 So today, we're going to talk a little bit about the effects of erosion, particularly the environmental effects of erosion, and how we calculate how much erosion might occur in a particular area. So first of all, erosion is caused by runoff. As part of our hydrospheric process, we have a number of different processes. But among those processes include the motion of water over the land surface, which is our runoff. And the amount of runoff is affected by a number of different things, the amount of precipitation, as well as other aspects that are removing water, transpiration, water being removed through plants, evaporation, water being removed into the atmosphere, and infiltration into the surface. The amount of water that travels across the surface, however, is what we call runoff. And it's a particularly important factor in both flooding when we have too much water in a location to deal with, and also in erosion, an erosion being the movement of sediment and soil from one place to another, as pictured here on the right. So now one of the things that affects erosion is the amount of energy. How hard the rain falls, for example, will determine how much the soil is affected by that rainfall, whether or not it hits and breaks up little pieces. And then as the raindrops splash, the soil can move up to three inches away, as it turns out. And then soil particles that are released from being connected to one another will actually flow with the accumulation of flowing water and be transported. Now this can be mediated or affected by the presence of other residue, leaf cover, vegetation, other things that will not only prevent the erosion by preventing the rainfall from hitting the soil directly, but will also tend to gather the water, pool the water, and slow the water down as it flows away and as runoff, and therefore leave behind some of the sediment. Another thing that obviously affects how much erosion occurs are the various conditions of the soil. One of the things we can do is we classify soil based on the size of the soil. Notice there's a little size chart here, large soil or large chunks of rock we consider to be gravel. As we get smaller, we go to sand and then silt and then to even smaller size, the finest size is something we call clay. There's something here we call loam, which is kind of a mixture of sand and silt and clay. Okay, when you mix in a different versions of those things. And usually when we're trying to classify soil, we're not going to have all soil of the same size, we're going to have different sizes that are mixed in together. There are ways we can go about classifying that. One of the ways is to use a chart like this one. Now this is an interesting chart because we're actually not using perpendicular axes here, but it's relating three percentages, the percentage sand, the percentage clay, and the percentage silt. In other words, those three different sizes are measured in percentages usually by volume or by weight. Actually, I'm pretty certain in this case that it's by mass. And what we can do in these cases, let's say for example, we have 40% clay. If we have 40% clay, we travel along that horizontal axis there. And we know that we also have, say for example, 20% silt. Well, then we travel along that axis and those two values will meet at a location that corresponds, well, 40 plus 20 is 60%. Well, the remaining 40% as evidenced on this axis is the percentage sand. And if we had that value, our classification according to this chart would be somewhere along the line. It's here between clay and clay loam. So since it's primarily clay, but maybe a little more since there's sand and silt mixed in, we might define that as being clay loam. Whereas there's other terms, for example, a very sandy loam would be a different mixture that was very little clay and quite a bit of sand with just a little bit of silt. We have maps. There's been extensive mapping of different kinds of soil. Here, for example, is the soil map of Durham, North Carolina, the county in Durham, and a bunch of different classifications of those. And the different types of soil will give us different information about how we expect runoff to behave. So there's a number of factors that contribute to erosion, which is what we'd like to talk about here. And we can use all those factors to make some estimates on how much sediment erodes off of a particular parcel of land. First of all, the biggest contributing factor is the climate. What's happening in the climate around you, the climate determines how much rainfall and how hard that rainfall occurs. It contributes to how dry or how wet the soil will tend to be. And ultimately is the major contributing factor to the amount of runoff. But obviously the conditions of the soil, as we just discussed, will come into play. The topography, whether it's steep or shallow, will obviously determine it. The steeper the more likely you are to have significant erosion. And then how the land is actually managed, either by putting crops on it, what's actually growing on it, the vegetation, and then the management support practices. Are there any things that you do? Now obviously this is applied quite a bit to farming because what happens in the case of farming is we often have the relatively rare case where we have a great deal of exposed soil. Most of the time when we're not farming, we don't have a lot of exposed soil, but that exposure is sort of a key contribution to erosion. So here's an example of an equation that we will often use to determine erosion and the amount of sediment that comes through it. Okay, and this equation was developed from different simulations, simulating rainfall and measuring actual erosion and different experiments that come along with it. And notice there are five different factors corresponding to the ones we just talked about are dealing with our soil conditions and our rainfall, et cetera, used to calculate an annual soil loss. And this is in this particular formula, usually measured in tons, so an actual weight of soil per hectare, which is an area of land. We're gonna talk about each of these factors. The first factor is this factor R. Different geographic areas have different climates and these different climates have different effects on how much you would expect to erode in sort of a general set of conditions. So you need a pattern of rainfall, different intensity of storms. Okay, the energy of these storms is gonna affect this factors, et cetera. Well, these factors have been measured, considered extensively, okay? And they change depending on where you are in different locations. You'll see, for example, in Las Vegas, we expect a very low level of erosion. Here we have a factor of eight, and this is sort of the base level of erosion, eight tons per hectare, sort of the base level that we're then gonna modify with some other factors. Whereas you'll see in Chicago, Illinois, okay, you'll see a number of 140, which is significantly more erosion. We can map factors like this. In fact, you can sort of see if you're looking for a particular area of the country, you can locate your county and using the various contours here, you can get a sense for what level you might expect or at least what base level you might expect of erosion. So let's say, for example, we were looking, I was actually grew up in New Hampshire, so if I was looking at the county where I grew up at a location up here, I would see that my erosivity factor R, sort of my base tons per hectare would be somewhere between 80 and 100. Maybe I would estimate that as being 80, 90 or 95 tons per hectare would be sort of the base erosivity for that area of the country. Now that's modified by a number of other factors, the first one being the condition of the soil. So we can look and see different soils have different erosion factors based on their potential to erode, okay. These factors are assigned to soils and arrange from basically 0.17 to 0.6. Organic soils can be as low as 0.2. In other words, things that have a large amount of organic material, rotting materials that tend to be very sticky tend to adhere to each other and have basically inhibit erosion. The higher this number, the more erosion we expect. And again, this reduces that sort of initial R by some factor, even at high values of K, this 0.6 would reduce that initial R value. The higher the factor, the greater the potential to erode based on soil conditions. Now that leads to a number of things that can actually contribute to the soil erodibility. And all of these are represented in this chart. Instead of using an equation, this is a kind of rare, well not rare, this was often used in many sort of engineering design things. And instead of using complex equations, we can instead use complex graphs. And those graphs can chain together a series of different factors to yield a final factor. Here's the example they use here. Let's say for example, you have a soil and you've measured some things about it. The first thing we would measure is the percentage of silt and very fine sand. And the example they use here, we say we have an example 65% silt or very fine sand. From there, we move across until we reach this set of curves that are moving from the upper left to the lower right. Those set of curves, we look for the percentage of sand. So we have some percentage of silt and we also have a percentage of sand ranging from 0.1 to two millimeters. And we look and we can see that there are curves for each of those things ranging from 0%, 5%, 10%. And the example here, we actually keep going until we reach the point and we're saying in the example that we have a 5% measurement of sand in this example. Well, from there you turn the corner and you move vertically above that point, that point that represents 65% silt and 5% sand. And we move up to this next curve which represents our percentage of organic matter. Well, organic matter again makes it very sticky. We see there are curves for 0% to 4% organic matter. And the example presumes that we have 3% organic matter. We touch that point in the curve on the 3% curve that corresponds with our other two measurements. And then from there, we once again move horizontally to yet another curve. Travel across to this curve. In this case, the place in this curve we're touching is a place that's as a number two on the soil structure. Well, there's a formula for soil structure here saying if we have fine granular material, that's what the number two represents here ranging from blocky, platey, or massive of the number four to very fine granular at number one. Well, if we are corresponding with number two, notice we now have one, two, three, four. This is the fourth factor that we're taking into account in this soil erodibility. We'll move down to our final factor here, which represents a chart of permeability where six is very slow ranging from one which is very rapid, a measure of permeability of the soil, which has something to do with the compactness of the soil. And we're assuming a value of four here, which is slow to moderate permeability. You can measure that by filling a column of soil and pouring water on top of it and seeing how fast that water moves through. And if we use that number of four, we once again now we're going to move horizontally back to this final measurement, which corresponds to a soil erodibility factor of 0.3. So again, this erodibility factor has a number of different characteristics associated with it. And this chart can capture all those characteristics in one factor, but uses graphical means instead of mathematical or formula means to do so. The next factor we look at is the length slope factor, basically having to deal with how steep the land is, okay? And in that case, there are two pieces that we care about here. We care about the gradient, which is the rise over run measured here as a percentage. So 0.5% is actually a value of 0.005 when we talk about the actual numerical value. And then you also care about the length of the slope in feet, how long is the area that you're considering. And we use different values here and we would consider some place on the chart that would represent both the slope length and the gradient of the slope. And that would give us a length factor that we would read from there. Other factors include what kind of management you had occur, cropping management. So the presence or absence of a crop or a crop residue will be a factor. For example, if you've taken everything off the land and you basically just have open dirt, you're going to have a substantial amount more erosion than if you have a crop that is on top of that and different crops grow differently. For example, corn tends to have a fairly strong stock that holds things together, but a fair amount of dry dirt around it, whereas grasses such as wheat or other crops like that are more close together and we're probably going to adhere to the soil a little bit better and hold it in place. So there are numbers that are given depending on the various factors. Notice a fallow field, a field that doesn't have any crop on it and sort of maintained as being tilled regularly and is basically soil has a value of one, whereas the other types of canopies will reduce the overall erosion runoff by various factors. Last but not least, there is contouring, which are certain methods that we can actually use here. Different methods of farming or different methods of caring for the land will actually help reduce the amount of erosion. For example, the idea of contouring or terracing are different ways of making sure that you have different levels or turning corners so things do not run in a straight row. For example, the idea of contouring is if I'm turning the corner here, then if water is running downhill, it's never going to be able to run perfectly along my rows. At some point it's going to, it might be running across rows here, but with rows at one location, but then across rows at another location and that helps minimize the effects of erosion. So again, we have this whole series of factors that we can use. They've been extensively studied and there are numbers that you can use for calculating these things. Some of the values like these ideas of land management and the crop may not come into play if you're talking about something like a soccer field, for example, they may be considered as relatively constant, but all of these factors can be used to calculate a value of erosion in tons per hectare.