What Happens When a Reservoir Goes Dry?

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,   technology, engineering and math, that is super  interactive. There are courses on logic, computer   science, math, and actually my favorite is this  one called the Physics of the Everyday that just   pulls back the scientific curtain on aspects of  the world that you only learned about in grade   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   pay anything to sign up, and the first 200  people that use the link will get 20% off an   annual premium subscription. Thank you for  watching, and let me know what you think.

Channel: Practical Engineering

Views: 2,094,468

Keywords: mega-drought, Lake Mead, water reservoir, Lake Powell, Lake Oroville, firm yield, snowpack, clearwell, water tower, bathtub ring, watershed, Optima Lake Dam, Beaver River

Id: wu9qy4DyKlo

Length: 13min 42sec (822 seconds)

Published: Tue Jul 19 2022

Reddit Comments

[u/No_Knead_Dan]

SS: Interesting video, but the guy never really answers the question. It really is just the Futurama meme of "we added bigger and bigger blocks of ice..."

Spend more and more money and energy trying to band-aid over the problem. But eventually the bill comes due.

[u/jeremiahthedamned]

there was an entire civilization in eastern iran around an inland sea that dried up after deforestation.