Water Erosion
Introduction
Water erosion occurs when water impacts the soil surface and displaces soil particles and when water flows over this surface, it mobilizes the soil particles causing the top soil to displace. While this process will always occur naturally, this natural process has been accelerated by agriculture. This is because most land used for agriculture will have most of the topsoil exposed to elements as vegetation is usually removed prior to farming or grazing thus water erosion is a problem. Water erosion results in the loss of topsoil, reduced crop yield due to the latter, damaged infrastructure, dispersal of weeds and silting of natural waterways. With the loss of topsoil, the subsoil would be exposed. Subsoil is very nutrient deficient and would not be suitable for use for farming as there wouldn't be enough nutrients to support the growth of the crop. Replenishing the topsoil naturally will also be difficult.
The 2001 National Land and Water Resources Audit showed that almost 40% of the continent is affected by sheet and rill erosion. 40% of Australia's land is facing medium erosion while 11% is facing high levels of erosion. The Murray-Darling Basin, a basin 1/7 of Australia's total land is facing erosion at 40% of its surface area. This has lead to excess soil salinity and water salinity. An estimated 4.4 billion metric tonnes of soil has been eroded since first European Settlement. These statistics show the extent of water erosion to Australia's lands and communities. It is also estimated that for every dollar that is spent for excess soil salinity, $5 is spent on water and wind erosion. The photograph below shows minor rill erosion which is a smaller scale of what is happening in Australia.
The 2001 National Land and Water Resources Audit showed that almost 40% of the continent is affected by sheet and rill erosion. 40% of Australia's land is facing medium erosion while 11% is facing high levels of erosion. The Murray-Darling Basin, a basin 1/7 of Australia's total land is facing erosion at 40% of its surface area. This has lead to excess soil salinity and water salinity. An estimated 4.4 billion metric tonnes of soil has been eroded since first European Settlement. These statistics show the extent of water erosion to Australia's lands and communities. It is also estimated that for every dollar that is spent for excess soil salinity, $5 is spent on water and wind erosion. The photograph below shows minor rill erosion which is a smaller scale of what is happening in Australia.
Interesting fact: Erosion comes from the latin word 'rodere' which means to gnaw. The term rodent also comes from the same latin word. There are some similarities between erosion and rodents!
Causes
Water erosion is caused by water, either from the rain or from artificial sources. Water erosion is made worse by unsustainable farming and grazing because both of these activities reduce the vegetation, which is vital as it protects the soil from receiving the true brunt of water. There are many stages of water erosion, each varying in size, severity and the impact of the ecosystem or economically.
There are 4 main types of water erosion in Australia: sheet and rill erosion, tunnel erosion, gully erosion and stream channel erosion.
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The diagram above shows the stages of water erosion. Water erosion starts with the splashing and the impact of the water droplets. This transits to sheet erosion, then rill and gully erosion. Finally the gully that was formed may transform into a running stream of water if enough rain persists.
Effects
Water erosion is in many ways very similar to wind erosion. Both types of erosion increase in severity when there is a lack of land cover (vegetation). Water erosion affects mostly farms however infrastructure is sometimes affected. Water erosion has the following impacts:
- Plant growth is severely affected by water erosion. Since water erosion normally washes away layers of topsoil, the nutrients within the soil is also washed away. Since the roots of most crops are not adapted to levels below topsoil, many plants have stunted growth levels when planted in areas that have sustained moderate to heavy water erosion. This results in damage to the environment and economical effects.
- Increased flooding is an effect of water erosion as the eroded topsoil can absorb water in the efficient manner while the subsoil underneath has reduced water soaking capacities. This results in increased flooding during heavy rains when the subsoil cannot absorb more water than the surface water collecting. This large collection of water can also cause damage to infrastructure such as roads and buildings vulnerable to flooding. The image on the right shows water eroding the soil and dirt underneath the road. This has resulted in heavy damages to the road.
- Water Quality is affected when the water carrying the eroded soil enters the waterway. This pollutes the water with soil particles and often, pesticides from the farms. Soil particles, especially nitrogen and phosphorus when diluted in the water, reduces water oxygen levels and water qualities. Fish that are living within the polluted waterway may succumb to these lethal conditions. These polluted waterways may also reach major rivers and oceans further spreading the eroded soils. This polluted water affects humans and the animals that utilize the water.
- Excess Soil Salinity is a major problem that is related to water erosion. The increase of water impacting the soil and reaching the water table raises the water level within the water table. This brings up the salt that resides on the surface of the groundwater stored within the water table. When the salt reaches the surface, the soil increases in salinity. This is a major problem that many areas around the world and Australia faces.
Effective Solutions
Water erosion can be prevented by many ways. Such measures include:
- Increase of vegetative cover- The main factor of water erosion is the fact that rain impacts the bare soil, allowing the soil particles to mobilize. A natural and efficient way in preventing the loss of topsoil is with the increase of vegetative cover in a particular area. On average, over 70% of vegetation cover protects the soil from surface water flows thus reducing the water erosion by over 80%. The plantation of vegetation or implementation of interceptors in up-slope areas will also prevent long runs of surface water up or down hill which is a major concern in mountainous regions in Australia.
- Surface water drainage- Another solution to face water erosion is the implementation of surface water drainage. These drainage systems reduce the velocity and volume of the water during peak flows, thus reducing the amount of soil being washed away at peak flows. The excess water is normally directed to water storage as diagram 1 shows, or natural drainage lines leading to a river mass with the latter being the preferred method due to the lower costs and complexity. Understanding the contour of the land would allow a farmer to decide which of the two would be the better choice. The understanding of the different soil types, basics of good crop residue and grazing animal capacities is also very important.
- Restrict stock access- The restriction of stock access requires pastured areas with livestock to be monitored regularly for erosion potential. When bare patches in the ground can be seen which is normally caused by erosion or physical erosion by the animals. Reducing stock rate, removing stock or simply fencing the 'at-risk areas' of the paddock is recommend to prevent further damage to be caused to the degraded area of land. While this option may not appeal to most people due to reduction of the short-term economic yield, in the long-term the costs and time required to reverse the damage would be multiplied. These bare patches are most likely to occur prior to opening rains and severe drought.
- Maintaining the soil condition- Maintaining good soil condition would prevent soil degradation and not allow the soil to degrade to a point that replacing or treating the soil would be required. This would be the cheapest in the long-term. Keeping the soil in good condition requires keeping the livestock or farming rate at a sustainable level and managing good soil structure and improving soil nutrients. Good soil structure and soil nutrient level can be maintained be visual assessment and the use of instruments.