Upstream Erosion and Agriculture

Increasing rates of sedimentation from anthropogenic sources is predominantly due to land use upstream; high sediment loads are caused by deforestation, poor agricultural practices and land development (such as roads, mining and other industry).

Most of the intensive land use on a global scale is given over to agriculture. Thus, agriculture is considered to be a major contributor of sediments to freshwaters. This is true in cases of bad agricultural practices (e.g., ploughing at right angles to waterways). In many cases, however, good agricultural practices (these are mainly to do with soil loss prevention techniques, e.g., terracing) have been shown to reduce the rate of sediment input. Soil protection is a prime concern to most agricultural practioners, and soil protection measures decrease rate of sediment input to below those due to natural processes. Thus, prevention of soil deterioration is beneficial to both agriculture and the environment.

Given below are the effects of aspects of agriculture on sediment input:

CONTENTS

1. Effect of Loss of Organic Matter

2. Effect of Irrigation

3. Use of Marginal Lands

4. Effect of Topsoil Loss and Soil Conservation

5. Effect of Land Clearance for Agriculture

1. Effect of Loss of Organic Matter

Soils with a high organic matter content are more cohesive and absorb more water, reducing run off. This is conducive to the growth of vegetation, which, in turn, further stabilizes the soil and contributes to the organic matter content. If soil fertility is reduced through lack of replenishment from upstream sediments, plant growth is reduced and the organic content of the soil steadily declines. As the soil loses organic content, its capacity to hold water and its cohesiveness is reduced and it becomes drier and more barren, increasing the erosion rate. Continued agricultural use of affected areas by, for example, harvesting crops, tilling soils or grazing by hoofed animals, increases the erosion rate of these depleted soils even further. These depleted sediments entering the waterways do little to replenish other soils further downstream. This often leads to smothering more fertile soils, further exacerbating the problem.

2. Effect of Irrigation

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If the methods of irrigation used are good, and used on suitable soils, irrigation can be of minor or even beneficial influence of erosion rates. Established irrigation practices (in some areas irrigation has been practiced, and has continued to be highly productive, for centuries) can be beneficial, however. Land leveling and construction of field bunds tend to reduce erosion rates. The successful raising of crops on irrigated land helps to stabilize soil conditions and maintain a high organic matter content, increasing soil cohesiveness. Irrigated land is, by definition, wetter. This makes the soil less able to absorb rainfall due to saturation, thereby increasing runoff. This could increase erosion rates in areas affected by soil degradation and loss of organic matter and therefore soil fertility. Problems can arise due to loss of soil fertility due to increased salinization of the soil, for example. As crop yield declines so does the organic matter input into the soil, increasing its erodibility, particularly in combination with the high runoff rate. Exacerbating this problem is the fact that previous successful years tend to attract increased human pressure on the local environment. People are attracted to successful areas, or a single farmer may simply increase agricultural pressure on a temporarily fertile area of land. With increased human population there is an increased use of livestock, wood and vegetation, for fuel, food and construction. All these can indirectly effect erosion rates.

3. Use of Marginal Lands:

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As population pressure increases there is a drive towards increasing food production, both at the subsistence level and for local, national and international markets. The more powerful agricultural bodies tend to control the most fertile and optimum areas for mass production of monocultural crops - mainly for the international market. This leads to subsitence farmers being increasingly driven onto more marginal lands. As these lands, by definition, are less productive, an increased effort is required in order to supply enough food. The methods used tend to be not sustainable as marginal lands tend to be steeper, more rocky, low topsoil and organic matter levels. All of these features increase runoff and erosion rates. Due to the nature of economics and control of land use, the more fertile lands in such situations tend to have relatively low density of people, while the more marginal tend to possess the higher densities.

4. Effect of Topsoil Loss and Soil Conservation

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It is evident that the poor farming practices that lead to increased sediment loads in local rivers and hence to coastal areas also result in the loss of usable land. The loss of fertile organic matter is of pressing concern to most farmers worldwide and most use some form of soil conservation in order to minimize topsoil loss for their own benefit. This, counter-intuitively, means that agriculture can effectively reduce sediment loads in rivers through increased stabilization of soils by sensitive tillage techniques and the addition of organic matter to erodable soils, thereby increasing soil cohesion. Hill slopes, in particular, can be subject to high background erosion rates, particularly gully formation. Areas where terraced farming is carried out, erosion rates can be effectively slowed to negligible rates in many instances. This is, of course, dependent on local variables and conditions, but it demonstrates that the relationship between agriculture and sedimentation rates are not necessarily negative and that already established practices for topsoil conservation effectively reduce the impact of agriculture on increased sedimentation rates.

5. Effect of Land Clearance for Agriculture

When land is cleared to make way for crops, there is typically a dramatic increase in sediments entering the river system. The longer the soil is left bare, the longer this effect lasts. Generally, cleared land is quickly replanted, however, with the new growth stabilizing the soil once more. This effect tends to be temporary and can be ameliorated with good agricultural practices and soil conservation techniques. Increased agricultural utilization of land tends to lead to increased population pressure, both locally and over the wider area. The influx of people causes a subsequent rise in construction of housing and road networks. The increase in developmental infrastructure has the effect of causing a more permanent increase in erosion rates. Thus, this may be considered an indirect negative impact of increased agricultural intensity.

While agriculture is responsible for only 10% or less of sediments deposited, when compared to natural rates of sedimentation, it may be that 10% is all that is required to upset delicate ecosystems such as coral reefs and sea grass habitats. These habitats are not only important to the fisheries sector of industry but also as systems of intrinsic value, and as natural coastal defences against coastal erosion.

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