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Predicting minimum achievable soil loss in developing countries
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.
... 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). ...
Riverbank erosion, aggradation, and river bed degradation are significant threats to most Indian watersheds. A quantitative valuation of the spatial distribution of soil erosion at a watershed level is vital for sustainable watershed planning and management practice. Very few studies can be found that focus on soil loss with sediment yield patterns in the upper Brahmaputra river basin due to massive erosion-deposition processes. Therefore, it is intended to study the soil loss and sediment yield pattern in the upper Brahmaputra river basin at the selected outlet of Majuli Island, located in South Asia. The principal objective is to evaluate soil erosion and sediment yield transported downstream of Majuli Island using the revised universal soil loss equation (RUSLE) and sediment delivery ratio (SDR) models, respectively, for 1979-2014. The study is conducted based on data availability and efficient models like RUSLE and SDR to measure soil erosion and sediment yield. Also, sediment yield maps for 36 years are prepared for the whole catchment considering rainfall as a variable. The maximum area contributing to soil erosion is observed around the hilly region having steep slopes. However, the highest sediment deposition is found near Majuli Island. This study would help take preventive measures for the erosion process and watershed management to minimize soil erosion in the watershed.
... The use of Digital elevation model (DEM) along with the advancement of satellite Remote Sensing (RS) and Geographic Information System (GIS) can be applied to provide a detailed assessment of soil erosion (Jain et al., 2001;Kouli et al., 2008). However, the problem of soil loss assessment and conservation practices in developing countries lies in the lack of field data or the ability to obtain high-resolution data to be used as an input for soil loss models (Phillips, 1989). ...
Soil erosion is one of the most critical environmental issues with severe consequences. Hence, it continues to be a significant limitation in the progress of many developing countries. Prediction and assessment of soil loss are, therefore, of utmost importance for soil fertility conservation, land and water management. Recent technological advances have provided useful models through which remotely-sensed data for a large scale area can be analysed and interpreted. The present study adopts a physiographically, biologically and climatically unique model for the assessment of soil erosion in the Indian Himalayan Region. The Revised Universal Soil Loss Equation model was applied in conjunction with Geographic Information System to estimate the average annual rate of soil erosion at both state and district levels in India. The model was deployed using coarse resolution datasets to identify specific areas vulnerable to soil erosion. In determining the spatial distribution of average annual soil erosion within the study region, all cell-based parameters of the model were multiplied in the specified 500 m × 500 m spatial resolution. The spatial pattern of annual soil erosion indicates that maximum soil loss occurs in northern and eastern states whereas low rates of erosion is observed in the eastern-most part of the study area.
... Examples involving soils can be found regarding historical preconceptions about tropical soils as fragile and infertile (based on assumptions of acceptable levels of stability and fertility), a notion now long refuted empirically within the soil science and physical geography mainstream (Schaetzl and Anderson, 2005: 388-392). Something similar has been accomplished regarding the (not very universal) Universal Soil Loss Equation, based on defining soil loss tolerance thresholds (or T factor) relative to crop yields in conventional agriculture. 1 Aside from there being no necessary relationship between crop yield and soil loss (Stocking, 2003), the equation has been found to need exceedingly burdensome local sampling campaigns (for calibration) beyond the capacity of most countries (Phillips, 1989). This is an example of technical correctives over the political biases (decision-making processes advantaging the wealthy) that inhere what tend to be regarded as neutral technical interventions. ...
Many urban areas feature soil pollution legacies that are augmented by recurring contaminant emissions. Because trace elements (like arsenic or lead) are not degradable like organic pollutants, trace element contamination is a lasting environmental threat to human health. Conventionally, the problem is assessed by comparisons with soil quality standards based on background soil concentration levels, which constitute a variety of baseline values. However, baselines are fraught with technical challenges and politically problematic assumptions. These are illustrated by soil quality standards (ambient and maximum allowable levels) addressing soil trace element contamination. These baselines used to assess toxicological threats (beyond soils) are not only contingent on ecological complexities and shifts, but also on national and institutional setting, testifying to their social, not just technical basis. As shown through two US examples, the latter social dimensions do not merely reflect differences in levels of risk acceptability. Soil quality standards can be used to pre-empt public debate over, among other matters, land use decisions. There are therefore both technical and political reasons to question such baselines, which end up becoming attempts to conceal uncertainties and partial or incomplete understandings. An ecosocialist Critical Physical Geography approach is explored that takes ecological and social contexts and their dynamism as primary means to assess trace element contamination.
... The concept of tolerable soil loss is concerned with limiting erosion to levels at which no irreversible degradation or productivity losses occur (Phillips, 1989). But natural erosion as a conservation goal has received criticism for its lack of realism because as Phillips (1989, p. 221) argued, " accelerated erosion is virtually inevitable whenever vegetation is periodically removed and soil surfaces disturbed. ...
... 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. ...
The present study assesses the impact of soil and water conservation (SWC) techniques on the hydrology of the Ronquillo watershed (42 km²) in the northern Andes of Peru. The hydrological model NASIM is applied to evaluate the scenario-based implementation of four different measures of soil and water conservation (terraces, bunds, afforestation, and check dams) with respect to its impact on catchment hydrology. The modeling results indicate that earth works and afforestation significantly affect flow volume, overland flow generation, and high flows. The impact of check dams on the stream low characteristics of the Ronquillo River is small. Earth works such as terraces and/or bund structures reduce surface runoff by 12–28%; however, flow volume diminishes by 6–14% as well. A loss of water for downstream usage of 1242–1643 m³ha-1y-1 is observed. Afforestation with eucalyptus and pine species reduces surface runoff by 9–11% and flow volume by 6–8%. Water loss due to enhanced interception and evapotranspiration rates of plantations compared to native species or crops amounts to 1062–2586 m³ha-1y-1. On average, high flow discharge is reduced by 2–31%. For a single runoff event of the peak over threshold time series, a maximum reduction of 48% in peakflow is observed. With respect to the appropriateness of SWC practices for water resource management in the Ronquillo watershed, earth works are more recommendable than afforestation because downstream water availability is less affected.
... Soil loss is a serious environmental problem in the world, and China is one of those countries which suffer such situation most. [1][2][3] .As the important factor of regional ecological security, soil erosion causes many problems such as degradation of land resource and ecological systems, this phenomenon becomes more distinct as the land-use change increases because it influences the impetus and resistance of soil erosion [4] . Land-use change will transform the types of the earth's surface, therefore, the impetus and resistance of soil erosion will be influenced, and it comes out to be the element of initiation and strengthening of soil erosion. ...
This paper improved the traditional method of researching soil loss changes based on land-use dynamic changes, geoinformation Tupu theory was introduced in to study the heterogeneity of soil erosion caused by land-use changes. With the example of Zhujiang Delta, we investigated the land-use changes and soil erosion changes over the period from 1998 to 2006. The results showed: the soil erosion intension aggravated in recent years, the average amount of soil erosion in 2006 was as much as 1.5 times of that in 1998; the change of land-use was the main reason for the change of soil erosion, and the diversity of soil erosion varied obviously with the transformation of land-use patterns; especially, soil erosion changed sharply with the mutual transformation between developing area or sand land and other land-use types correspondingly.
This book provides a theory of the principles behind the interdependence of different aspects of nature. Climate, vegetation, geology, landforms, soils, hydrology, and other environmental factors are all linked. There must be some general principles about the way in which Earth surface systems operate, and the ways in which interactions of the bio-, litho-, hydro-, and atmospheres manifest themselves. Yet there may be inherent limits on our ability to understand these interactions using traditional reductionist science. This book argues that the simultaneous presence of order and chaos reflects fundamental, common properties of Earth surface processes and systems. It shows how and why this is the case, with examples ranging from geological time scales to microscale examinations of process mechanics.
This book provides a theory to overcome the problem of identifying the principles behind the interdependence of different aspects of nature. Climate, vegetation, geology, landforms, soils, hydrology, and other environmental factors are all linked. Many scientists agree that there must be some general principles about the way in which earth surface systems operate, and about the ways in which the interactions of the biosphere, lithosphere, hydrosphere, and atmosphere manifest themselves. Yet there may be inherent limits on our ability to understand and isolate these interactions using traditional reductionist science. The argument of this book is that the simultaneous presence of order and chaos reflects fundamental, common properties of earth surface processes and systems. It shows how and why this is the case, with examples ranging from evolutionary and geological times scales to microscale examinations of process mechanics.
Soil and nutrient losses, under different management options and scales, for cropland and rangeland were investigated in Lesotho, Southern Africa. An approximation of soil formation was estimated with a simple weathering experiment. The replacement cost of lost soil nutrients was calculated and the concept of soil life applied to the results. The various approaches and scenarios included in the calculations result in predicted crop yield losses of between 1% and 5% per year. The two most common erosion prediction equations in Southern Africa, USLE and SLEMSA, were tested on the results. The ease of validation inherent to the SLEMSA equation is the most remarkable fact from these tests. Once the erodibility factor is established, SLEMSA fairly accurately predicts soil loss from a variety of land uses and management options, both over entire seasons and from single storms. However, the USLE equation proved to be the best tool for single-storm, unvalidated prediction.
Based on the measured data of soil and water loss in the monitoring stations in different relief areas in Zhangjiakou, the spatial suitabilities of water and soil conservation measures were researched. The results showed that, in different relief sections of the sloping lands, the benefits of water and soil conservation for level terrace was the highest, which was larger than 90%, and that for the planted grass was the lowest, while that for the economy forestry was the median. For the waste land, the benefits of vegetation measure combined with level trench and/ or fish-scale pits, over 75%, was the best, those of natural restoration and planted grass measures occupied the median level with large spatial and temporal differences, and the main reasons could be attributed to the diversities of vegetation quality and soil properties. During the initial period of closure measure to controling soil and water loss, eco-recovery must depend on engineering measures to provide growing environment to vegetation.
Soil erosion has been the most prominent issue in Chinas ecological environment. The rocky mountain area of Northern China is an important typical area of soil erosion and one of the most dangerous regions of potential erosion in China characterized by more stones in thinner soils and the land surface is easily reduced to sand gravel or rocky desertification. Concentrated rainfall with frequent rainstorms, broken topography with steep slope and highly dense gully are important natural factors resulting in soil erosion. However, unreasonable land utilization especially slope farmland aggravates soil erosion. Soil erosion results in land degradation, threatens regional water resources security, intensifies regional poverty and restricts socio-economic development. Sloping cultivated land transformation will be the necessary choice for soil and water conservation and ecological construction in this region.
As a case study on analysis of soil erosion change, in Hulunbeir, Inner Mongolia, China, was once one of the most severely eroded regions in semi-arid and semi-humid China. However, its soil erosion is becoming a matter of concern for more and more people in recent years. The main body of vegetation is Daxinganling virgin forests and Hulunbeir grassland, However, large areas of forest were destroyed and excessive grazing in pastureland, which resulted in the increase of soil erosion. Fortunately, the government has taken corresponding measures to prevention and control soil erosion in recent years. In this study, based on TM imagines, and employing ArcGIS 9.3 technique and empirical Revised Universal Soil Loss Equation (RUSLE) model, this paper studied the dynamics of soil erosion at the Hulunbeir from 1995 to 2005 years. Use of the Markov model to predicted the soil erosion in Hulunbeir next 15 years. The results show that the main erosion types arte the tiny degree erosion as clue, above the light degree erosion is 10 percent of Hulunbeir area in the past 10 years from 1995 to 2005. The net decrease of the tiny degree erosion in 2000 is 534.33km2 compared with 1995, and the area of above the light degree erosion increased 580.02km2 from 1995 to 2000. This explains that the soil erosion of Hulunbeir area shows the increasing tendency in the past 5 years. The net increased of the tiny degree erosion in 2005 is 8326.69km2 compared with 2000, and the area of above the light degree erosion is decreased by 8360.54km2. This shows the decreasing tendency of the soil erosion intention in Hulunbeir region. And comparing to 2000 year, the soil erosion have undergone great changes. Then, the development and the future tendency of soil erosion in 2010, 2015 and 2020 year that uesed simulated with Markov process, the soil erosion intensity for Hulunbeir area will be alleviated, among which above the light degree erosion land area wil- be decreased by 5.4%, 4.4% and 3.9% proportion to that of 2005. Indicating that soil erosion had been substantially controlled.
Remote sensing information services have characteristics of multi-dimension, heterogeneity, and high dynamic performance, so it is very difficult to realize online search by use of UDDI registry of business search engines. It has become a bottle-neck restricting the farther development of remote sensing information services. Considering the characteristics of remote sensing information services, the paper introduces remote sensing information ontologies into the online search of the remote sensing information services and proposes a semantics clustering based and multi-QoS (Quality of Service) constraints method to improve the performance of remote sensing information services discovery. Experimental results show that the method proposed in the paper can attain well search performance.
The fact that the subjects of science are abstract and complex creates the necessity of supporting the science education with concrete materials. The subject of this study is to determine the effect of the models, one of the instruments used for educational purposes. The study aims to elucidate the effect of the models, created by the students related to mitosis and meiosis, upon academic success. The working group was divided into two as experiment (N=32) and control (N=24) groups. The students in the experimental group have been taught with the traditional teaching method and then they made models related to mitosis and meiosis afterwards. The control group on the other hand was only taught with the same method without making any models. The data obtained from these two groups showed there were was no significant differences as regards to their pre tests scores. However final tests results were significantly in favor of the experimental group.
In arid and semi-arid areas, a large area of soil erosion in the upper reaches will not only aggravate drought of atmospheric and soil, but also induce deterioration of grasslands and desertification. In this paper, as the Shiyang River upper reaches a case, we analyzed how the changes of land use and land cover influence the soil erosion, based on remote sensing satellite date and DEM, using ArcGIS software and soil erosion formula (RUSLE). The results show that: (1) the soil erosion intensity of farmland was the most serious among the vegetation types. The ability of soil and water conservation of forest, shrub and high covered grassland was the best, and which can effectively prevent the soil loss caused by the increase of slope gradient. (2) Lapping the land use changes map, soil erosion changes map and the slope map, the result indicated that reclamation high covered grassland to farmland in the areas of slope gradient exceeding 15 was the most mainly cause.
Five major factors affect erosion and transportation processes by water: climate, relief, soil, vegetation and man's activities. In order to discern climatic differences in erosion, soil loss models and erosion maps are analysed. Global and regional maps of sediment yield are reproduced and the construction methods and the reliability of the maps are considered. Fournier's and Strakhov's maps are compared with regional maps in order to evaluate the reliability of the global maps.
The main conclusions of this study are that rainfall erosivity is great in Af, Am, Aw and Ca climates and lower in other climates. Low sediment yields are found in Db, Dc, and Dd cclimates. High values occur in Cs, Ca and, in many cases, in Da climates. Graphs of relations between precipitation and sediment yield indicate high sediment yield in climates which experience seasonal rainfall and in BS climates. The map analysis indicates both high and low sediment yields in BS climates. Graph and map analysis indicates increased erosion in mountains.
ABSTRACT EQUATIONS describing overland flow depth, rainfall induced,soil,detachment,and,sediment,transport capacity,on,interrill,areas,were,identified.,The,Darcy- Weisbach,equation,which,included,a parameter,for predicting,flow,resistance,caused,by rainfall,was,used,to calculate,depth,of overland,flow.,Soil,detachment,was determined,from,an equation,incorporating,raindrop induced, impact pressure estimates. The product of a soil transport factor, bottom shear stress and flow velocity,was,used,to,calculate,sediment,transport capacity.,Nondimensional,forms,of the,model,equations were,evaluated,using,existing,experimental,data.
A sediment budget for a drainage basin is a quantitative statement of the rates of production, transport, and discharge of detritus. To construct a sediment budget for a drainage basin, one must integrate the temporal and spatial variations of transport and storage processes. This requires: recognition and quantification of transport processes, recognition and quantification of storage elements, and identifi- cation of linkages among transport processes and storage elements. To accomplish this task, it is neces- sary to know the detailed dynamics of transport processes and storage sites, including such problems as defining the recurrence interval of each transport process at a place.
Soil erodibility measurements using field runoff plots were made for three contrasting soils in Nigeria. Field experiments were conducted over a period of 3 years from 1980 to 1983. The soil at Onne in eastern Nigeria is developed on coastal sediments and is classified as a paleudult. The soil at Ikom derived from basalt is a rhodustalf, and that at Jos on basement complex is an oxic paleustalf. Three runoff plots, each measuring 25 m x 4 m, with appropriate collection tanks and multidivisor system were established at each site. Rainfall intensity was monitored for each experimental site using a daily recording raingauge. Also monitored were the initial soil properties and their alteration over the 3-year period. Estimates of soil erodibility were made using the nomogram developed by Wischmeier et al. (1971). Field measurements indicate soil erodibility to be 0.015, 0.04 and 0.04 (in SI units) for Ikom, Onne and Jos soils, respectively. Soil erodibility as estimated from the nomogram is 0.039, 0.025 and 0.18 for Ikom, Onne and Jos respectively. The considerable differences between the measured and estimated values of soil erodibility indicate the necessity for appropriate modifications to adapt this nomogram for tropical soils.
A model is developed for predicting annual soil loss from field-sized areas on hillslopes. The model comprises a water phase and a sediment phase. In the sediment phase erosion is taken to result from the detachment of soil particles by rainsplash and their transport by runoff. Splash is related to rainfall energy and rainfall interception; runoff transport capacity depends upon the volume of runoff, slope steepness and crop management. Rainfall energy and runoff volume are estimated from annual rainfall amount in the water phase. The predicted rate of soil loss is compared with a topsoil renewal rate to determine changes in topsoil depth over time. Model validation was carried out by comparing predicted and observed values of annual runoff and erosion for 67 sites in 12 countries. As an example of how the model can be used, a 100-year simulation exercise is presented for soil erosion under shifting cultivation in Peninsular Malaysia.
Des travaux de terrain ont été menés dans trois pays (Côte d'Ivoire, Haute Volta, Niger) afin de fournir davantage de donnéees concernant les différents facteurs qui inter viennent sur l'érosion hydrique en Afrique de l'Ouest. Deux types de simulateurs de pluies ont été utilisés. Une grande attention a été portée aux courbes intensité-durée, à l'énergie cinétique des pluies, et à la gamme d'humidités avant les averses. Les traitements ont été choisis en fonction des systèmes culturaux qui prédominent dans les diverses régions d'études. Les résultats sont conformes à ceux obtenus sous pluies naturelles. Ils expriment l'importance des éléments grossiers superficiels quant à la conserrvation des sols. En milieu tropical humide une protection naturelle est assurée par le couvert végétal qui compense ainsi les effets néfastes des pluies très violentes. L'influence des teechniques traditionnelles de lutte antiérosive est liée aux quantités d'eau infiltrée:à partir d'un cer tain seuil, les billons s'écroulent et l'érosion subit alors une vive augmentation. De même l'effet de l'utilisation de résidus de culture comme paillage dépend de la texture du matériel pédologique superficiel. Ainsi, l'utilisation, sans discernement, de l'équation universelle des pertes en terres dont les facteurs sont supposés indépendants peut être très hasardeuse. (RESUME D'AUTEUR)
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.-from Authors
During the 1983-84 agricultural year, data were collected at 100 field sites to assess the amount and spatial distribution of soil loss in Rwanda. Erosivity, erodibility, topography, crop cover and conservation, the factors determining erosional rates on agricultural lands were measured and evaluated. The results were used to calibrate the USLE for the country's agro-ecologic conditions. Correlations between the measured and estimated USLE values were statistically significant. This permits soil loss to be estimated for the 20 000 fields comprising the complete national agricultural survey. Mulching contrasted to engineering methods is found to be the most effective conservation practice even on slopes exceeding 30°. -from Author
The contribution to soil loss made by splash, rainwash, and wash was separately collected for each of four sample types, distinguished by surface moisture and coherence characteristics, at fourteen sites within S Ontario. Rainfall or its equivalent was induced at an intensity of 50 mm hr-1 by means of the Toronto-pattern field-rainfall-simulator. The collected data suggests the relative ranking of process is first, rainwash; second, splash; and third wash. Further, it is evident that entrainment by splash and transportation by wash is not the dominant form of erosion. -from Author
Criticises the classic approach to soil conservation in developing countries, whereby when projects go wrong, farmers are blamed and also suffer the consequences. Points out that the value judgements of experts are different from those of land users in developing countries, that local society and the farming system are complex, multifaceted units, and that erosion's impacts are most devastating in the tropics and developing countries. Looks at various factors which would influence a soil conservation project: the value of money in LDCs; impacts on local society; the indigenous farming system; economic evaluation of projects; national and international costs. Gives case studies from Zimbabwe and Lesotho. -after Author
Yang's (1996) sediment transport theory based on unit stream power is one of the most accurate theories, but in his equations the use of product of slope and critical velocity instead of critical unit stream power is not suitable. Dimensionless critical unit stream power required at incipient motion can be derived from the principle of conservation of power as a function of dimensionless particle diameter and relative roughness. Based on a lot of data sets, this new criterion was developed. By use of this new criteria, Yang's (1973) sand transport formula and his 1984 gravel transport formula could be improved when sediment concentration is less than about 100 ppm by weight.
Recent research has helped explain how sediment source is affected by soil rilling, slope length, slope steepness, surface cover, and other factors. Division of the source between that from interrill areas and that from rill areas provides an approach that is very useful, analytically and experimentally. The significance of each of the above factors on rill and interrill erosion is discussed. Graphs and tables represent data.
Examines the soil erodibility factor K in terms of its prediction from basic soil properties and the effect on antecedent conditions. Improvements for estimating the K factor are suggested. -K.Clayton
Current attempts to assess soil loss tolerances (T values) are based on faulty premises concerning rates of topsoil development and mineral weathering processes. Previous attempts to estimate soil tolerance values are reviewed and it is shown that T-values have evolved through short-term economic considerations of declining agricultural productivity due to excessive soil erosion and are not derived from scientific criteria alone. Attempts are now being made to collect data on soil formation rates. -R.H.Johnson
A physically based, computationally simple, analytical framework is developed for predicting the effects of topography on erosion and deposition on two-dimensional and quasi three-dimensional non-planar hillslopes. It is then extended to apply to more complex slope geometries found in real three-dimensional terrain using a digital terrain model. An example is given of predicted erosion and deposition patterns on an experimental catchment at Wagga Wagga, Australia. Predicted zones of high erosion show good agreement with the long term pattern of erosion and deposition in the catchment, especially the observed locations of gullies and zones of severe sheet erosion.
Scientific planning for soil and water conservation requires knowledge of the relations between those factors that cause loss of soil and water and those that help to reduce such losses. The soil loss prediction procedure presented in this handbook provides specific guidelines which are needed for selecting the control practices best suited to the particular needs of each site. The procedure is founded on an empirical soil loss equation that is believed to be applicable wherever numerical values of it factors are available. KEYWORDS: TROPAG textbar Miscellaneous subjects textbar Climatology textbar Land Conservation and Management textbar USA (Mainland).
Flows with predominate flow direction are governed by the de Saint Venant flow equations. The kinematic wave approach retains the mass balance but considers pseudo-uniform flow conditions instead of the full momentum ba- lance. This approximation is particularly well-suited for overland runoff pro- cesses. The zero-inertia approach may be regarded as an intermediate formula- tion which retains the effect of surface slope but neglects dynamical flow prop- erties. This investigation considers in detail the differences between the three of the aforementioned approaches under pseudo-steady flow conditions. The result indicates application limits of the two simplified wave theories when compared to the de Saint Venant flow equations and may be regarded as a useful decisive criterion.
Starting from the basic erosion principles, an upland soil erosion model to predict soil loss by overland flow from individual storms on forested hillslopes can be derived in the form
where Qs is total soil loss for a storm event, n is roughness coefficient, x is down slope distance, Kf is soil erodibility factor, S is slope, α is slope exponent and Q is runoff. Values of n and α are to be determined for different environments and are 0·58 and 2·1 for a mixed pine forest ecosystem. A significant correlation (r = 0·933, n = 96) fits between the observed and predicted values using this expression, and the model fitting is good.
Soil loss and runoff were measured from concave, uniform, and convex slopes subjected to simulated rainfall. Fluorescent glass particles and microrelief measurements were used to determine soil movement patterns in the slopes. For slopes of equal average steepness, a concave shape greatly reduced the total sediment loss in comparison with that from either a uniform or convex slope. Average slope steepness was not a good indicator of soil delivery past a given point except for a uniform slope. In general, soil losses from irregular slopes depend on the steepness of a short section of that slope immediately above the point of measurement. Soil movement off the plots was primarily transport by raindrop splash to a rill system and then transport down the slope by runoff in the rill system. A breakdown of the rill system occurred on the bottom of the concave slopes because of decreasing local steepness, resulting in sheet flow and sediment deposition.
Since stream power ω and sediment transport rate i are different values of the same physical quantity, namely, the time rate of energy supply and dissipation, it is rational to relate one to the other. The experimental relation has been difficult to interpret because of the spurious curvature of log-log plots in which a constant threshold stream power of zero is involved. The substitution of an excess power ω − ω0 removes this curvature, and existing data on laboratory bed load transport rate measurements ib suggest a general empirical relation: ib ∝ (ω − ω0)[(ω − ω0);0]½. Existing laboratory data have also shown clearly that at any given value of ω − ω0 the bedload transport rate ib decreases as an inverse function of the ratio flow depth to grain size Y/D. The East Fork River (Wyoming) project has recently enabled bed load sampling devices to be calibrated, so reasonably reliable measurements can be made in natural rivers. The uncertainties in the measurement of the corresponding river power are discussed, and a simple data reliability test is suggested. Data covering three seasons collected from both Snake and Clearwater rivers appear to be reliable. Though there is much scatter due to day variations in the river conditions, these data, together with data on an imtermediate scale from East Fork River and on a small laboratory scale, conform with startling consistency to the following general empirical relation: ib/(ω − ω0) ≈ [(ω − ω0)/ω0]½(Y/D)−⅔ over a 2 million-fold range of stream discharge. The degree of consistency of the above empirical relation with the theoretical relation deduced previously (Bagnold, 1973) is discussed, as are also some morphological implications of the dependence of ib on the depth to grain size ratio Y/D.
Theoretical equations for calculating the unit stream power of both sheet and rill flow were developed and used to predict the sediment transport capacity of such flows. Independent data sets from three sources representing both finely aggregated clay soils and coarse textured nonaggregated soils, sheet, rill, and composite sheet rill flow systems, and a range of slopes were used to test the utility of the method. The results were very good and demonstrated the simplicity and robustness of the method. For shallow overland flow the best results were obtained when the critical unit stream power at incipient sediment motion was treated as a constant value that was independent of slope. The results also suggest that a unique value of critical unit stream power for rill initiation exists that is independent of soil type. For noncohesive loams or fine sands and finely aggregated clay soils the sediment transport capacity can be accurately predicted from a knowledge of the physical characteristics of the soil or bed material alone. For aggregated clay soils this requires information on the aggregate size distribution and the effects of soil particle size differentiation as flow rates and unit stream powers increase with the transition from sheet to rill flow.
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
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.
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.
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.)
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.
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
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.
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.
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.
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.
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.
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.
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.