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Erosion affects crop productivity and remains the largest cause of water pollution on Earth, depositing nutrients, sediments, pesticides, and fertilizers into water supplies. There are two types of erosion: natural and human-induced. Erosion prediction and the need for soil conservation became a focus in the twentieth century under President Roosevelt’s New Deal, which helped spread the word about the threats of erosion.
One of the less-appreciated constants of world history has been soil erosion, because its effects may be unnoticed before crop productivity wanes. Soil erosion causes damage in two main places: where the removal occurs and where the sediment deposits. Where the erosion occurs, it removes particles, organic matter, and important nutrients since many dissolve into water. Thus the problems of on-site soil erosion are the physical loss of the medium of plant growth, nutrient depletion, and either land abandonment or the cost of conservation and reclamation. Severe erosion has removed as much as 50 meters of soil and sediment (or more) from surfaces, creating canyons where cornfields existed a few decades before. The off-site problems of erosion are at least as severe and include water pollution, sedimentation, and property burial. Indeed, soil erosion creates the largest water pollution problem on Earth by carrying nutrients and fertilizers, sediments, and pesticides into stream channels. Sedimentation fills up channels that must be dredged, or the channel capacity decreases, which cuts down on its holding capacity and increases flooding. Sedimentation has also buried whole towns and covered many valleys with several meters of often much less fertile sediment.
We can view the history of soil erosion as spanning several periods. It started long before human history as geological or “natural” erosion, which is generally a slow process, but given enough time it carved mile-deep and spectacular canyons. This soil erosion occurred in several temporal modes, but it was generally slow and steady over millions of years, though it could be episodically rapid and discontinuous. A second wave started with human-induced or human-accelerated erosion, when humans became technologically advanced enough to disrupt the surface vegetation through fire and the girdling of trees. Evidence suggests that cooking fires go back over 1 million years, but evidence indicates that the use of fire to control vegetation, and thus causing erosion, clearly started as a hunter-gatherer phenomenon in the Pleistocene era (about 60,000 BCE) in what is now Tanzania.
Significant soil erosion started when humans domesticated animals and plants, removed vegetation from larger areas, and thus intensified land use. This erosion presumably began with domestication and concentrated settlement around ten thousand years ago in the Near East and later elsewhere. A third period of erosion probably started with more active trail formation, continued active removal of vegetation for settlements, and soil manipulation for seedbeds. The first actual evidence for erosion seems to lag behind the earliest evidence for agriculture. This lag is about one thousand years in Greece, where the first erosion occurred in some regions about 5000 BCE. The lag also occurred in Mesoamerica, where evidence for agricultural-induced land-use change occurred around 3600 BCE, but the first wave of sedimentation from erosion occurred by 1400 BCE. Generally, this early erosion accelerated with the Bronze Age civilizations of Eurasia and the Early Preclassic (before the first millennium CE) Americas as pioneer farmers ascended from the river valleys and lowlands and deforested steeper slopes in Mesopotamia, Mesoamerica, the Mediterranean, China, and the Indus Valley. Soil erosion waxed and waned in ancient cultures after this period, depending on soil conservation, climate change, and land-use intensity. In some parts of the Classic Americas (about the first millennium CE) in Mesoamerica and the Andes, soil conservation features sustained heavy soil use with high populations, though some studies argue that high soil demands and insufficient conservation figured in declines and collapses. The evidence for the Mediterranean is variable; there is some evidence for soil stability and some for erosion and sedimentation during the highly populated and intensely managed Hellenistic and Roman periods.
A fourth period of world soil erosion occurred with the vast breaking up of new lands around the world that resulted from colonial settlement during the sixteenth to the twentieth centuries. For the first time in history, large areas of previously uncultivated land fell under the plow in the Americas, Oceania, Siberia, Asia, and Africa. Moreover, farmers used to the relatively mild climates and low slopes of western Europe began to farm areas on steeper slopes with much more intensive precipitation or drier and more wind-erosionprone conditions. These farmers were pioneers who came with little knowledge about their environments and ignored what conservation indigenous people had practiced. This ignorance led to devastating rates of soil erosion and lost land productivity.
The final period of world soil erosion came after World War II, with the expansion of mechanization and population growth fueled by better food and medicine. What once had been remote or marginal lands, such as steppes and tropical forests, became farmable due to the imperatives of high populations and the growing markets for tropical crops like coffee and bananas. Expanding populations and a variety of displacement processes drove farmers into lands extremely susceptible to erosion, such as that in the mountains of Central and South America, Africa, and south and east Asia. The mechanics of soil erosion alone explain why recent agricultural and wood-cutting expansion upslope into hills of Haiti, Rwanda, Madagascar, and Nepal have made human-induced soil erosion the largest agent of geomorphic change on the Earth today.
Unfortunately, even recently in the United States, with its large conservation efforts and scientific capability, almost one-third of its agricultural lands are eroding significantly faster than soil is forming. Despite this, American land continues to maintain its productivity, but at the cost of several immediate and long-term problems: sedimentation (the largest water pollutant) and its ecological impact; cost of fertilizer; and more fuel use.
Soil Erosion Processes
We cannot understand the history of soil erosion without recognizing the processes of soil erosion. Soil erosion is the movement of soil particles by wind and water moving in flows, streams, and waves. Geomorphology is the science that studies the processes and forms of the Earth’s surface. Other geomorphic agents also sculpt the Earth’s surface over time, including glaciers, chemical dissolution, mass movements or landslides, and of course tectonic and volcanic activities. For the most part, humans speed up Earth surface dissection in some places and sedimentation in others, playing their part in such processes as landslides, sinkhole formation, and soil, stream, and beach erosion. Soil erosion can start with raindrops that fall up to about 32 kilometers per hour and impact a soil surface, dislodging and splashing particles of mineral and organic matter upward. These particles will land slightly downwind, but this will only lead to slow creep if the vegetation cover is substantial or water does not run over the surface.
This runoff or overland flow is the second important step in erosion, and it only happens when rainfall or water delivery to a point occurs faster than soil pores can take in water (infiltration). Runoff may also occur with snowmelt or ice melt and cause accelerated erosion on surfaces from which humans have cleared vegetation or which has been plowed. Initially with runoff, water flows over the surface and removes thin layers of soil, by raindrops dislodging particles and by the force applied by water flow. This occurs first as sheet erosion as planar flows remove particles evenly from the surface, except for the more resistant soil pedestals that are often left behind as testament to former soil surfaces. This interrill erosion occurs in the belt of no channels on the upper slopes, and can be insidious because it leaves only subtle clues but may cause high soil particle and nutrient losses.
Rills (small streams) start to form downhill from interrills where flow converges and start to dissect soil channels in three directions: headcut, downcut, and laterally cut. Rills can remove large quantities of soil, including whole sections of topsoil, but farmers can plow these out, though the plowing itself may loosen and make soils prone to erosion again. With more flow and greater turbulence, channels enlarge and tend to form in the same slopes where flows concentrate. Since plowing can expunge these larger rills, but they return in the same slope position, they are called ephemeral rills. These areas can be tilled out or left vegetated.
Gullies on the other hand are mature channels that have back, down, and laterally eroded over so large a space that normal tractors cannot plow them out. They can also be formed by waterfall erosion (runoff falls from one surface to another and undercuts the headwall surface), or by piping (water flowing underground intersects the surface, forming a surface outlet channel that erodes a larger area and undercuts surface soils that collapse along the subsurface channel). Gullies often start out being narrow and widen by channel flows undercutting their sides. Water flowing in these channels carries water in suspension and as bed load, rolling, creeping, and saltating (bouncing) downstream.
Human landscape alteration also increases the size and frequency of mass movements on slopes, stream bank erosion, coastal erosion, and wind erosion. Wind soil erosion occurs at natural and accelerated rates as well and over a large part of the Earth, especially on flat, drier, sandier, and less-vegetated areas. The key factors in wind erosion are surface cover, soil coherence, and wind intensity and duration. In many areas where all of these conditions prevail, such as in Loess Plateau of China, which has had among the highest rates of erosion for millennia, water erosion is also very high. The processes of wind erosion starts with sediment load in a channel being carried in suspension by winds fast enough to hold up particles or those particles being rolled or saltated along the ground. Over 90 percent of the sediment is carried less than 1 meter above the surface, and all soil textures (clay, silt, and sand and even gravel) can be carried by wind, depending on aggregation, shape, and density. Winds tend to carry the larger particles like sands over shorter distances as creep or saltation. They can and do carry clays over thousands of kilometers, but clays also cohere into large enough clods that they resist deflation. Thus under normal winds, silt and fine sand is often the texture size that deflates, suspends, travels, and drops out into deposits at predictable distances from the point of erosion. These deposition areas build up and become the world’s extensive loess (wind-deposited, loamy soil) deposits, like those in China, central Europe, the Mississippi Valley, and the Palouse region of Washington State, often fertile but highly erosive landscapes.
Measuring and Predicting Erosion
Humans have recognized the on-site and off-site problems due to soil erosion for millennia. Terracing started at least five thousand years ago, and structures to divert runoff were common in many ancient societies. Yet it was not until the early twentieth century that policy makers and scientists recognized the need to predict soil erosion. In 1908, President Theodore Roosevelt recognized that soil erosion was among the most dangerous environmental challenges. But the affective response to soil erosion in the United States only came during the mid-1930s in the form of the Soil Conservation Service (SCS)—formerly the Soil Erosion Service of 1933 and now the Natural Resources Conservation Service. The SCS was the largest factor in the spread of soil conservation in the United States, made possible by President Franklin D. Roosevelt’s New Deal and enthusiastically championed by H. H. Bennett, the first and most prominent director of the service. The New Deal spread the word about erosion and conservation through funding rural development, art, and science. For example, it organized conservation demonstrations and the Civilian Conservation Corps projects that built check dams and terracing around the United States. The New Deal also used science and scientific management, building predictive models by collecting more than eleven thousand so-called plot years of erosion data from around the United States under different land uses and constant slope lengths and distances. (Scientists can measure erosion using many techniques that have helped them understand both natural and accelerated rates of soil erosion. Measurement has focused on pin studies on natural slopes that have recorded truncation of soils under a variety of land uses and rain intensities and under physically and mathematically simulated conditions.)
Scientists led by Walter Wischmeier at Purdue University forged the plot data into the Universal Soil Loss Equation (USLE), a predictive equation that could be used by farmers and scientists to estimate and compare soil erosion under different crop types and conservation practices. The equation applies well to the regions in the United States from which it was empirically derived, and many studies have adapted it to many other parts of the world with variable success. The equation predicts sheet and rill erosion based on six variables: rainfall intensity, soil erodibility, slope length, slope gradient, crop types, and conservation practices (RKLSCP). Scientists further adapted the USLE into the Revised USLE (RUSLE), which is based on the same set of factors. These equations have become codified as tools for policy and as important foundations of conservation planning for many land uses and are now available for use around the world from the U.S. Department of Agriculture’s Agricultural Research Service (2006). Many scientists have also worked on a variety of physically based or process-oriented models that attempt to simulate the natural, physical processes of soil erosion, such as detachment. This next generation of models, such as the Water Erosion Prediction Process (WEPP) model, should more accurately predict more types of erosion and deposition across a landscape from sheet, rill, and channel erosion.
Soil Erosion in Perspective
Soil erosion has ramped up and waxed and waned through five major periods in world history. Despite twentieth-century advances in understanding soil erosion and conservation in the United States and other developed nations, the rates of soil erosion have not really waned in much of the developing world in the last half-century. Indeed, humans today, through soil erosion, are the leading geomorphic agents on the Earth. The periods when soil erosion ramped up came as the result of technological breakthroughs and population expansions that allowed humans to alter the landscape: applying fire, domesticating animals, centralizing habitation and intensifying farming, expanding onto steeper slopes, and creating a greater demand for tropical crops. In many cases of severe soil erosion, pioneer farmers broke new lands with little understanding of them. History also shows that soil conservation arose at different times, curtailing soil losses and developing stable soil use during periods of increased population growth. The problem has always been how to sustain and preserve soil while speeding up the conservation learning curve of pioneer settlers.
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