What Happens When a Reservoir Goes Dry?

What Happens When a Reservoir Goes Dry?
Reservoirs are a solution to the tremendous variability in natural water supply, but what happens when they stop filling up?
People use water at more or less a constant rate and yet, mother nature supplies it in unpredictable sloshes of rain or snow that can change with the seasons and often have considerable dry periods between them. If the sloshes get too far apart, we call it a drought. And at least one study has estimated that the past two decades have been the driest period in more than a thousand years for the southwestern United States, leading to a so-called “mega-drought.”

In June of 2022, the level in Lake Mead, the largest water reservoir in the United States
formed by the Hoover Dam, reached yet another all-time low of 175 feet or 53 meters below full,
a level that hasn’t been seen since the lake was first filled in the 1930s. Rusted debris, sunken
boats, and even human remains have surfaced from beneath the receding water level. And Lake Mead
doesn’t stand alone. In fact, it’s just a drop in the bucket. Many of the largest water reservoirs
in the western United States are at critically low storage with the summer of 2022 only just getting
started. Lake Powell upstream of Lake Mead on the Colorado River is at its lowest level on record.
Lake Oroville (of the enormous spillway failure fame) and Lake Shasta, two of California’s
largest reservoirs, are at critical levels. The combined reservoirs in Utah are below 50%
full. Even many of the westernmost reservoirs here in Texas are very low going into summer.
People use water at more or less a constant rate and yet, mother nature supplies it in
unpredictable sloshes of rain or snow that can change with the seasons and often have
considerable dry periods between them. If the sloshes get too far apart, we call it a drought.
And at least one study has estimated that the past two decades have been the driest period in more
than a thousand years for the southwestern United States, leading to a so-called “mega-drought.”
Dams and reservoirs are one solution to this tremendous variability in natural water supply.
But what happens when they stop filling up or (in the case of one lake in Oklahoma), what happens
when they never fill up in the first place? I’m Grady, and this is Practical Engineering.
On today’s episode we’re talking about water availability and water supply storage reservoirs.
This video is sponsored by Brilliant,
the best way to learn math and science through problem solving. More on them later.
The absolute necessity of water demands that city planners always assume the worst case scenario.
If you have a dry year (or even a dry day), you can’t just hunker down until the rainy
weather comes back. So the biggest question when developing a new supply of water is the firm
yield. That’s the maximum amount of water the source will supply
during the worst possible drought. Here’s an example to make this clearer:
Imagine you’re the director of public works for a new town. To keep your residents hydrated and
clean, you build a pumping station on a nearby river to collect that water and send it to a
treatment plant where it can be purified and distributed. This river doesn’t flow at a
constant rate. There’s lots of flow during the spring as mountain snowpack melts and runs off,
but the flow declines over the course of the summer once that snow has melted
and rain showers are more spread out. In really dry years, when the snowpack is thin,
the flow in the river nearly dries up completely. In other words,
the river has no firm yield. It’s not a dependable supply of water in any volume. Of course, there is
water to be used most of the time, but most of the time isn’t enough for this basic human need.
So what do you do? One option is to store some of that excess water so that it can keep the
pumps running and the taps flowing during the dry times. But, the amount of storage matters.
A clearwell at a water treatment plant or an elevated water tower usually holds roughly
one day’s worth of supply. Those types of tanks are meant to smooth out variability
in demands over the course of a day (and I have a video on that topic), but they can’t do much
for the reliability of a water source. If the river dries up for more than one day at a time,
a water tower won’t do much good. For that, you need to increase your storage capacity by an
order of magnitude (or two). That’s why we build dams to create reservoirs that, in some cases,
hold trillions of gallons or tens of trillions of liters at a time, incredible (almost unimaginable)
volumes. You could never build a tank to hold so much liquid, but creating an impoundment
across a river valley allows the water to fill the landscape like a bathtub. Dams take
advantage of mother nature’s topography to form simple yet monumental water storage facilities.
Let’s put a small reservoir on your city’s river and see how that changes the reliability of your
supply. If the reservoir is small, it stays full for most of the year. Any water that
isn’t stored simply flows downstream as if the reservoir wasn’t even there.
But, during the summer, as flows in the river start to decrease,
the reservoir can supplement the supply by making releases. It’s still possible that in those dry
years, you won’t have a lot of water stored for the summer, but you’ll still have more than zero,
meaning your supply has a firm yield, a safe amount of water you can promise to deliver even
under the worst conditions, roughly equal to the average flow rate over the course of a dry year.
Now let’s imagine you build a bigger dam to increase the size of your reservoir so it can
hold more than just a season’s worth of supply. Instead of simply making up a deficit during the
driest few months, now you can make up the deficit of one or more dry years. The firm yield of your
water source goes up even further, approaching the long-term average of river flows, and completely
eliminating the idea of a drought by converting all those inconsistent sloshes of rain and snow
into a perfectly constant supply. Beyond this, any increase in reservoir capacity doesn’t contribute
to yield. After all, a reservoir doesn’t create water, it just stores what’s already there.
Of course, dams do more than merely store water for cities that need a firm supply for their
citizens. They also store water for agriculture and hydropower that have more flexibility in their
demand. Reservoirs serve as a destination for recreation, driving massive tourism economies.
Some reservoirs are built simply to provide cooling water for power plants. And, many dams
are constructed larger than needed for just water conservation so they can also absorb a large flood
event (even when the reservoir is full). Every reservoir has operating guidelines that clarify
when and where water can be withdrawn or released and under what conditions and no
two are the same. But, I’m explaining all this to clarify one salient point:
an empty reservoir isn’t necessarily a bad thing.
Dams are expensive to build. They tie up huge amounts of public resources. They are risky
structures that must be vigilantly monitored, maintained, and rehabilitated. And in many cases,
they have significant impacts on the natural environment. Put simply, we don’t build dams
bigger than what’s needed. Empty reservoirs might create a negative public perception.
Dried up lake beds are ugly, and the “bathtub ring” around Lake Mead is a stark reminder of
water scarcity in the American Southwest. But, not using the entire storage volume available can
be considered a lack of good stewardship of the dam, and that means reservoirs should be empty
sometimes. Why build it so big if you’re not going to use the stored water during periods of drought?
Storage is the whole point of the thing… except there’s one more thing to discuss:
Engineers and planners don’t actually know what the worst case scenario drought will be over the
lifetime of a reservoir. In an ideal world, we could look at thousands of years of historical
streamflow records to get a sense of how long droughts can last for a particular waterbody.
And in fact, some rivers do have stream gages that have been diligently collecting data for more than
a century, but most don’t. So, when assessing the yield of a new water supply reservoir,
planners have to make a lot of assumptions and use indirect sources of information.
But even if we could look at a long-term historical record as the basis of design,
there’s another problem. There’s no rule that says the future climate on earth will look
anything like the past one, and indeed we have reason to believe that the long-term
average streamflows in many areas of the world – along with many other direct measures of
climate – are changing. In that case, it makes sense to worry that reservoirs are going dry.
Like I said, reservoirs don’t create water, so if the total amount delivered to the watershed
through precipitation is decreasing over time, so will a reservoirs firm yield
That brings me to the question of the whole video: what happens when a reservoir runs out of water?
It’s a pretty complicated question, not only because water suppliers and distributors
are relatively independent of each other and decentralized (capable of making very different
decisions in the face of scarcity), but also because the effects happen over a long period
of time. Most utilities maintain long-term plans that look far into the future for both
supply and demand, allowing them to develop new supplies or implement conservation measures
well before the situation becomes an emergency for their customers. Barring major failures in
government or public administration, you’re unlikely to turn on your tap someday and
not have flowing water. In reality, water availability is mostly an economic issue.
We don’t so much run out as we just use more expensive ways to get it. Utilities spend
more money on infrastructure like pipelines that bring in water from places with greater abundance,
wells that can take advantage of groundwater resources, or even desalination plants that can
convert brackish sources or even seawater into a freshwater source. Alternatively,
utilities might invest in advertising and various conservation efforts to convince their customers
to use less. Either way, those costs get passed down to the ratepayers and beyond.
For some, like those in cities, the higher water prices might be worth the cost to live in a
climate that would otherwise be inhospitable. For others, especially farmers, the increased cost of
water might offset their margins, forcing them to let fields fallow temporarily or for good.
So, while drying reservoirs might not constitute an emergency for most individuals,
the impacts trickle down to everyone through increased rates,
increased costs of food, and a whole host of other implications. That’s why many consider
what’s happening in the American southwest to be a quote-unquote “slow moving trainwreck.”
In 2019, all the states that use water from the Colorado River signed a drought contingency
plan that involves curtailing use, starting in Arizona and Nevada. Those curtailments
will force farmers to tap into groundwater supplies which are both expensive and limited.
Eventually, irrigated farming in Arizona and Nevada may become a thing of the past.
There’s no question that the climate is changing in the American Southwest,
as years continue to be hotter and drier than any time in recorded history. It can be hard to
connect cause and effect for such widespread and dramatic shifts in long-term weather patterns,
but I have one example of an empty reservoir where there’s no question about why it’s dry.
In 1978, the US Army Corps of Engineers completed Optima Lake Dam across the Beaver
River in Oklahoma. The dam is an earth embankment 120 feet (or 37 meters) high and over 3 miles or
5 kilometers long. The Beaver River in Oklahoma had historically averaged around 30 cubic feet
or nearly a cubic meter per second of flow and the river even had some major floods,
sending huge volumes of water downstream. However, during construction of the dam,
it became clear that things were rapidly changing. It turns out that most of the flows in the Beaver
River were from springs, areas where groundwater seeps up to the surface. Over the 1960s and 70s,
pumping of groundwater for cities and agriculture reduced the level of the aquifer in this area,
slashing streamflow in the Beaver River as it did. The result was that when construction was finished
on this massive earthen dam, the reservoir never filled up. Now Optima Lake Dam sits
mostly high and dry in the Oklahoma Panhandle, never having reached more than 5 percent full,
as a monument to bad assumptions about the climate and a lesson to engineers,
water planners, and everyone about the challenges we face in a drier future.
Drought seems really simple when you’re just looking at the level in a reservoir, but I hope
this video helped you appreciate the technical complexity in developing and managing water
supply for a large area that involves hydrology, geology, meteorology, climatology, and of course
a lot of civil engineering. In fact, I’ve found through everything I do that the all the best and
most important projects combine the expertise and knowledge of lots of different fields of study.
And the way you get exposed to those different fields isn’t by reading or watching videos.
It’s by doing the thing, coding the program, by building the project. That’s why I’m so thankful
to have Brilliant as the sponsor of today’s video. Brilliant is a learning platform for science,
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school (for example, one of my favorite obsessions – the weather). If that sounds interesting to you,
go try Brilliant yourself completely free at brilliant.org/PracticalEngineering. You don’t
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