Course overview: What is Hydrology? - CE 433, Class 1 (12 Jan 2026): transcript

Okay, welcome. Let's get started. Actually, I guess the first thing I need to do is take the attendance. So, let me pause this. All right. Well, let's talk about hydraologic engineering. Uh, this is a civil engineering design elective. And that change happened a couple of years ago. Prior to maybe two years ago, the way that the course was taught was just as a civil engineering elective, but I reoriented things so that you could get some design experience related to hydrarology. But that's not going to be until the second half of the class because a lot of the things that you need to do to design in hydrarology are dependent on you understanding the science of hydrarology. And so this course is going to be a little bit different probably from a lot of the 400 level civil engineering classes that you're taking because we're going to be learning fundamental science in this class where for a lot of your 400 level

classes probably you've done the fundamental science a long time in the past. You know um you did uh structural analysis, you did materials and now you're moving on to the design in those areas. And so I just wanted to acknowledge that things are going to be a little bit different in this class. And there are some really important concepts that we have to go over before you can move into design. So why is hydrarology important? Let me dim the lights a little bit. I'll have to draw the blanks after class so that there's not so much glare. Anybody know where this is?

>> Yeah, that's just right around the corner. That's we're on Hal Greer and we're looking at here's the steel of West Virginia. So, if you go straight through and then turn right than the engineering building. So, we're all familiar with the location, but something's a little different compared to the ordinary day. Um, this I took this photo on a rainy day. It wasn't as deep as it looks. You're not supposed to drive through standing water, right? So, being the bad boy that I am, I had to drive all around and get as many pictures as possible because I'm loving it. I'm loving this flooding.

It's so good as a lesson for hydraology. And it's not the first time I've done it either. I've taken pictures like that in a lot of cities where they've heavy duty rain. So the point is hydrarology is important in urban environments like um here in Huntington. Do you know why it floods so badly in Huntington? Like what's the issue? Anybody know? >> Many impervious materials. >> Imperous materials. That's a huge part of it. We've got roads. We've got roof area. We've got sidewalks. And water doesn't pass through those things. At least it's not supposed to. If water passes through your roof, you're probably going to call a roofer and get it fixed. But imperous area causes runoff in an urban environment. Um, but not every city has flooding like this. I mean, it rains in a lot of places. And so why is it that when it rains in, let me pick a city, Washington DC, they

don't have flooding like this? Well, in Washington DC, it's because they're made of money in DC. And if they got a problem, you know, it's so visible to the people who control the purse strings that they spend it so that it's fixed. Both Washington DC and Huntington have been around a long time. Part of the issue is the age of the city. here in Huntington when they were designing the uh pipes that go underground to carry water, they weren't necessarily thinking 100 years in the future, they weren't thinking that oh all this area is going to be developed. So part of the issue is just the populations they had in mind when they sized the infrastructure and the contributing area to the flows were different then they are now. So, it's partly a timecale problem.

Also, when they put in a lot of the the pipes in Huntington, that was before cars were invented. And so, you know, um they probably weren't thinking that there was going to be so much paved roads, you know, like dirt roads were still common back 100 120 years ago. And dirt roads don't um cause as much runoff as imperous surfaces like asphalt and concrete. So part of it is because of just the the proliferation of impervious area over time and then also back in those days they had what's called a combined sewer where waste water and storm water goes through the same pipe.

Nowadays it's far more common to separate storm sewers and sanitary sewers which is one of the great euphemisms of civil engineering a sanitary sewer. Sometimes you call things the exact opposite of what it is or what they're doing. And sanitary sewer is one of those. But anyways, urban areas, hydraology is important. Rural areas, hydrarology is important. This is a photo that I took in my backyard. This is my neighbor's property. My house is up the hill. Um but um I've often wondered why my neighbor is so fixated on leaf blowing. Like the guy spends 8 hours on a Sunday blowing leaves and I wondered like, you know, like a pile of leaves kill his family or something. Why is he so fixated on getting those leaves out of here? And so I like I've just kind of wondered it over the years. And then what I realized is no, actually he's doing it for a

reason. It's because this hollow the the water through here goes into an underground pipe that runs under his garage and under his house. And he knows that if that pipe gets clogged that then the water's going to go over the ground and scour the foundation of his house and scour the foundation of his garage. And so what I realized I guess I could have just asked the guy, "Hey, why do you leaf blow for eight hours every Sunday?" No, it's because he's worried about that pipe clogging and he's worried about flooding and so he's doing the preventative maintenance to make sure that uh water doesn't flow through his garage. So that's just a small example of how hydrarology is important in rural areas. It's also important because it's in rural areas that we do things like grow and irrigate crops. Um, it's important in rural areas because, well, most of the United States

area is rural. Like connecting between cities is roads that go through areas that aren't very highly habited. And so the hydraology affects the roads that connect cities. And so it's an infrastructure issue as well. So in this class, this semester, there's a couple of big questions that we're going to be tackling. And the science that I teach you is not just because it's g whiz kind of interesting science. It's in the service of you understanding [clears throat] the fundamentals so that we can address the big questions of the course. And so here it is. If I had to distill it down to one sentence like what is this class about for an area that we're interested in?

What are the runoff characteristics? Runoff is the water that happens when it rains. So, we want to know when it rains, what are the characteristics of the runoff? I'm not talking about its flavor. I'm talking about like how much water is there going to be when it rains and when is it going to get here. So, we're interested in the timing and the quantity of water during storm events because it's that variability that we have to deal with as civil engineers.

The hydraologist talks to the hydraulic engineer. The hydraologist says, "Here's how much water there's going to be." And then the hydraulic engineer figures out a way to move it from point A to point B so that it doesn't cause problems. So the runoff characteristics that we're going to try to describe using principles of science is let's think about the rainfall. What do we have to know about the rain to describe runoff? Well, we want to know how much is it going to rain in a certain place and how quickly or how often do big storms come through. So, return period is a way of describing the size of storms in terms of how likely they are to occur in a given year. Have you ever heard of a hundred-year storm before? People always talk about, "Wow, the hundred-year storm came through." What they're talking about when they say that is a storm

that's so big its probability of occurrence is once every hundred years. Now, that doesn't mean it's like following a schedule and it's on the calendar. It's just a statistical description. But that's just an example of one way that people talk about rainfall quantities is they'll talk about it in statistical probabilities or sometimes it's important for us to know about rainfall. It's temporal distribution.

This is a $5 phrase. You're getting your money's worth today tuitionwise just by learning what temporal distribution is. Because it'll make you sound smart when you're on the job when you say, "Well, let's consider the temporal distribution of the rainfall." It just means temporal distribution is when over time is it raining. Have you ever noticed that when it rains at first it just kind of sprinkles for a minute or two and then it pours really hard and then it goes back to like a less intense precipitation then it finishes by sprinkling a little bit more. Like if I have to go somewhere and I look outside and it's pouring just absolute buckets I usually just wait five minutes and then it calms down a lot. So temporal distribution is there are studies that evaluate the intensity of the rainfall not only on the short term like in the course of a an hourong storm how much of that precipitation occurs within the

first five minutes within the next five minutes and so on. There are what are called rainfall hytographs that describe that. But there's also longer term temporal distribution studies that look at, for example, in the Pacific Northwest, how much of the rain do they get in the winter versus the summer and how that's different to storms that occur in the Midwest of the United States where rainfall is more focused in the summer months than in the winter months and so on. So that's temporal distribution is just how spread out something is over time.

how spread out something is over time. So knowing that, what do you think spatial distribution is? How spread out something is over >> space. >> Oh, you guys are smart. It's going to be such a good semester. It really is. Yes, that's right. How things are spread out over space. And so around here, the most detailed rain gauge we have is at the Huntington airport. at the Huntington airport. I think that they are recording the data in like five minute intervals and I think you can download it aggregated in 15minute intervals, maybe even the five, I don't know. There are other rain gauges though where it just says like a once a day total, you know, like the 24-hour total.

I've seen storms before where I had a rain gauge set up at my house in Ona. I live over by the mall, roughly speaking, over by the mall and it's about 20 miles from the airport and I've seen storms where the rainfall intensity at the airport was really, really high and at my house we got no rain at all. So that's an illustration of spatial distribution is just how are you going to predict the rainfall at the area that you're interested in when you're measuring it at just a a few selected locations and there has to be some way for us to interpolate the rainfall in kind of a repeatable and organized and defensible way. If you don't have exact rainfall data for the location you're interested in, how do you extrapolate?

And that's an important part of hydrarology because there's never data exactly where you want it. If you're doing a construction project at a certain site, they don't have a rain gauge there that goes back a 100red years. Like in the state of West Virginia, there's probably only 20 rain gauges in the whole state with 100 years worth of data. Maybe less than 20. And if you think about how many what thousands of square miles of area there are in the state and we're relying on 20 rain gauges that go back a 100 years.

Extrapolation is a really important part of characterizing rainfall data. But it's not just the rain that we're interested in. We're also interested in soil and how infiltration occurs. Infiltration is when the precipitation that falls out of the sky hits the ground. When it hits the ground, it can do, broadly speaking, two things. It can either run off or it can infiltrate. Infiltration is penetration of the moisture down into the soil. And so, like the things that we have to know to be able to predict infiltration is we have to know what kind of soil is there.

And around here, we have from a hydraology standpoint or soil. We've got class C and class D soil, which is a lot of clay, plastic soil, shale, rock, stuff that isn't very good at infiltrating water. So, it tends to promote runoff. The the soil becomes saturated really easily, which is along with the uh steep slopes one of the things that causes waterheds in this area to be very flashy. Flashy wersheds are ones that have a very quick response to a rainfall event.

We also want to know about the land use because what you've got built on top of the land also influences how well water is going to infiltrate because water's going to infiltrate worse if you've got concrete on top of some soil compared to if you've got Bermuda grass on top of some soil. So vegetation [snorts] and uh also human disturbance of the soil. If you've got heavy machinery driving over it, then that can affect the infiltration properties as well.

Anticcedant moisture is talking about how much moisture was in the soil before the rainfall event you're interested in. So if you're going to be simulating like a 2-in storm that happens today, how much did it rain in the last 5 days? how much water is already in the soil that we have to account for when we are characterizing runoff. There's also abstractions that we're going to learn about this semester.

Things like the evaporation of water from both plants, rivers, and soil interception, which I guess it's kind of similar to football. In football, interception is when something like takes the ball out of the air, the other team. Interception is when it's raining and instead of hitting the ground, the rain falls onto like a building or trees.

So, um, that reduces the amount of precipitation that actually makes it to the ground. So, you've probably stood under a tree before in a storm. Don't stand under a tree if there's lots of lightning, though. But under a storm, like if it's if it's raining, a certain amount of the moisture is going to have to wet the leaves and it won't get to the ground in the short term. And so we need to account for things like that if what we're trying to answer is when it rains, what does the runoff look like? All of these things are going to come into play in how much runoff there is and how quickly the runoff arrives at a certain location.

Um and then if we want to actually visualize that if we want to generate something called a hydrograph. Hydrograph. Hydro is water. Graph means graph. So a hydrograph is a graph of water. So it's something like a hydrograph would look like this where we've got time and then we've got flow. And so if I've got a road, here's a road.

And then there's like a river. I'm looking down from above. Here's a river. And I need to put in a culvert here, right? Want to put in a culvert, which is just a pipe. I want to put in a pipe so that the river can go under the bridge. So there's like a whole watershed upstream of this location. this this location of interest where the culvert is. That's what I want to know. How much water is going to make it to that culvert? How quickly is it going to get there? What's the flow rate? Because I want to make sure that the pipe is big enough. Well, to predict that, I have to take some way to account for all of these things like rainfall, infiltration, and abstractions, I need like a process to turn that into a hydrograph. And the hydrograph is just an estimate of when the water will arrive. Like I could actually measure the water at that location. There are such thing as stream gauges. You've probably heard of a stream gauge before.

Maybe you've even seen one. In the state of West Virginia, there's about 120 stream gauges that are collecting data. And most likely there's not going to be a stream gauge at the location that you're interested in. And so you can't use real world data to do your design of how big should that covert be. You're going to have to predict. So you're going to use a model. Do you know what a model is? Like little airplane model is like a simulation of reality. When you build a model plane or a model car, it's like a scaled replica. And so models are a lot of different things. like someone who's walking around with like a new clothing design, they're a representative of what you supposedly are going to look like if you buy those clothes.

The models that I'm interested in for this class for hydrarology, I want a model that predicts the hydrograph. So, I want my model to I want to tell it about the rain. I want to tell my model about the soil. I want to tell the model about any abstractions that exist. And then the size of the basin. I want to tell it things about this location and I want it to predict how much flow is going to exist at that location. And so the runoff modeling that we're going to do will allow us to come up with the hydrograph and then tell us what's the peak the peak flow rate.

So we'll we'll spend a lot of time this semester learning the fundamentals so that we can eventually predict the peak flow rate and then that tells you how big the pipe needs to be. So you know your boss is saying how big should the pipe be that goes under that road? You got to take a step back. There's a lot of calculations that go into it. But then once the water gets into the river then there's also the effects of routing. And routing is just like adjustments that we make for the timing and delay of how quickly the water could arrive at a certain location. So it's just extra considerations like the travel time as the water is flowing through the stream like the velocity of the stream based on the slope and how rough the stream is. It's also related to uh what's inside of the watershed.

Like if you've got a whed with lots of thick leaf litter and trees and branches and all the creeks are clogged with all that vegetative debris, then the water moves through the wershed more slowly than like my neighbor's holler where he's leafblown and removed every stick and every twig. That's good because like that debris doesn't end up in his cover, but then it also allows the water to travel a lot faster to the culvert than it otherwise would. And so you're going to learn about the implications of that this semester. If the water's traveling quickly to the location of interest, then it turns out that the peak is even higher than it would have been if there was some sort of a dampening delay as the water flows over the surface of the watershed.

And what we're going to do with all that is things like sizing coververts, figuring out how deep the water would be in a stream, like how high the bridge can go. We're going to be doing analysis of what sort of flood prevention measures could be taken to limit blood damage. This is a really broad field. hydrarology. I love it a lot. I think it's really interesting. I'm glad that you decided to take this class. I hope that you like it just a fraction as much as I like it. Um I remember when I was a student taking this class, I read the textbook because I thought it was interesting. Does that just make me the biggest nerd ever?

Right. Embarrassing to admit. I wanted to point out that I've got the uh syllabus and schedule on uh on Blackboard. I've handed out a copy to you. We're just going to really fly through a couple of things on the syllabus on schedule related to policies. So, if you'll glance at that as we go over it. I wanted to mention that um my office hours are Monday, Wednesday, and Friday from 3:00 to 5 and I'm here a lot more than that. So, if the doors to the civil engineering department are open, feel free to stop by if you've got a question. You can also send me an email just to like schedule an appointment if you want to be sure. But, uh, feel free to pass by and if I've got the time, then I'll be happy to help you out outside of these office hours. You can also email me or give me a call on Teams. I'll be happy

to help you. Um, we do have a book and it's required to get. You need to buy the book. It would be a struggle to pass the class if you don't get the book. So, please buy the book. Water resources engineering, Larry Mays. Um, and then you'll also need to have a Windows laptop this semester because there's some software that we're going to be working with that doesn't have an Apple Mac version. So, it only runs on a Windows computer. So, if you have a Mac computer, you'll either need to set up whatever virtualization so you can run a Windows environment on it or just borrow a computer from somebody. Um, so here's the learning outcomes for the course and where you're going to practice it and where it's going to be assessed and so on. Um, there's actually two classes happening right now. I don't know if you're aware of that, but right now I'm so good that

I'm simultaneously teaching civil engineering 433 and environmental engineering 670 at the same time. Three of the students in this class are taking that graduate class and they have slightly different grading scale than those of you who are taking the undergraduate class and they have an extra assignment as well. Uh but for those of you taking the undergraduate version, here's how your grade is going to be determined. We've got a couple of design projects at the end of the semester that you'll learn more about. The uh the grad students don't have attendance points.

They have a research paper that they have to write for that 5%. Whereas those of you who are taking the undergraduate version, just get it for coming to class. So, um a lot of the policies are the same from one class to another. For example, I'm going to be dropping the one lowest homework assignment grade from calculating your uh your scores. Late work is not accepted. [clears throat] Um the assignments are going to be due on uh the dates indicated here. At the end, there's a schedule that shows all the submission deadlines and the topics of the assignments. And so the assignments will be due at 11:59 p.m. on the date that's indicated.

And if you have late work, Uh, that's too bad. I have strictly I have a strict that's too bad policy, [laughter] but I do drop the lowest assignment. So, you know, you don't have to do one of them. Um, please don't cheat in the course. Read over the academic integrity expectations and let me know if you have any questions about that. The the most important bottom line is that it's important for you to learn the material and it's best to do that if you work independently on the assignments. And it's okay to check your work with other students and to help them find mistakes if if they've made an error, but you can't um sit there at the same time and work on the problem. And so that like one person's really just copying off of another person because then it's not a good learning experience for the person who was just kind of um not going through the same intellectual struggle as the one who was uh figuring all the things out. So, be cautious about

collaboration because it can be a good thing if you do it correctly and it can be a negative thing if you don't. Um, for the undergraduate class, here's the scale of how many unexcused absences you can have to and not have it affect your grade. Otherwise, the uh 5% attendance goes down by one per missed class. from there. Um, I have a moderate use AI policy in this class, meaning there are certain ways I encourage you to use it and other ways that uh you can't use it. And so, you can't use AI for any uh work on the class project. You can't use it for solving your homework assignment. Um, you can ask it questions about hydrarology and it'll actually probably do a pretty good job explaining things.

And so that would be a nice way to use AI is just to uh have it offer its own point of view on certain concepts maybe that you don't understand yet. But when you do use AI for something that you submit, you need to uh acknowledge by putting in just a brief statement to explain which platform you used and in what way you used AI for the thing that you've submitted. Any questions?

I'm sorry to say I'm going to miss class on Wednesday and Friday. I'm going to Hawaii, but it's not going to be fun. I'm going there for a trail race, so it's going to be torture. It's not going to be fun. So, >> that was awful. >> Well, I'll let you know. I'll let you know if I survive. >> What's that? >> I'm going to stay over the water. >> Stay over the water. No. No. No. Nothing like that. No. But I will be studying the watersheds as I'm there. Yeah, my race goes through um like a jungle. So I'll try and study the uh the hydraology of it. I'll be reflecting on hydraology during the race. But the point is so Wednesday and Friday, see on the schedule where it says online, um those classes are already on YouTube.

I've already got them set up. And so if you go to Oh, not that again. If you go [clears throat] to uh here, yeah, class two, class three, there's the link. It's recorded already. There's also this concepts and terminology page. Um, it is a bunch of words that you need to be able to explain, define, use in a sentence and so on. So, um, it's a pretty long list. You need to know all this stuff. And a lot of it I'll teach you in class. Some of it you'll have to just like look it up in the book and learn what that word means by reading the definition in the book.

So, um, the exams are going to be a mix of problem solving and concept questions. And so the concept questions will largely draw from the concepts on this page. I wanted to point out that that's here and I'll print a copy and bring it to our next class. Uh our next class is Wednesday of next week because you know on Monday university's closed no classes. So on this week, Wednesday, Friday, lectures online.

And then Monday of next week, no class, Martin Luther King. Wednesday, we'll be back in the class together. Okay, so that goes over the schedule. Oh, by the way, your first homework assignment, if you look at the schedule on the syllabus, the first homework assignment, which is available now, is due on Wednesday the 21st of January. So, um, let's just take a look at that.

Here it is. Assignments homework one, precipitation. So, the [snorts] questions on this precipitation assignment are related to the recorded lectures that are on YouTube. So uh and also related to some things in the book. Okay. So let's talk a little bit more about the water cycle and hydrarology.

Show you some interesting pictures from the area here. Uh let me dim the lights a bit so that this is easier to see. Don't fall asleep. I know it's after lunch. Um, so when uh when flooding occurs, it goes to the low-lying areas and here in West Virginia that just happens to be the most convenient place to build a house unfortunately because um you know to build up on the hillside it's a lot more expensive because you got to put in a road and then you've got a steep road that you've got to get up which is tricky when it snows and it's slippery and then you know you've got to pave pave the road and it's expensive. So, a lot of people in West Virginia live in the flat land, which generally is flat because it's next to a stream that's kind of carved uh the mountains out of the way. So, flooding is a real issue

throughout the state and there have been some high-profile storms and floods over the years. Most recently, a really significant one was in 2016 and um I did some research for the West Virginia Department of Transportation um in the past where we studied some watersheds that experienced a lot of damage in that 2016 flood. And so I'll talk more about that in specific when we do some of the watershed modeling using WMS this semester.

Hydrarology is important in urban areas. We already talked about that. It's important in rural areas. It's important in places where it's wet. It's also This is a picture in uh Richwood. I don't know if this video is still working anymore. Let's see if this video is still available. Yeah, I think it is. That's good. We just have to wait for the commercial limits like we've got. And um here in West Virginia, the typical annual average precipitation is about 40 ines per year. And um hydrarology is important in aid regions as well. This is a photo I showed if you're in the hydraulics class, I showed this picture.

I did some research when I lived in the UAE related to flooding in the mountains of the UAE which interestingly share a lot of characteristics with the mountains of West Virginia in terms of their size, their slope, the size of the watershed um upstream of a certain location. What's different obviously is a really different vegetative profile like no vegetation versus heavily forested.

But in um in aid environments, water isn't something that they're just trying to get rid of as easily as possible. They're trying to capture and make good use of the water if they can. And so what this is a picture of is this is a groundwater recharge dam that they built in the interior of the United Arab Emirates. Uh this is about an hour's drive from Dubai to here. And um so what they did was they built a dam just so that when it rains they can capture it instead of it just going immediately to the sea. They want to capture it and try and force it down under underground because um groundwater was a really important source of water in the UAE historically. They've mostly exhausted their aquafers in the UAE. Now it accounts for less than 5% of the water that's consumed in the country. Most of the water there is desalinated which is very expensive but for historical reasons and to try and prevent

subsidance and to maybe enhance a little bit of resilience they've built a series Here's another one a different groundwater recharge dam. The problem that they face though is that when it rains there's often a lot of sediment in the water as well. like it's kind of turbid. And so all of the water gets to this low point and it stops moving and then the settlement the sediment goes to the bottom of the ground. And um it's a really fine material that clogs the pores. And so they're getting probably more evaporation than they are recharge, unless they're constantly scraping the uh the sediment off the bottom of their of the dam to try and like enhance the uh the recharge.

Here's some pictures that I took when I lived in the UAE of flooding, you know, people in uh parking lots who obviously weren't expecting a rainy day and the damage it caused. car so new knew it still has plastic on the seats, but now it's uh underwater and people driving through water just like I did there. This was another rainy day that caused some issues. Um, a lot of the intersections in the UAE don't have storm drains set up. And so what they'll do when it rains is, you know, some of the low-lying intersections, they have to pump them out with trucks. And unfortunately, cars are driving fast even though it's raining and there's a lot of accidents. And so you'll always see these pumper trucks kind of destroyed because they're out when it's wet. People are driving crazy when it's wet and uh the inevitable collisions occur. It's also kind of a risk for mosquito breeding. You know, to have

standing water, it can be a health concern. Um, mosquitoes most recently were in the news by uh carrying what's called the Zika virus. I remember my brother lives down in Miami, Florida, and they were really worried about Zika and um mosquitoes that carried it about 10 years ago. It was in the news a lot. It would cause havoc on campus. This is the university that I worked at over in the United Arab Emirates. They had this really nice granite walkway. It was like stone all over campus, but it gets super slippery because it's like polished super slick and then it gets wet and it's kind of dangerous. So uh on the spectrum of hydraology feeding data into h uh hydraulic engineering um we are going to mostly this semester focus on surface hydrarology. We'll have

just a small amount of subsurface hydrarology. So subsurface is water moving through the soil and rocks and aquifers. So we'll spend a little bit of time learning about the well equations, but most of the time we're going to be doing uh the movement of water over the surface. And then the information that's generated from hydrarology is used in open channel flow. So like sizing of channels and in pipe flow where we're sizing coververts in order to uh move water from place to place pipe networks and um we're going to spend some time early in the semester talking about the water cycle and uh just the the different places that water moves and what drives the water cycle is solar radiation in the form of evaporation in

the ocean which the moisture evaporates gets into the air and it passes over the land and then from there there's lots of different things that can happen. Um it can precipitate in the form of water meaning rain. It can precipitate in the form of snow. Uh there can be hail. We're going to learn a little bit about like what causes those gigantic hailstones that happen occasionally during big storms. like why is it that it is uh chunky solids of ice that comes down through the air sometimes rather than rain. Um we're going to be talking about the relative quantities of water in the different places and how fresh water is actually a very scarce resource and compared to the vast quantities of water that we can't use. like most of the water that exists in the world is in places that it's not convenient for us to access or it has minerals in it that

make it very expensive to purify. So this is just showing the relative amounts of water where for 100 units of precipitation on the land that requires 424 units of evaporation from the ocean because most of the Earth's surface is water. So when there's evaporation from the ocean, most of that evaporation is going to come out of the air as precipitation on the ocean. But when it precipitates onto the land, some of the water that precipitates onto the land gets down into the subsurface. Some of it runs over the surface and gets back into the ocean again. So, this is just kind of quantifying in relative terms how much water is in various places.

And um let me just jump ahead here. Okay, this is the last slide that we'll look at today. And this is what I was talking about a few minutes ago. H just helps you to appreciate how scarce fresh water is. The water that we've got in the Ohio River seems like a lot, but of all of the water on Earth, uh 96.5% of it is in the ocean. And it's not easy for us to use ocean water because of the salt that's in it. And so desalination is a very energyintensive process. And u even the most efficient desalination produces water that's more than 10 times more expensive than the water that we can get from the Ohio River. You know the Ohio River has to go through a treatment process as well

before we can drink it. It doesn't have to be treated before we irrigate with it, but it has to be treated before we can drink with it. But then in arid places like the UAE, they have to desalinate the water to drink it and also to irrigate with it. So they do have some farming over there, but the expense and inefficiency of farming farming is compounded by the fact that they're having to desalinate for their irrigation water. So of the water on Earth that is fresh water, only 2.5% of the water on Earth's surface is fresh water, meaning it doesn't have salinity in it.

Most of that is in very inconvenient places. It's in glaciers and ice caps. So, we can't really use the water in Antarctica or in the Arctic. It's just it's in the wrong place. It's frozen. Um, and a lot of the groundwater that exists is so deep that it's not economical to pump it up. Like lifting water from underground is very energy intensive as well. So there's a certain depth beyond which it's just not economically viable to retrieve groundwater. So that a lot of the groundwater we can get, much of it we can't. And so if you look at the 2.5% of the Earth's water that is fresh, only 1.3% of that is fresh water that's at the surface. So meaning water that's in lakes and rivers.

Um, and even a lot of that is uh is tied up as ice and snow. Like so ice and snow that's not in the ice cap or in the glaciers. So it's at the surface of uh let's say Minnesota or Canada or something. Um so the point is that fresh water is very scarce. It's a resource that we have to appreciate and try and protect. And that'll be another uh minor thing that we talk about this semester is uh preserving the quality of water.

Um and we'll mostly tie into that when we're talking about um storm water detention and like storm storm detention facilities like um ponds for developments. like when there's a a new commercial development or residential development going in, we generally have to put storm water ponds to collect the water. All right, it's 251. First day of class, I'm already holding you over. It won't happen again.

It won't happen on Wednesday or Friday because remember, we've got an online class those days. So, let me know if you've got any questions. We'll meet again on Wednesday of next week and your first homework assignment is due Wednesday night next week. See you then.