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Soil Erosion

John Wiley & Sons

Chapter One

Imagine a small group of processes that have operated on the land since the first rains and winds millions of years ago and in all but the coldest and driest regions. These processes are largely responsible for the shape of the Earth's land surface. They divide the land into drainage basins, sculpt the mountains and the valleys, and form the hillslopes and stream channels. They are capable of stripping the fertile topsoil from the land, topsoil that was tens, hundreds, or even thousands of years in the making. They are capable of destroying the productivity of the land in just a few years or even months, quite literally taking the food from the mouths of men, women, children, and other fauna. This small group of processes is known as erosion and sedimentation, and includes detachment, entrainment, transportation, and deposition of soil and other earth materials.

On the basis of its temporal and spatial ubiquity, erosion qualifies as a major, quite possibly the major, environmental problem worldwide. Due to its temporal and spatial ubiquity, together with its numerous impacts, erosion is an essential research topic for physical and social scientists alike. Serious efforts by farmers, miners, contractors, and the personnel of several government agencies arenecessary to protect the soil by means of effective erosion-control programs and to minimize both on- and off-site damage resulting from erosion and sedimentation.

Today, the rate of soil erosion exceeds the rate of soil formation over wide areas resulting in the depletion of soil resources and productive potential (Figure 1.1). This disparity between erosion and soil-formation rates usually is the result of human activities. As the global population increases and the demands for food, shelter, and standard-of-living expectations increase, soil depletion proceeds at faster rates and over wider areas. In the United States alone, about 57.3 million acres (23.2 million hectares) of fragile highly erodible cropland was determined to experience excessive erosion, and about 50.5 million acres (20.4 million hectares) of non-highly erodible cropland was determined to have erosion that exceeded the tolerable soil-loss rate [U.S. Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS), 1997b revised, 2000]. The soil-loss tolerance is the "maximum level of soil erosion that will permit a high level of crop productivity to be sustained economically and indefinitely" (Wischmeier and Smith, 1978); the utility and limitations of this concept are discussed in Chapter 8. The tragedy, of course, is that the technology exists to control erosion rates in nearly all circumstances.

During the past 30 years, many studies have documented the magnitude of soil-erosion problems, expressed as billions of tons of eroded soil or billions of dollars of erosion and sedimentation damage each year (summaries by Lal, 1994a; Morgan, 1991). Most authors acknowledge that these data are imprecise, due to the temporal and spatial variability of erosion processes, the paucity of accurate erosion measurements, extrapolation of data from small plots to continental scales, and the conversion of erosion and sedimentation rates into monetary units (Boardman, 1998; Crosson, 1995; Lal, 1994a; Osterkamp et al., 1998; Pimental et al., 1995; Ribaudo, 1986). Nevertheless, by any measure, soil erosion is a monumental problem throughout the world, threatening ecosystems and human well-being.

The average erosion rates for very large areas misrepresents the true dimensions of erosion problems. An average value disguises the areas of low and high erosion rates. Some parts of a large area frequently experience low erosion rates that may not be problematic or may be controlled through minor and inexpensive modifications of cultural practices. Other parts of a large area often experience high erosion rates that require substantial efforts and resources to control erosion. Often, a small proportion of a land area is responsible for a large proportion of the total erosion and sediment yield. Erosion control targeted toward the areas with the highest rates can markedly reduce erosion averages.

In most cases, long-term soil productivity and long-term sustainable agriculture require soil-erosion rates that do not exceed soil-formation rates. Soil productivity is the capacity of a soil, in its normal environment, to produce a particular plant or sequence of plants under a specified management system (National Soil Erosion-Soil Productivity Research Planning Committee, 1981). Soil productivity can be maintained and even enhanced, at least in the short term, through the use of high-yield plant varieties, pesticides, and fertilizers. These practices, however, are not economically feasible in some cases, reducing farm-business profitability, and sometimes cause other environmental problems, such as water pollution due to the transport of pesticides and fertilizers in the runoff from fields. Further, the transfer of these technologies to the "developing world" often is limited by economic and other cultural conditions.

The development and management of effective erosion-control programs require a thorough understanding of erosion processes, the ability to measure and estimate erosion rates accurately, and a knowledge of the theory and practice of erosion-control techniques. The goal of this introductory soil-erosion textbook is to (1) provide a fundamental knowledge of erosion processes, measurement, estimation, and control; (2) identify sources of more detailed information; and (3) lay the foundation for a career in erosion research and soil conservation.

PHYSICAL AND ECONOMIC SIGNIFICANCE OF EROSION

Soil erosion affects the land and its inhabitants in various direct and indirect ways. In this section, the physical and economic ramifications of erosion are discussed and social issues are addressed later in the chapter.

Changes in Soil Properties

Soil properties are the product of pedogenic (soil-forming) processes, frequently modified by human activities. Properties such as material strength, infiltration capacity, and plant productivity are altered by erosion processes (Appendix A). Soils possess strength properties that largely determine the ability to resist stresses. These properties often change in the long term as the result of weathering, pedogenic processes, and decomposition of organic matter, as well as in the short term as a result of seasonal climate conditions. Accelerated erosion removes the upper layer (A-horizon) of the soil, exposing the underlying layer (B-horizon) that may possess different strength properties and, hence, different abilities to resist the stresses imposed by gravitational forces, raindrop impact, and surface runoff.

Another important property of soils is the infiltration capacity, defined as the maximum rate at which water enters the soil. The infiltration capacity of the soil divides precipitation into soil moisture, groundwater, and surface runoff (Appendix B). Soil moisture binds soil particles together, reducing wind erosion rates. Water flowing beneath the surface produces pore-water pressures that reduce the friction between soil particles, making the particles more susceptible to gravitational and erosive forces. Subsurface flows may emerge downslope as seepage and contribute to surface flows. Once the precipitation rate exceeds the infiltration capacity of the soil, runoff collects and flows across the land surface, generating the hydraulic forces that erode and transport sediment from hillslopes and through stream channels. The infiltration capacity of the A-horizon often is substantially higher than the infiltration capacity of the B-horizon. When erosion removes the A-horizon, exposing the B-horizon, a precipitation event of given intensity produces greater runoff volume and velocity, due to the lower infiltration capacity, and causes higher erosion rates, depending on the susceptibility (erodibility) of the B-horizon material to erosive forces.

Soils possess physical and chemical properties that strongly influence vegetation growth. Productivity suffers from too much or too little water. Slow-draining soils may become waterlogged, akin to overwatered household plants, making the soil suitable only for hydrophytic (wetland) plant types. Fast-draining soils may not retain adequate water to support other than xerophytic (desert land) plant types. The water-holding capacity of a soil is related to the particle-size composition. Fine-textured soils possess greater surface area to which water molecules can adsorb and be stored than do coarse-textured soils. As a rule of thumb, soils composed of 70% or more sand-size particles are considered to be droughty. When fine-size particles are removed from the soil by water and wind erosion processes, the water-holding capacity of the soil decreases. The decrease in water-holding capacity adversely affects plant growth if water becomes a limiting factor. In addition, the reduction of soil depth due to erosion decreases the volume of soil involved in water and nutrient storage.

Fertility is the capacity of a soil to provide the quantities and balances of elements and compounds necessary for plant development. Plant nutrients are stored and cycled through the upper layers of soil. Removal of these layers by erosion diminishes the quantities of nutrients available for plant use. Some plant nutrients and pesticides (including insecticides, herbicides, fungicides), whether naturally occurring or applied by the farmer or reclamation specialist, are adsorbed to sediment particles and transported from the site by runoff and erosion. Figure 1.2 shows the spatial variability of potential pesticide runoff in the United States.

Economic Consequences of Erosion

The economic consequences of erosion can be examined at local (field, farm, or construction site), regional, and national scales. Changes in the physical and chemical properties of the soil affect farm-business profitability by reducing crop yields or increasing management requirements to maintain yields. According to the USDA, NRCS (1997a), soil erosion continues to threaten the productive capacity of nearly one-third of the cropland and at least one-fifth of all rangeland in the United States Soil erosion reduces crop yields by reducing soil organic matter, water-holding capacity, rooting depth, and the availability of plant nutrients, as well as degrading soil structure and altering the soil texture (Weesies et al., 1994).

It has proven somewhat difficult to accurately document the relationship between soil erosion and land productivity because both soil erosion and productivity rates are influenced by numerous conditions that vary temporally and spatially. For example, Olson et al. (1994) observed that the variables complicating the relationship between soil erosion and soil productivity include (1) landscape position and hillslope components, (2) surface and subsurface water flow, (3) natural versus artificial erosioncontrol treatments, (4) soil properties, and (5) past and present land management. In addition, climate variability from year to year affects both soil erosion and productivity during field studies, complicating data analyses and interpretations.

Soil erosion is an insidious process that attacks the most productive topsoil layer first and may cause decreasing productivity at imperceptible rates over extended periods. Thus, the decline in soil productivity often is masked by planting high-yield crop breeds and by increasing the applications of fertilizers and pesticides where the financial resources are available to permit these investments (Follett and Stewart, 1985). Before the widespread use of commercial fertilizers, loss of topsoil reduced yields 50% or more compared to yields from soils with little topsoil loss (Weesies et al., 1994). Extensive analyses revealed that the reduction in crop yield depends on soil and climate characteristics as well as fertilization rates. Soil erosion affects farm-business profitability, in both the short and long term.

The cumulative effects of high erosion rates may have national and regional consequences. Since 1982, cropland in the United States, declined by about 11 million acres (4.5 million hectares) or 2.6%, pastureland declined by almost 12 million acres (4.9 million hectares) or 9%, and rangeland declined by about 11 million acres (4.5 million hectares) or 2.6%, while forestland increased by 3.6 million acres (1.5 million hectares) or 0.9% (USDA, NRCS, 1997b, revised 2000). Former croplands in the northeastern and southern states now support forests. Many acres (hectares) of Mississippi River bottomland forests and Great Plains grasslands are now croplands (USDA, NRCS, 1997a). In fact, there are several reasons for these land-use changes, including changes in agrobusiness economics, soil depletion due to past erosion, and the influences of erosion-control programs, in addition to competing land uses, especially urban and suburban development.

In an effort to protect soils vulnerable to water and wind erosion, the Conservation Reserve Program (CRP) was created in 1985. As of 1997, 32.7 million acres (13.2 million hectares) were enrolled in the CRP program and taken from production. The estimated average annual sheet and rill erosion from cultivated cropland was 3.1 tons/acre (7 metric tons/ ha) per year, while the sheet and rill erosion from CRP land was 0.4 ton/ acre (0.9 metric ton/ha) per year. Estimated average annual wind erosion from cultivated cropland was 2.5 tons/acre (5.6 metric tons/ha) per year, while wind erosion from CRP land was 0.3 ton/acre (0.7 metric ton/ha) per year (USDA, NRCS, 1997b).

Conservation reserve programs that retire erosion-vulnerable lands from production are feasible in the United States, due to the abundance of fertile croplands coupled with the economic and technical resources to maximize productivity on other lands. Today, each acre (hectare) of cropland produces nearly three times as much food and fiber as that which was produced by that acre (hectare) in 1935 (USDA, NRCS, 1997a). As a result, the demands of the U.S. marketplace are satisfied at prices lower than those for other industrial countries. The question is whether or not high levels of productivity are sustainable through future generations. There are about 108 million acres (43.7 million hectares) of cropland in the United States where the annual soil-erosion rate exceeds the soil-loss tolerance rate (USDA, NRCS, 1997b). Figure 1.3 shows the spatial variability of cropland erosion in the United States.

The situation, however, is very different in other parts of the world.

Continues...

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Excerpted from Soil Erosion by Terrence J. Toy George R. Foster Kenneth G. Renard Excerpted by permission.
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