Causes and Effects of Water Erosion

Causes and Effects:

I would like to use the following diagram in which I'll explain what I mean with Catchment system dynamics.

I view this as a system in which processes and feedbacks interact. The system in terms of water and sediment and the movement within that is impacted by natural and human drivers. So we have natural drivers, such as a wildfire or climate change or land cover changes, or human drivers, such as Catchment management or land and water management. And if you understand how the proces and feedback within such a system work, then you can also know what the consequences of such drivers would be. So, for instance, how much sediment will be accumulating downstream, or how much water will be available for crop growth. And if you then really fully understand how the processes interact with these drivers, then you can also try to manipulate using specific management strategies to come to a sustainable situation. I want to use for that this diagram from Professor  Lane, which was introduced already in 1955. So it is a very old diagram, but I still think that it clearly shows how water and sediment interact with each other within a Catchment system.

So you have on one end of the balance the amount of water, and on the other end of the balance you have the amount of sediment and the size of the sediment. And on the side of the water you also have the slope of the stream. So what happens for instance if your system always tries to be in an equilibrium, and what happens if your system comes in disequilibrium? For instance by climate change, you get less water available because the rainfall decreases, then this end of the balance becomes lighter and this arrow will then go this way and your system in the river will aggravate. So, more sediment will be deposit in there. On the other hand, you can for instance have a situation in which you have less sediment available. For instance, because of reforestation, because the amount of sediment coming from you hill slopes will decrease. This means then that this part of your balance will become lighter and your system will degrade in your river system.

One of the key drivers that are very clearly to be seen in a natural system, is a wild fire. If a wild fire occurs, your system basically becomes an outdoor laboratory. All your vegetation is gone and therefore your availability of your sediment will be much higher. So, you will have a lot of accumulation of sediment in the base. And off-site effects of this situation will be quite fierce. You can see here in this picture. You can see the off side sediment accumulation here in the base, which does not only involve sediment, but also other types of debris.

Also climate change can have such an impact. Either by creating droughts or creating floods. So much more off-site water availability. Then you have also agricultural management or land management, which has a big influence on the balance between water and sediment. It basically always boils down to whether you have the availability of your sediment to be moved and the amount of water that has the capability to move the sediment. So if you for instance in a agricultural management system have a cover crop in between your vines, as in this vineyard is shown, then the availability of your sediment will be less.

If you, in this picture, see that you are plowing your field, this means that the availability of your sediment must be much higher. But also the roughness of the system will be much higher so again that has an impact on the balance between the available sediment to be transported and the capacity of the water that transport it. 

And then this sediment of course will be moved downstream, creating a problem off-site. Here, you can see that the sediment has been accumulating in the drainage ditch next to the road creating a big financial problem, because of course now this sediment needs to be removed and that is very costly.

Reducing the Effects of Water Erosion:

I'll be discussing ways of reducing the effects of water erosion. It’s avoiding erosion from the start, so it’s really about three basic principles, that I will explain that are important here. First of all, it’s the land capability. That has to do with the soil type, the slope and the location of the land. So is the crop that we want to grow is it actually suitable for the soil type that we have, given the climate that we have? Or, the slope, is it not too steep for agriculture at all? So, land capability. The next one is surface cover. Well, surface cover is really a vital issue. It can be living plants, it can be residues of crops, but they protect the soil against the impact of the rainfall. And also they provide more time for water to infiltrate into the soil. The third one then, is runoff. If runoff does occur, then we should try and reduce it as much as possible, so that it does not get an erosive pulls. Also, by considering these 3 principles of ensuring the soil is appropriately used, maintaining soil cover and minimizing overland flow, will help to minimize water erosion.

Soil and water conservation measures are appropriate techniques or approaches to prevent the loss of soil or water. There’s a vast number of options that we have, so we have to look at the local soil type and the slope again to determine in the end what is the best choice for a measure to take at a certain location. And also remember that there’s very many developments all over the world but they still always need to be adapted to the local situation to work. So, there are four different categories, that are structural measures, vegetative, agronomic and management measures. And I will give you an example and explain them each.

To start with structural measures, they are mostly well known because you see them very often in the field. So, they are terraces, like you see in the picture, can also be stone bunds, gabions or half moon structures. They are relatively permanent in the field. They reduce usually the slope, the steepness of the slope, thereby increasing the infiltration and reducing erosion. However, what is a disadvantage of them is that they are quite expensive to install. Another issue of structural measures, is their maintenance. So, you have for instance here a gabion, which is the wall inside that gully, what will happen here is that the gully actually will move around it. So, the structure didn’t really work in this case.

And the maintenance is an issue, because often a project lifetime is short, only a couple of years maybe, so as long as there’s money, they can be maintained. However, if the project stops, there is no money or incentive anymore, to keep the maintenance and then they may collapse. And that’s a waste of money of course.

Vegetative measures, as the word implies, they employ some kind of vegetation. So either shrubs or trees, or grasses. For example, grass strips along the contour of a hill slope or a grassed waterway.

And what they do is they reduce the speed of water when it flows over. An other example is agro-forestry. So that’s a whole combination of different types of vegetation that works together to protect the soil.

Third one is agronomic measures, as they are called. It’s actually everything the farmer does on the field, so ploughing or mulching. As opposed to the structural measures, that are permanent, these type of measures are usually not. So, because the farmer has to use the field every year, they will be repeated every season or every year. Mulching is leaving crop residues purposely on a field, and so they protect the soil from the raindrop impact. Remember that surface cover was a very important issue. So they are left on the field after the harvest. However, an issue here is actually the looks of the field. Well, farmers like their fields to look good. To look nice, neatly ploughed. If this mulch is left on the field, it may look a little bit messy.

At last, there are the management measures. They are different from the others, in the sense that they have a fundamental change of the land use. So, examples are rotational grazing or area closure, but also prescribed fire, stall feeding or compost. So different types of land use. And that implies that they reduce the intensity of the land being used. So remember about the capability of the land? That’s an issue here. However, you should also keep in mind here, that the cattle needs to graze somewhere.

So, if you close an area somewhere on one place, you need to provide an alternative on another location. Or you might run the risk of replacing one degradation spot by another location.

However, whatever is selected should be appropriately adapted to local conditions and be well maintained. If this is achieved then the measure or measures can reduce the impacts of water erosion and ensure the sustainability and productivity of the soil for life.