3Nature Sustainability. The water footprint of water conservation using shade balls in California
A study published in Nature Sustainability revisited the sustainability of the Los Angeles shade ball deployment and questioned whether the water saved through evaporation suppression was greater than the water consumed and polluted in the manufacturing process. The researchers pointed out that in a world where water is used in almost every production process, even water conservation can carry a water cost.3Nature Sustainability. The water footprint of water conservation using shade balls in California
This does not necessarily mean shade balls are a net negative. The calculation depends on where the manufacturing water impacts occur versus where the conservation benefits are realized. Water used in petrochemical extraction in one region is not interchangeable with drinking water saved in a Los Angeles reservoir. Drought-stricken Southern California values a gallon of local reservoir water very differently than a gallon of process water used at a refinery in another state. But the analysis is a healthy reminder that the full environmental cost of any technology includes its supply chain, and shade balls are no exception.
Why Not Just Cover the Reservoirs?
If the goal is to keep sunlight off the water, why not build a roof? Many water utilities have done exactly that, constructing covered or underground storage facilities. The main obstacle is cost. A floating cover or fixed roof for a large reservoir can run into the hundreds of millions of dollars, depending on the size and engineering requirements. The Los Angeles Reservoir, for instance, covers about 175 acres. Building a structure over that footprint is a major civil engineering project.
Shade balls cost roughly 36 cents each when LADWP purchased them in bulk. At 96 million balls, the total came to about $34.5 million, a large number but a fraction of what a permanent cover would cost. The tradeoff is durability: shade balls have a useful life of roughly 10 years before UV degradation and physical wear require replacement, whereas a properly built cover can last decades. Over a 25-year horizon, the cost comparison narrows, and the environmental footprint of repeatedly manufacturing and disposing of millions of plastic spheres adds up.
Some utilities have opted for floating modular covers, which are rigid panels that lock together on the water surface. These last longer than individual balls and provide better sealing against both light and evaporation, but they cost more up front and are harder to install and maintain. Shade balls occupy a niche where the problem is urgent, the budget is constrained, and the reservoir geometry allows for effective ball coverage.
Algae and Biological Effects
Blocking sunlight has consequences beyond chemistry and evaporation. Algae need light to grow, and open reservoirs in warm climates are prone to algal blooms that can create taste and odor problems in drinking water, clog filtration systems, and in some cases produce toxins. By cutting off the light supply, shade balls can suppress algal growth without the need for additional chemical treatment like copper sulfate, which is commonly used in reservoirs to control algae.
However, covering a water surface also changes the ecosystem below it. Reduced light and altered temperature profiles affect dissolved oxygen levels, microbial communities, and the overall ecology of the water body. For drinking water reservoirs, which are engineered systems rather than natural habitats, this is generally acceptable. But applying shade balls to a natural lake or pond would raise different ecological questions. The technology is best suited to managed water storage infrastructure, not wild waterways.
Research into floating ball systems has examined biological water quality parameters at various coverage levels, suggesting that the relationship between coverage and biological outcomes is not a simple on-off switch.2Agricultural Water Management. Effect of floating balls on evaporation inhibition, surface energy balance and biological water quality parameters at different coverage fractions Partial coverage may still allow enough light for some algal activity, while full coverage more thoroughly suppresses it. This connects back to the practical point about deployment density: the biological benefits, like the evaporation benefits, scale with how completely the surface is covered.
Do Shade Balls Heat the Water?
This is one of the most common objections raised when people first learn about shade balls. If the balls are black and absorb sunlight, don’t they get hot and then transfer that heat into the water? It seems intuitive that a layer of sun-baked black plastic would warm the reservoir rather than cool it.
The reality is more nuanced. The balls do absorb solar energy and get warm to the touch. But because they are hollow, thin-walled, and sitting on top of the water, most of that absorbed heat radiates and convects back into the air above rather than conducting efficiently into the water below. Meanwhile, the balls are preventing the far more significant thermal input that would occur if sunlight directly penetrated the water surface. Water absorbs solar radiation very efficiently, especially in the infrared wavelengths. Blocking that direct absorption more than compensates for the modest heat conducted through the plastic shell.
Net temperature measurements from deployed shade ball reservoirs have generally shown that covered water stays cooler than uncovered water under the same conditions. The effect is not enormous, but it runs in the right direction. Cooler water also has lower evaporation rates, creating a feedback loop that reinforces the conservation benefit.
What Happens When Shade Balls Reach End of Life
HDPE is one of the more recyclable plastics, commonly labeled as recycling number 2. In principle, shade balls that have reached the end of their useful life can be collected and recycled into other HDPE products. Whether this actually happens at scale depends on the economics of recycling in the region and whether the carbon black pigment creates complications for recyclers.
There is also the question of microplastic generation. Plastic floating in sunlight for a decade, even UV-stabilized plastic, undergoes some degree of surface degradation. Whether shade balls shed meaningful quantities of microplastic particles into drinking water reservoirs has not been extensively studied in the public literature, and it is a question that deserves more attention as deployments age. HDPE is considered one of the more chemically stable plastics, and it does not contain the plasticizers associated with PVC or the bisphenol compounds found in polycarbonate. But “more stable” is a relative term when you are talking about millions of objects floating in a drinking water supply for years.
For a technology that was adopted quickly to solve an urgent compliance and conservation problem, these longer-term questions about material degradation and disposal were secondary concerns at the time. As shade ball deployments mature and approach their replacement cycles, the waste management and microplastic dimensions will likely receive more scrutiny.
Where Else Shade Balls Have Been Considered
After the Los Angeles deployment went viral, interest in shade balls spread to other contexts. Mining operations have explored floating covers for tailings ponds, where sunlight can drive unwanted chemical reactions and evaporation exposes concentrated pollutants. Agricultural water storage in arid regions, particularly in the Middle East, Australia, and parts of Africa, has also been a target for floating evaporation barriers, though modular floating panels sometimes compete with balls in those markets.
The concept has also been studied for smaller-scale applications like irrigation ponds and industrial cooling water basins. In each case, the same basic physics applies: block sunlight, reduce evaporation, suppress photochemical reactions. But the economics shift depending on the value of the water being protected, the climate, and whether cheaper alternatives like shade cloth or floating covers make more sense for the specific geometry involved.
One context where shade balls are clearly not the answer is flowing water. Rivers, streams, and canals with significant current would simply carry the balls downstream. They work because reservoirs are still bodies of water with defined boundaries that keep the balls contained. Even in reservoirs, strong sustained winds can push balls to one end, temporarily exposing parts of the surface. Some installations have used floating booms or barriers to keep balls distributed more evenly, adding another layer of infrastructure cost.