Soil and Water Quality An Agenda for Agriculture (1993) / Chapter Skim
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9 Fate and Transport of Sediments
9 Fate and Transport of Sediments
Pages 337-360
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From page 337... ...
Sediment deposition in stream channels, irrigation canals, reservoirs, estuaries, harbors, and water conveyance structures reduces the capacities of these water bodies to perform their prime functions and often requires costly treatments. In many instances, the sediments removed from upland areas and channels and subsequently transported downstream carry adsorbed chemicals that exacerbate water quality problems at points downstream.
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From page 338... ...
There are, however, even fewer research data on the definition and control of the wind erosion process than there are for water erosion (Lal, 1988~. Although the basic principles governing wind erosion process and control are similar to those governing water erosion, the specific cause-effect relationships and the effectiveness of wind erosion control practices have not been as widely investigated as have those for water erosion.
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From page 339... ...
The universal soil loss equation (Wischmeier and Smith, 1978) and the revised universal soil loss equation (Renard et al., 1991)
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From page 341... ...
Effective water erosion control requires simultaneous efforts to increase the degree to which and the length of the season during which the soil is covered by plants or plant residues and to decrease the volume and energy of runoff water. Effective wind erosion control also depends on increased amounts of soil cover and reductions in the energy of wind that is in contact with soil particles.
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From page 342... ...
The problems are especially severe along the bluff of the Mississippi and Yazoo river floodplain, where steep gradients promote channel erosion and the low gradients on the flood plain cause the sediment to be deposited. Flood control reservoirs also induce sediment deposition within upstream channels.
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From page 343... ...
Similarly, the wind erosion equation predicts wind erosion rates as the product of five values representing soil erodibility, soil ridge roughness, climate, field length, and vegetative cover. Since the original and revised universal soil loss equations and the wind erosion equation were first introduced, their systematic approach has had a tremendous effect on erosion technology and conservation planning.
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From page 344... ...
The more versatile WEPP model incorporates many of the scientific advances that have been made since development of the universal soil loss equation and is based on the physical principles and processes of soil erosion by water. A companion effort by other Agricultural Research Service, Soil Conservation Service, and university investigators is under way to improve the predictive capability of sediment loss from wind erosion, culminating in the wind erosion prediction system (WEPS)
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From page 345... ...
are capable of addressing hydrology, erosion, and sediment transport but are intended for the simulation of discrete events. In other instances there is a need for continuous simulation of runoff and sediment yields from watersheds and river basins where the impacts of conservation practices must be assessed.
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From page 346... ...
The review of Nearing and colleagues (1990) is based on experience through the development of the WEPP hill slope profile erosion model, which is a computer-based technology for estimating rill and interrill soil losses on hill slopes.
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From page 348... ...
With the new simulation models, we can begin to address more fully temporal and spatial distributions of soil loss and sediment yield, confidence limits for our erosion estimates, and probabilities of meeting conservation goals with given management systems. Larson and colleagues (1990)
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From page 349... ...
A major problem with modeling upland erosion and sediment transport where concentrated flow begins involves the hydraulic transport process. Most sediment transport processes for upland erosion models are taken from those developed for stream flows.
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From page 350... ...
As the new wind erosion prediction technology becomes operational, considerable work will need to be done to develop the data bases required for its implementation over the wide range of environmental conditions that occur in the United States and worldwide. As with water erosion, wind erosion prediction technology will require development of associated technologies such as expert systems, digital elevation models, and geographic information systems.
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From page 351... ...
Farming Practices The effects of different types of plant cover, tillage, and cropping systems have been evaluated on erosion plots and watersheds and by using rainfall simulators and wind tunnels. Various types of conservation tillage practices have been developed and evaluated.
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From page 352... ...
areas if I: Act: ^~m~ ~s~i~-e~s~S~ ~ Ajar hat!
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From page 353... ...
Table 9-2 compares the amount of surface soil cover, soil erosion, and runoff from a rainfall simulator for three different wheat tillage systems. Increased soil cover was found to greatly reduce both water runoff and soil erosion.
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From page 354... ...
Reprinted with permission from 13 Soil and Water Conservation Society. TABLE 9-3 Runoff and Soil Loss from Watersheds under Conventionally and Conservation Tilled Systems Conservation Tilled Watershed Conventionally Tilled Watershed Runoff Soil loss RunoffSoil loss Year (mm)
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From page 355... ...
Channel Management Channel erosion tends to increase when there are low sediment loads from decreased upland erosion and tends to decrease when there are high sediment loads from increased upland erosion. Thus, erosion control on farm fields and upland areas, such as that which might result from the use of conservation tillage or grassed waterways, may result in excessive channel instability if runoff is also not controlled.
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From page 356... ...
Upland erosion control programs may also reduce runoff rates and amounts, but it may be accomplished at the expense of reduced water supplies downstream. Channel maintenance programs suffer from the perceived idea that engineering efforts associated with, for example, bank stabilization, grade control, dredging, and energy control are aesthetically unacceptable and destroy wildlife and biological habitats.
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From page 357... ...
Upland treatments may increase the retention times of water runoffs and thus reduce the flood peaks and their associated erosive capacity through channels and limit the delivery of sediments to channels subject to deposition. Channel maintenance involves control of the energy of the flowing water in such a way that erosion of the stream banks and streambed is minimized in an environmentally and aesthetically acceptable way.
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From page 358... ...
In addition to climatic variability and hydrologic differences, some areas of the United States have influent rather than effluent streambeds, which change the nature of downstream impacts of channel instability. Channel Dynamic Conditions The channel dynamic conditions must be incorporated into water resource analytic models in sufficient detail to permit meaningful assessment of the role of engineering structures and channel heterogeneity.
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From page 359... ...
In more arid regions such as those in the western United States, influent streams that result when the water table is below the level of the channel bed result in unique problems in maintaining stable channels. Vegetation in such zones is controlled by the water table depth, which in turn affects above- and belowground biomass and the natural stabilities of the streambeds and stream banks.
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