Article

Predicting minimum achievable soil loss in developing countries

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Abstract

Soil erosion is recognized as a critical global environmental problem, but soil loss prediction methods and conceptual frameworks for conservation goal-setting are often inappropriate in developing countries. The minimum achievable soil loss (MASL) concept was developed as an erosion-control goal-setting framework which recognizes the constraints on erosion control in developing areas. A physical model for relative soil loss assessment was developed for implementing the MASL concept. The model is based on Bagnold's stream power concept and a kinematic approximation of surface runoff. It is used to determine minimum achievable soil loss reduction rates for given situations and has input requirements which are appropriate for situations where data and data-acquisition are limited. Tests using data from Malaysia show that the model gives results comparable to those of existing empirical soil loss estimation methods with more stringent data requirements and more limited ranges of applicability.

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... There is a small, but systematic difference in the calculated distances between the two: on average, the distances calculated from the maps with the 20x20m cells are 3.4 % higher. (Phillips, 1989;ISSS, 1996). ...
... A simple model based on these factors is shown in Table 8 (Phillips, 1989 Kwaad, 1998b). The roughness coefficient for forests was not taken from Table 8, but modified. ...
... Factors that determine the Manning roughness coefficient according toPhillips (1989). Very irregular. ...
... Recent advancement in studies shows that satellite imagery and a platform like geographic information system provide a detailed assessment with higher accuracy in estimating a river's soil erosion and sediment flow (Jain et al., 2001). Also, there is a lack of field data or high-resolution data in developing countries as input data for soil erosion models (Phillips, 1989). In erosion risk models, the main problem is validation for comparing calculated soil erosion with measured soil erosion data due to scarcely available information (Lazzari et al., 2015). ...
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... The estimation of Manning's n was based on a modification of the general model n = f (soil surface irregularity, obstructions, vegetation cover, and characteristics) as described by Phillips (1989) using: ...
... As no roughness values for bunds and terraces are available in the literature, alternatively the Manning´s roughness coefficient (n) is determined by applying a step-wise additive/weighted estimation method (Arcement and Schneider 1989). This method computes the Manning´s roughness coefficient by assessing the surface irregularity, the obstructions to overland flow, the proportion of basal ground cover, and the type and height of the vegetation (Engman 1986;Phillips 1989 to the arithmetic mean of the upper and lower contour line values. As a result, a stepped terrain morphology is obtained. ...
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The length-slope factor in the USLE is a purely empirical relationship that was derived from an extensive data base. A physically based length-slope factor was independently derived in this paper by using unit stream power theory to describe the erosion processes associated with sheet and rill flow on hillslopes. It was shown that the two length-slope factors are equivalent. Therefore, the USLE length-slope factor is a measure of the sediment transport capacity of runoff from the landscape, but fails to fully account for the hydrological processes that affect runoff and erosion. - from Authors
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A new infiltration equation, derived exactly from a recent quasi-solution of the governing differential flow equation, is capable of expressing eleven previously published infiltration equations. On the basis of least squares fitting, the new equation in three-parameter dimensionless form expressed each of the eleven previous equations within ±0.31% over all times. If only one parameter was retained and fitted in the dimensionless form, nine of the eleven equations were still expressed within ±2.5%. This one-parameter dimensionless form implies the dimensional infiltration equation I = (S/A0)]1 - exp (-A0t½)] + Kt, which is therefore proposed as a realistic blend of rigor and utility for the complete range of time t(0≦t ≦∞), where I is the cumulative quality of water infiltrated, S is the sorptivity, A0 is a constant arising from the quasi-solution, and K is the sated (satiated, near-saturated) hydraulic conductivity. Because of its broad matching capability, this equation is deemed to hold considerable promise for describing and fitting experimental infiltration data, whether from the laboratory or the field.
Article
Hydraulic roughness coefficients have been derived from runoff plot data originally collected for erosion studies. The data were collected from different agricultural and natural surfaces by applying constant rainfall rates from rainfall simulators. The derived roughness coefficient is actually an "effective" roughness coefficient that includes: the effect of raindrop impact; the effect of channelization of flow; the effects of obstacles such as litter, crop ridges, rocks, and roughness from tillage; the frictional drag over the surface; and the erosion and transport of sediment. A ready reference of friction factors for overland flow is presented in tabular format with a description of the various surfaces and land uses. Surface conditions varied from very smooth asphalt to extremely rough, litter-strewn agricultural and rangeland areas.
Article
Although rill and gully erosions in Zimbabwe are under control, sheet erosion is still a problem, with estimated rates of soil loss of 50 to 80 t/ha/year. Discusses development of the SLEMSA model building framework, to predict sheet erosion losses, from arable lands in areas of high intensity rainfall. Within the physical systems of (crops, climate soil, topography). There are control variables for modelling purposes. These include rainfall energy/interception soil erodibility, slope steepness/length. The resulting sub-models are then combined to give a bed estimate of mean annual soil loss. The purpose of the soil life-span model is to provide a measure of the consequences of an existing soil loss rate. Presents an example application of this model to a communal area, noting the benefits of crop ridges on contoured land. (C.J.U.)
Article
A flexible system of computer grid-square mapping of soil erosion promoting parameters is presented. Maps of potential soil erosion risk and actual soil erosion have been produced using this system. The method is intended for developing countries and has been used for two different projects in Sierra Leone. Constraints on the choice of parameters for environmental mapping and on erosion rate verification in developing countries are discussed.
Article
The emerging perception that soil conservation policy should reduce erosion damages rather than erosion per se is itself shaping new perspectives on soil loss tolerance values, on the relative importance of on-farm and off-farm damages, on the proper targets of conservation efforts, and on the voluntary versus regulatory approaches to conservationa policies. -from Author
Article
In this paper the transporting capacity of thin flows, in the laminar and transitional flow regime, is studied. Experiments were carried out on irregular as well as on plane beds, using two totally different set-ups. The results of these two types of experiment were convergent. In both cases, sediment concentration was clearly related to grain shear velocity and unit stream power, expressed as the product of mean velocity and slope (Yang, 1973). The data agreed with those of Kramer and Meyer (1969). For a sandy bed, the unit stream power relationship was able to predict reasonably well the sediment concentrations measured on a mulched surface. For laminar and transitional flows, both the unit stream power and the shear velocity are related in the same way to slope and unit discharge. The unit stream power is a parameter which in particular can be very easily measured and might therefore become useful in obtaining a quick estimate of the transporting capacity of a thin flow. However, before a sediment transport equation for thin flows can be developed, more information is needed about the influence of the flow regime and grain size and density.
Article
A standardized nomenclature is presented that clarifies the differences among measures of stream power. It is advocated that the term stream power be used in a conceptual context only. The recommended terminology for the various measures of this general concept is total stream power for the power of an entire reach or stream, crosssectional stream power for power per unit length, mean stream power for power per unit wetted area. and unit stream power for power per unit weight of water.
Article
Soil conservation programs and management of sediment pollution in streams are both concerned with land conservation and water quality, but specific management goals are usually governed by separate considerations. Geomorphic concepts, especially the fluvial sediment budget, can be used to examine the relationship between upland soil erosion and sediment pollution in waterways. An example of such an application is given for the upper Tar River basin of North Carolina, showing that achieving soil loss goals will not prevent continuing high stream sediment concentrations and water quality deterioration. The budget concept can also be used to determine whether direct control of sediment input to waterways is needed in addition to on-site erosion control. Usually such applications require assumptions of a constant qualitative character of the sediment budget.
Article
Land surface topography significantly affects the processes of runoff and erosion. A system which determines slope, aspect, and curvature in both the down-slope and across-slope directions is developed for an altitude matrix. Also, the upslope drainage area and maximum drainage distance are determined for every point within the altitude matrix. A FORTRAN 66 program performs the analysis.
Article
The development and use of indices of soil erodibility during the past thirty years are reviewed. The ambiguous record of some of the indices is noted, and a theoretical assessment is made of their validity and limitations. Testing of index efficiency against a standard reference of soil loss from samples subjected to artificially simulated rainfall is described. The efficiency of each index is measured by statistical analysis, and the most efficient indices of soil erodibility are shown to be percentage-weight of waterstable aggregates (W.S.A) > 3 mm, percentage-weight of W.S.A. > 0.5 mm, erosion ratio, surface-aggregation ratio, modified surface-aggregation ratio, and clay ratio. Further statistical analysis is used to show that percentageweight of W.S.A. > 3 mm, is the only completely reliable index of erodibility.
Article
Recent assessments of both the on-farm and off-farm costs of soil loss and sedimentation have emphasised the relevance of research in the field of erosion and sedimentation and this has been further underscored by an increasing awareness of the importance of sediment-associated transport in the movement of contaminants through terrestrial and aquatic ecosystems. In response to these challenges, research has expanded and several new foci of interest can be identified. This review considers three which are seen to be of particular significance. The first relates to a growing interest in the physical and chemical properties of fine grained-sediment, both in terms of the role of erosion processes in controlling the properties of sediment at source and the transport of particulates through river systems. The second reflects the increasing evidence of the importance of storage and remobilisation in the movement of sediment through both small drainage basins and large river systems. The third represents the potential offered by lake sediment studies as a means of reconstructing past patterns of sediment yield and sediment sources.
Article
The linking of on-site rates of erosion and soil loss within a drainage basin to the sediment yield at the basin outlet, and improved knowledge and representation of the associated processes of sediment delivery, represent a major research need within the field of erosion and sedimentation and also an important scale problem in drainage basin studies. This paper reviews the limitations of the sediment delivery ratio concept by considering the problems of temporal and spatial lumping and its blackbox nature. Some recent advances in our understanding of the sediment delivery system and its modelling are described and the lack of detailed empirical investigations is highlighted. The significance of recent concern for the role of sediments in the transport of nutrients and contaminants to sediment delivery studies is introduced, and the need for further work in this field is emphasised.
Soil resistance to water erosion and its dynamics
  • Burykin
Burykin, A. M. (1988) Soil resistance to water erosion and its dynamics. Soviet Soil Science 20, 93-103.
Soil-conserving versus soil-degrading crops and soil management for erosion control
  • Lal
Lal, R. (1977) Soil-conserving versus soil-degrading crops and soil management for erosion control. In Soil conservation and management in the humid tropics (D. Greenland and R. Lal, eds), pp. 81-86. New York: Wiley.