At some point in your high school education, perhaps in a chemistry or physics class, you were probably told, “evaporation is a cooling process.” It is. And that is good. Evaporation of your perspiration cools your body, enabling you to maintain your temperature in just the right range. But evaporation is also a mass loss process, and just as evaporation of your perspiration without replenishment will eventually result in your demise, so too can the water lost by evaporation from a lake result in the effective elimination of a lake or reservoir, a thing of some concern in locales where a lake is used to provide water for human use. Evaporation is invisible to us, the molecules of water on a lake surface transformed from the liquid to the vapor phase, silently drifting into the atmosphere where they may fall again as rain, typically very far away, and often where the droplets do little to help replenish lakes and reservoirs, as when rain falls over the ocean.
Evaporation is a big deal. Even in wet climates, such as South Carolina where I live, a lake can lose over 50 inches of water to evaporation in a year. Near my home there are three interconnected reservoirs, Lakes Hartwell, Russell, and Thurmond, each formed by a dam on the Savannah River, the river that forms the border between South Carolina and Georgia for most of its length. Lake Hartwell experiences an average annual evaporation rate of about 52 inches per year, which is likely close to the average for the other two reservoirs. If you take this evaporation rate and multiply it by the surface area of just these three reservoirs, you get 661,400 acre-feet of evaporative loss per year. That volume is equivalent to slightly less than 29 billion cubic feet, or 216 billion gallons. That is a lot of water. It is equal to 11 percent of the total volume of these reservoirs when they are full. At Clyo, Georgia, near where the Savannah River enters the Atlantic Ocean, the total annual outflow is 8.2 million acre-feet/year, meaning the loss due to evaporation, just from these three reservoirs, is equivalent to 8 percent of the outflow of the entire Savannah River.
The above example pertains to the wet southeastern United States. If we now look at the much drier western part of the United States, where relative humidities are much lower and evaporation rates much higher, the impact of evaporation is even more dramatic. Indeed, in the 17 western states, annual evaporation from impoundments is 15.6 million acre-feet, an amount comparable to all of the water stored in all of the reservoirs of California (in 1965). Lake Mead by itself, which provides water for Las Vegas and several other cities, and where the annual rainfall is a paltry four inches, experiences an evaporation rate estimated at 76.0 inches per year. Multiplying this rate by the surface area of Lake Mead gives a volume of 1.0 million acre-feet/year lost to evaporation. By way of comparison, the entire state of Nevada is permitted to remove 300,000 acre-feet per year from the Colorado River. In other words, astoundingly, the amount of water lost to evaporation from Lake Mead is over three times larger than what Nevada is allowed to remove from the Colorado River.
Given the large amounts of water lost to evaporation, it is not surprising that valiant attempts have been made to reduce or eliminate evaporation from lakes and reservoirs, particularly in the West. In drought-riven years, water managers in dry climates have resorted to desperate measures. In one well-publicized situation in 2015, the Los Angeles Department of Water and Power, in response to a drought, dropped 96 million floating black balls called shade balls onto the surface of a reservoir in an effort to reduce evaporative loss. The balls covered the water surface leaving little space in between, serving to protect the water from solar heating and reducing the flow of water vapor from the water surface, thereby reducing evaporative loss. The program was implemented on several reservoirs in the Los Angeles water system, and images of the 4-inch diameter plastic balls covering these reservoir surfaces became a brief media sensation. Though the shade balls on some of these smaller Los Angeles reservoirs have since been replaced by floating covers, the large 175-acre Los Angeles Reservoir continues to be protected by shade balls, where they have reduced evaporation, by 85 percent to 90 percent, an effective solution if not a cheap one.
Another way to reduce evaporation is to simply prevent an air/water interface from forming in the first place. Although reservoirs are a staple of water-resources management, there are other ways to store water, namely by taking available water and pumping it into the ground. There, safely contained in an aquifer, evaporation cannot steal water away. Indeed, by pumping water down through porous rock beds, and then pumping it back up some distance away, water is not just protected from evaporation, it is made cleaner, the porous rock serving as a filter, removing pollutants and microorganisms.
But the existing water system in the United States already relies heavily on reservoirs. Lake Powell and Lake Mead and the thousands of other reservoirs used throughout the nation represent an enormous taxpayer-financed investment in large infrastructure, and the water providers who use them are not going to abandon them any time soon. Furthermore, the population of the United States seems to continue its ever-southward migration with well-watered cities such as Detroit, Chicago, and Buffalo losing people each year to dry southern and western locales such as Dallas, Phoenix, Las Vegas, and others. In the Great Lakes, the United States is endowed with one of the largest lake systems in the world, a truly massive volume of fresh water. Yet, as a population, we are fleeing that wondrous supply of water at breakneck speed. So, like it or not, the reservoirs we’ve built in the South and West are not only important, they are becoming more important with each passing year.
Periodically the question arises: what can be done to eliminate or reduce the large evaporative losses that reservoirs experience? This question is ignored during years of water plenitude, but returns during drought years. It certainly gains more urgency as droughts become longer, more frequent, and more severe as time passes.
Imagine you are the head of a local water provider and you are standing on the shore of your city’s reservoir. It is July, it is hot, and the reservoir level is low. The water surface lays beneath the hot sky, evaporation occurring unimpeded, the reservoir measurably dropping each day. “Isn’t there a way to turn off this evaporation?” is surely the question that runs through the minds of those responsible for providing water to cities and municipalities during droughts. Short of spending millions of dollars to manufacture black shade balls, is there some other way to prevent all of this water from leaking up into the sky? Is there some kind of skin or film that could be spread across the immense area of a reservoir to hamper or stop evaporation and protect the water, at least until the rains come? Though riddled with caveats, the answer to that question is actually yes.
In the last chapter we examined surfactant monolayers, these single-molecule-thick films that water is never free of. Though just one molecule thick, they have a powerful impact on surface tension and can imbue the water surface with an elasticity that is otherwise absent and that profoundly affects how water absorbs heat and dissolved gases. But under the right conditions, certain classes of surfactants can do even more—they can also effectively stop evaporation. This is especially true of monolayers referred to as solid-phase surfactant monolayers.
Surfactants have a molecular structure that consists of long hydrocarbon chains. For many surfactants these long chains show no real structure on the water surface—they are more or less randomly oriented with respect to each other. However, solid-phase surfactant molecules orient themselves vertically, their chains perpendicular to the water surface. This is because one end of the hydrocarbon chain is hydrophilic and therefore wants to attach itself to the water, while the other end is hydrophobic and seeks to be as far from the water surface as possible. By pointing straight upward, both ends of the molecule get their way. In this configuration, the molecules are aligned parallel to each other like blades of grass on a lawn, each molecule packed closely to its neighbor. Viewed from above, the molecules are located in a grid pattern, forming a two-dimensional crystal. In addition to causing the film to attain a lateral rigidity, much like its three-dimensional cousin, this molecular structure provides virtually no space between the surfactant molecules. Water at the surface is forced to travel along the narrow passages between these long molecules, severely limiting evaporation. Hence, a solid-phase surfactant film, once formed, effectively shuts evaporation down.
The ability of solid-phase monolayers to reduce the rate of evaporation has been known for some time. However, a period of relatively intense research occurred in the 1950s and 1960s. The suppression of evaporation by monolayers was vigorously investigated by groups in the United States and Australia; laboratory studies and field campaigns were conducted to determine if and how suppression of evaporation by surfactant monolayers could be successfully implemented on lakes and reservoirs. Interestingly, or you might even say disturbingly, many such studies were conducted on reservoirs that were actively being used as the water supply for cities, making guinea pigs of the residents.
Let’s look at what was done, and learned, here in the United States. Many of the evaporation-suppression studies were conducted by the Bureau of Reclamation. Especially notable was the deposition of monolayers on Lake Hefner, a 2500-acre lake in Oklahoma City, which is part of that city’s water supply. Experiments were conducted on the lake for 86 days using a surfactant mixture composed primarily of hexadecanol with smaller amounts of tetradecanol and octadecanol. These surfactants are relatively harmless and are commonly used in consumer products such as shampoos, lotions, and cosmetics. Still, spreading these chemicals over a reservoir every day for 86 days, at times covering as much as 89 percent of the surface seems a bit risky. Nevertheless, this was done, along with similar studies at Ralston Creek Reservoir, a 150-acre lake that is part of the Denver water system, on a portion of Lake Mead, and on other reservoirs. The material deposited was applied in a variety of ways, often blown on as flaked material from a boat or sprayed as a slurry of the surfactant. A large amount was applied, considering the fact that the desired result was a layer one molecule thick. In the Lake Hefner study, 0.3 pounds of material were deposited per acre per day. This translates to an awful lot of material. If we use the 89-percent-coverage value cited above, this means deposition as high as 668 pounds per day. The average coverage on Lake Hefner was 10 percent, but still this corresponds to deposition of 75 pounds of material per day, adding up to 6450 pounds, slightly more than three tons for the 86-day study. It is easy to see that this practice would, if widely implemented on lakes throughout the west, lead to a huge consumption of materials.
In the laboratory, spreading a solid phase monolayer is relatively straightforward, and these monolayers do reduce evaporation dramatically. But the water surface one obtains in a laboratory tank is very different from that on a lake or reservoir. On such a large area, the monolayer is spread outward from many flakes or pieces of bulk surfactant (depending on how it is deposited). Where these individual monolayers meet, gaps inevitably form where evaporation is still able to occur. Moreover, in the outdoors, many factors cause the dissipation of surfactant films once formed. Ultraviolet degradation, bacterial degradation, and especially wind and waves, all serve to reduce the coverage. To effectively use surfactants to control evaporation would require continuous application potentially via boat or other movable platforms, or perhaps via multiple installed and partially automated devices. All of this equipment would require maintenance, upkeep, and resupply, again requiring boats. The whole scheme turns out to be daunting logistically and, inevitably, costly. Thus, not surprisingly, the use of surfactant monolayers to control evaporation has not been embraced and has not enjoyed significant implementation beyond field studies.
Today the use of surfactants in evaporation reduction is limited primarily to what are called liquid pool covers, surfactant solutions spread over a pool to reduce evaporation, thereby keeping the water warm. Though not employed in any significant way, the fact that monolayers were considered a potential method for reducing evaporation on reservoirs despite their many drawbacks reveals how truly constrained our freshwater resources have become.
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But, do we really want to suppress evaporation in the first place? We began this chapter by noting that evaporation is a cooling process. So, without it, the temperature of lakes and reservoirs would rise. If we were able to suppress evaporation from reservoirs on a large scale, what would be the impact of the increased reservoir temperatures, particularly for very large reservoirs that host complex ecosystems? Perhaps the failure of surfactant monolayers to work effectively is the best thing that could have happened to the ecosystems of the lakes and reservoirs humans use.
And perhaps it is wise to step back even further and ask whether the formation of reservoirs was a wise thing to do in the first place. This is certainly an academic exercise given the prevalence and continuing construction of reservoirs of all sizes throughout the world. But still, it is a question worth asking. Evaporation shows that there is a bit of irony associated with reservoirs—we build them to hold onto the water that would otherwise flow via rivers out to the ocean. But, in creating these reservoirs we have also created enormous water surfaces, vast air/water interfaces that didn’t exist before and where prodigious quantities of water are lost via evaporation, evaporation that didn’t occur before they were built. Of course there is a net gain. The amount of water humans are able to withdraw and use from these reservoirs is much larger than would otherwise be available, evaporation notwithstanding. These reservoirs wouldn’t have been built in the first place is this wasn’t the case. But still, it forces us to ponder the results of all that extra evaporation into the atmosphere. What are the long-term consequences of this additional evaporation, and how can we even quantify it? If one searches the journal literature about reservoirs and evaporation, a whole host of articles focusing on the impact of climate change on evaporation from reservoirs are found. But there is a lack of research on the converse, that is, on how evaporation from reservoirs impacts and has impacted the climate. In holding onto all of this freshwater, we are also forcing a very large amount of it up into the atmosphere. One wonders what the consequences are and will be.