 So let's talk a little bit about surface characteristics and how they affect runoff. So when water falling out of the sky as precipitation and will think mostly in terms of rain falling out of the sky encounters a solid surface, a number of possible things can happen. So if we have a solid surface here, a few things that can happen. One of the things that happen is the water can go through the surface. It can infiltrate and fit in between the small little voids, the spaces inside that surface. It's one of the things that can happen. So infiltration. Another thing that can happen to the water is it could get captured. For example, it could not actually filter in, but it could get captured in some sort of bowl area and get stored. Now that could be because this is what we call an impervious surface, where there's no way to infiltrate, to filter in through it, or it could be just because it takes a long time for it to filter in. And so some of the water stays there while the other water filters in and gets out of the way. You could also have adsorption. And notice I use the prefix add, which means sticking on or outside, as opposed to ab, which means to come inside. A sponge absorbs stuff. It brings water inside, whereas as opposed to adsorption is something that's actually sticking on the outside. And when we think about it as adsorption, is we think about little particles, small particles of rock, actually get coated with water because water is actually a relatively sticky substance. And that coating keeps the water there, the stickiness of the water attaches to the rocks. Basically, this happens to anything. When you get out of a shower, you need to towel yourself off because the water remains, there sticks there until it has an opportunity to either be removed or evaporate or something of that effect. And so those are three things that can keep the water in the general area, okay? But any water that doesn't either go in or stay in one place is going to tend to flow off of the surface. And this flow is what we call runoff. And we're interested in runoff because ultimately that runoff is going to be, go into streams and rivers. It can result in flooding, which is when we have too much water in an area or it can be redirected to some sort of reservoir, a place where we actually want to keep the water. And since water is something that's rather important, as engineers, we're interested in sort of figuring out where the water goes so we can guide it to a place where we may either want to store it or use it. So there's a number of characteristics of the surface that affect how much runoff occurs and how much of these other sort of things occurred to the water, okay? Primary characteristics, characteristics that we're interested in looking at and measuring. One of these characteristics is something called permeability, okay? Permeability is effectively a measure of how easy it is for the water to get in between the cracks in the surface, okay? You can have surfaces that, as we said before, are impermeable and impermeable surface is something that doesn't allow anything to seep into it. Most of our impermeable surfaces are man-made. There are a few natural things, rocks, okay? Certain rocks and things like that will prevent water from flowing through them or at least in any timeframe that's considered, that we would consider. However, most impermeable surfaces are man-made surfaces. Things like rubes, where we do not want water getting into our houses, so we specifically make it so that it's impossible for the water to get through the roof or things like asphalt or roadways and stuff like that. So impermeable surfaces obviously determine how much goes in versus how much runs off, okay? There's also a measure of what we call porosity, okay? So for porosity, if we think about soil on a very small level as being very teeny tiny particles, the porosity is a measure of how much space there is between the particles. The more space, the more water can go in there and be stored in that space, more water can seep in. And in general, the more space, the easier it is for the water to flow through, okay? Now, porosity and permeability associated with it depends on a couple of factors, but the biggest factor that sort of affects porosity is whether or not the materials are what we call sorted or whether they are graded. And these are both quantities that have to do with the size of the particles. Sorted particles, well basically if you have different sizes, sorted particles mean that you have particles that are all similar size, whereas a material that's graded is going to have a mix of sizes. For example, my pictures here, I have nicely stacked little groups of particles, okay? They're all very similar sized. If you have those stacks of particles that are all similar size, if you look carefully and analyze them, what you'll find out is that if you have similar size, you end up with similar porosity, well size and shape. For example, if we think about these as being a circle, any circular particle that's stacked with other circular particles is going to take up a certain amount of space. If we think about it in two dimensions, we can see that the area of the circle is pi r squared, whereas the area of the box that contains it is, well each side is two r is gonna be two r quantity squared. And so that means the ratio of how much stuff there is filling the box to how much empty stuff there is in the box is something like pi r squared over four r squared or pi over four. That's actually what we call the solid fraction is the amount of the square that's filled up, okay? The empty space, the void space would be equal to one minus pi over four, that fraction, and that would give you a fraction that would be representing how much empty space there is there, and this is called the porosity, a measure of how much empty space there is. Well, if everything's exactly nicely packed, you have values that you can easily calculate. And notice this does not depend on the size of the particles. Large particles will pack and leave this fraction, but small particles will pack and leave the same fraction just more of them. But what happens if you get something that's graded? Graded means we have a mix of sizes. If you have a mix of sizes, then the teeny tiny particles of gravel fit in and fill in the spaces among the bigger ones. And you actually have less overall space. So porosity depends on the size of the particles you have and then the mix of the various sizes that you have. And that affects whether or not water can stay in there and or how easy it is for water to seep in, ultimately affecting both the infiltration and the runoff. Couple other factors. You have the permeability, you have the porosity, and again, these are factors of the soil itself. You also have the type of soil. As we mentioned, certain types, well, water can absorb and it's sort of attached to water particles. Well, certain types of soils can be stickier, can interact with water or can absorb or hold the water more carefully or some can even absorb the water, bring the water inside. So the type of soil will also affect how likely it is for the water to stay there versus runoff. Okay. The Soil Conservation Service, SCS, Soil Conservation Service, actually classifies soil into four types. A, B, C, and D. Where should those types sort of represent runoff potential, the potential of the water to runoff? And I believe they go from low runoff potential, A, where it's more likely to seep in, to high runoff potential, low to high runoff potential based on things like the porosity and the type of soil. And they've taken, I believe it's 4,000 different soil types and put them into one of these categories. So often you'll look up the soil type that's in your area to sort of help determine some of the factors for runoff. So permeability, porosity, soil type, okay. I should go ahead and put type of soil. There are also some other things that we actually want to measure, conditions in the area. For example, our soil moisture. In other words, how much water is already in the area? Is it wet or dry? If it's already wet, that means that there doesn't, none of the water that's flowing in is going to be captured in absorption as much because there's already water there and there doesn't need to have no more room for the water to stick. So the existing soil moisture or existing storage is actually an important piece, okay. The existence of vegetation. Plant life affects the amount of water that's going to stay locally because the plants will absorb and use some of the water. Also the presence of roots and even dead plant life is going to sort of affect the soil characteristics and whether or not they contain or keep the water in place. So the presence of vegetation is also another piece that we need to look at. Finally, two things about the watershed itself. Watershed characteristics. If a watershed, the area that the water is falling on has a certain size or shape that will ultimately affect the runoff characteristics because the water will all be guided to some location. For example, if a watershed is fairly round, then the water is all going to tend to meet at a location at about the same time. So we're not just thinking about when the water arrives from the sky, but we also think within the water's arriving from other runoff portions and the combination of the runoff when it all meets together is going to affect the flow characteristics for how it runs off. That's going to be different for a watershed that is long and thin. That in that case like that, the water might arrive and take longer to travel to different spots in the watershed and therefore might arrive at different places and you get different amounts of water arriving at different time. So the characteristics of the watershed are important. And then the last important piece for us to consider in this case is going to be the slope of the area, the slope of the entire watershed. Obviously the steeper the bank is, the more likely is that the water is going to be redirected and runoff before it has a chance to infiltrate and run through. If it's a relatively low gradient, a low slope, then it has more opportunity to both adsorb and to infiltrate. So this is a list of the characteristics that we must think about and consider when we're trying to calculate how much runoff will occur as a result of a precipitation event.