Around the world, there are 16.7 million reservoirs with
a surface area greater than 100 square meters.
Many of these are located in arid or semi-arid areas, and that means evaporation.
How much water evaporates away depends on the location, geography
as well as the local conditions, but it can be quite significant.
Considering how much of the United States is water-stressed,
we cannot really afford to let 20, 30, or even 40% of this water vanish before our eyes. Right?
So in this video we are going to talk about reservoir evaporation suppression
techniques - and also touch briefly on the controversy of shade balls.
## Beginnings
So how big is this problem? How much water are we actually losing anyway?
Part of the reason why evaporation effects have been largely ignored
in water management has partly to do with how difficult it is to measure and calculate. But
once you are capable of measuring this, we can see that these losses are significant.
In the state of Queensland in Australia, the estimate is that 1 billion cubic meters - 40% of
the total capacity of 2.5 billion - is evaporated away. While in Spain, that number is about 10%.
And over in the United States, a study of 200 reservoirs in Texas
showed an annual loss of about 20% of the reservoir's active storage capacity.
Studies of evaporative losses in Lake Superior and Lake Tahoe
average some 40-60% of the total water budget.
In the case of Lake Tahoe,
more water is evaporated away into the air than is actually consumed.
All of this water evaporating away has significant economic effects,
especially since much of the reservoir water is used for irrigation and agriculture.
Going back to the Queensland example, the amount of water that packs up and
goes into the air is estimated to be enough to irrigate 125,000
hectares each year - worth almost $375 million of agriculture.
As various areas in the world find themselves increasingly hit by long,
brutal droughts, evaporation suppression technologies become increasingly feasible.
## Factors of Evaporation
The physics of water evaporation are deceptively simple.
The rate at which water evaporates off a reservoir is determined by the magnitude
of the differences in the vapor pressure between the water on that reservoir's surface and the air.
The bigger the difference, the faster evaporation happens. So the fastest
evaporation rates occur when the reservoir water is warm and the air is cold, dry, and windy.
But translating this physics principle into real life is tricky because the
real world is complicated and full of feedback mechanisms.
For instance, the process of water evaporating consumes energy,
which decreases the water body's temperature - negatively affecting the evaporation rate.
And the reservoir's unique local properties have a significant role to play as well.
These include the reservoir's surface area and its volume. A deep body of water with
a small surface area evaporates more slowly than a shallow one with a wide surface area.
The local landscape and climate has a big part to play as well. Unhindered high winds
accelerate evaporation rates, but many reservoirs exist in areas of complex
geographies - like valleys and gorges - and vegetations - like trees and such.
Another important factor takes into account when the reservoir is holding water. For
Australia - which is on the southern hemisphere - the months from December
to February are the summer months, and that is when the reservoirs dry up the fastest.
## Penman Monteith
The complexity of this stuff makes it hard to measure and model. The most
commonly cited baseline model is the Penman-Monteith model.
This is a formula standardized by the Food and Agriculture
Organization of the United Nations - originally designed to calculate
how much water evaporates out of the soil or the cavity of a plant.
The Penman-Montieth is not an easy equation to understand,
but it takes into account wind speed, air temperature, water temperature, solar radiation,
and reservoir depth to give us an approximation of evaporation losses between two points in time.
The difficulties of collecting data for this equation and calculating its
results has resulted in several studies trying to create "approximate" equations
that take into account fewer of these variables. To varying effects more or less.
## Chemical Monolayers
The first modern "evaporation suppression methods"
begin with what are called "monolayer molecular films".
These are a chemical form of evaporation suppression in which we create a single,
insoluble chemical layer. There are two ways it keeps water from evaporating away:
First, it sits on top of the water surface and physically
prevents the water molecules from escaping into the air.
Second, the oil reduces the surface tension of the water,
which lowers the available surface area of water available to evaporate away.
Chemicals like these have been researched since the 1920s. But it got a lot of publicity in the
1950s when the Australian government tried it - reporting a 30% reduction
in evaporation rates. This caught a lot of people's attention around the world.
Generally, the chemicals chosen are fatty alcohols - long-chains
of carbons derived from natural fats and oils.
One widely used commercial solution is called "WaterSavr". This is available as
a white powder blend. You put it on the water and it creates an invisible film
that spreads itself out on top of a water surface via a special chemical reaction.
WaterSavr is fully biodegradable within 2 days. So people often
buy little machines that float around, automatically spreading it on the water
whenever needed. They have been deployed in Australia, Singapore, and the United States.
When used in ideal conditions, it regularly cuts evaporation rates by an estimated 30%.
There are no recorded negative effects on water safety, quality or the environment.
That is great, but real world gains are highly dependent on the weather conditions. High winds,
bacteria, intense sunlight, and dust particles can weaken their
effectiveness. We need more than just a chemical layer.
## Throwing Shades
Shades seem to be the most obvious solution. If we feel hot under the sun,
then we go under the shade. Do the same for our water.
Studies have shown that throwing shade can be very effective. One
trial done in 2005 by the Queensland Department of Natural Resources and
Mines used cables to suspend a black single-filament cloth over the reservoir.
The framework is free standing, with the cables anchored to the wall. And
it looks like a huge spiderweb with cables crossing over at multiple points - meaning
that they are best installed when the reservoir is empty.
This shade cover seems to work quite well. The trial evaluation found that the shade cloth
reduced evaporation rates by 60-80%. Other floating covers act basically like a lid,
and so can do even better, up to 90% reduction especially during the summer months.
But being a physical item, there are maintenance and damage concerns to consider. The frame
is estimated to last about 30 years, with the cloth itself estimated to go about half of that.
When it rains, the water gathers on top of the shade covers, causing it
to sag and maybe break while attracting animals like ducks. Damage from weather,
wind, and animals - all this adds additional cost.
The maintenance and economic concerns of physically applying shade covers
to reservoirs make them difficult long-term or economically prudent
propositions. Particularly for very large, heavily used reservoirs.
## Floating Shades
So what about having the shades float right on top of the water?
This is the concept of floating shade covers.
There are two types of that floating shade cover, a continuous one and a modular one. The
continuous one is just one piece of plastic, wax, or foam - usually polyethylene plastic.
The modular one involves individual items that freely float around. It can be shaped like a
basin, hexagon or ball, and experiments have shown that they can cut evaporation rates by some 80%.
There are also biological floating shade covers, aquatic plants like duckweed and
water lily. A study in Thailand found that using duckweed can reduce evaporation rates by 10%.
Though it should be noted that some water plants actually accelerate water
evaporation - their big leaves suck it up and it evaporates out of their pores.
And randomly introducing aquatic plants to warm water reservoirs has water quality effects.
One notable type of floating shade cover is the floating photovoltaic panel. There
is a double benefit here of reducing evaporation and generating electricity,
with the downside of coverage and cost.
## The Shade Ball Controversy
Before we move on, we should discuss the California shade ball controversy.
A floating shade ball is a small sphere about 10 centimeters wide and made of
high density polyethylene. Their effectiveness in significantly cutting evaporation rates has
held up in various trials in Spain, China, Egypt, Brazil, and Australia.
Based on these results, in 2008 the California government proposed the idea of putting 400,000
shade balls in the Ivanhoe reservoir in 2008.
A few years later in 2014-15, they put nearly 100 million shade balls in their
largest reservoir. This was largely for three reasons. One, they help to prevent evaporation.
And two, to prevent the formation of a hazardous chemical called bromate,
which forms when the chlorine in the water is exposed to sunlight.
And three, they help keep birds away. Apparently the original idea came from a biologist who saw
airports using "bird balls" to keep birds from gathering in bodies of water at the airport.
California officials mentioned that the shade
balls saved about 1.1 billion liters of water a year from evaporating away.
Which is good. Californians need every drop for those big lawns of theirs.
The controversy - if I suppose one can call it that - was whether or not making the shade
balls used up more water than they saved from evaporating in the reservoir. As I mentioned,
the balls are made of polyethylene - which requires natural gas, oil and energy to make.
Several calculations found that the balls use more water than they save unless they
are used for about 3 years or more. But Los Angeles removed many of these shade
balls in 2017 and replaced them with shade covers - floating or otherwise.
So during their 1.5 year deployment from August 2015 to March 2017, they might have
saved 1.7 billion liters of water, but used up 2.9 billion in their manufacture. A net loss.
I wouldn't quite say that was the fairest assessment. These shade
balls are rated for about 10 years. The main issue seems to be that many of the
shade balls were removed from some reservoirs after the drought ended.
There are some other concerns with the balls. Most people generally don't like how they look. And I
do admit that they are kind of ugly floating there on the water doing plastic ball things.
Furthermore, people worry about the potential health effects associated with drinking water
that has had plastic sitting in it for a long time. For instance, the concern that these shade
balls can add more microplastics or disruptive chemicals like bisphenol BPA into the water.
That all being said, these are effective. And as far as I can tell, shade balls are still present
in a few southern California reservoirs that are too large to have shade covers.
## Windbreaks
Since wind is so important to the evaporation process,
adding wind breaks can - in theory - help with evaporation.
Several studies in Australia found that if you plant a bunch of trees near a dam or reservoir,
you can reduce wind speeds at the water level by 80% or so - depending on the height of the tree.
This correlates to an evaporation reduction of about 4-8%, which does
not sound like a whole lot considering the other measures we have here.
And since trees are big and all, this can only be economically feasible for smaller reservoirs.
## Bubbles
Bubbles. This one might seem a little random but bear with me here.
The idea behind bubble injection is that during the summer,
the water near the surface gets warmer.
A natural barrier called a thermocline forms
between this layer of warmer water and the colder water down below.
This natural phenomena is referred to as stratification and it prevents the
water from mixing together. So what we are trying to do here with the
bubble plumes is to break up this stratification and mix the water.
This idea was first suggested back in 1962 and was first deployed at Lake Wohlford in California.
The trial result found an estimated improvement of about 6% annual water
evaporation reduction - a 15% reduction during the summer
months and an actual 9% increase in evaporation during the winter.
This works best with deep reservoirs over 18 meters deep where you are more likely to
get the cold water layers. And unfortunately, you do tend to mess with local ecosystems with
this method, destratifying water layers can affect fish species.
## Managed Water Recharge
Here is a wild one. If you are worried about your reservoirs drying up,
then why not move them underground?
This is where you purposefully inject water back into the ground in order to recharge
an aquifer. The methods used depend on the local climate and the geology.
The most common mechanism for this is to use an injection well, injects stuff down a well.
Such equipment is usually used for enhanced oil recovery though rather than aquifer recharge.
The Gulf Countries have been particularly interested in this - which makes sense
considering their climate. It helps them store water especially for agricultural use.
Case studies in Abu Dhabi have found that some 88-100% of the
water can be recovered without a large detriment to water quality.
## Conclusion
Evaporation suppression techniques need to be implemented as part of a greater strategy
that also includes water conservation, recycling, and more. That much is true.
That being said, the continued challenge of estimating evaporation losses
significantly influences policies to address them.
The Venture Capitalist John Doerr wrote a book called "Measure What
Matters". You can't economically address evaporation until you can measure it.
Recently, new innovations in remote sensing and machine
learning have shown high potential to address these with greater accuracy.
I am particularly impressed by a 2019 paper by two researchers at Texas A&M University,
who used Landsat data to estimate evaporation losses of 721 reservoirs
down to the local level - making it extremely practical for management.
To conclude this rather dry video, I wanted to leave us with a joke about
water evaporation. But it just seemed to vanish into thin air.