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Assefa M Melesse – One of the best experts on this subject based on the ideXlab platform.

  • effect of rainfall intensity slope and Antecedent Moisture content on sediment concentration and sediment enrichment ratio
    Catena, 2012
    Co-Authors: Mengistu B Defersha, Assefa M Melesse

    Abstract:

    Abstract The transportability of sediment by runoff and its potential for subsequent deposition is dependent on the size and distribution of the transported material and the sediment concentration. The size distribution of the eroded sediment is also expected to vary depending on which erosion process is predominant. This study discusses the results of laboratory analysis to evaluate the effect of rainfall intensity, slope steepness, soil types and Antecedent Moisture content on sediment concentration, runoff coefficients, and sediment enrichment ratios. To achieve the objectives, laboratory experiments were conducted using FEL-3 rainfall simulator and erosion pan. Rainfall intensity of 120, 70, and 55 mm/h was applied sequentially at 9, 25 and 45% slope for three soil types found in Alemaya watershed, eastern Ethiopia Alemaya Black soil (Soil A), Regosols (Soil B), and Cambisols (Soil C) varied from clay to sandy clay loam in texture with wet and dry Antecedent Moisture contents. The rate of sediment concentration significantly varied with Moisture contents. For Soil A, sediment concentrations (24.3 g/l) from an initially air-dry surface and (18.7 g/l) from an initially wet surface were observed. Wetting decreased sediment concentration of Soil A, B and C by 23, 45.7 and 1.3%, respectively. Sediment concentration didn’t show any trend with rainfall intensities. For Soil B, the highest average silt enrichment ratio of 2.15 was observed followed by average clay enrichment ratio of 1.61. For Soil A and Soil C, average clay enrichment ratios of 1.13 and 1.91 were observed, respectively. Initial Moisture contents had effect on the enrichment ratio, however, the actual effect varied with soil type and the size of the particle. Results of the study indicated that the actual effect of slope as well as rainfall intensity on sediment concentration and sediment yield vary with soil types and Moisture contents.

  • The effect of slope steepness and Antecedent Moisture content on interrill erosion, runoff and sediment size distribution in the highlands of Ethiopia
    Hydrology and Earth System Sciences, 2011
    Co-Authors: Mengistu B Defersha, S. Quraishi, Assefa M Melesse

    Abstract:

    Abstract. Soil erosion is a two-phase process consisting of the detachment of individual particles and their transport by the flowing water. This study discusses the results of laboratory experiments in which for three soils, the runoff depth, sediment yield, splash erosion and sediment size were measured. Rainfall intensity, slope and Antecedent Moisture contents were varied in the experiment. The soil types ranged from clay to sandy clay loam (Alemaya Black soil, Regosols and Cambisols). Rainfall was applied for six sequential 15-min periods with rainfall intensities varying between 55 and 120 mm h−1. The three slopes tested were 9, 25, and 45 %. Results show that as slope increased from 9 to 25 %, splash erosion and sediment yield increased. An increase in slope from 25 to 45 % generally decreases in splash erosion. Sediment yield for one soil increased and one soil decreased with slope and for the third soil the trend was different between the two initial Moisture contents. Sediment yield was correlated (r = 0.66) with runoff amounts but not with splash erosion. Interrill erosion models that were based on the flowing water and rainfall intensity fitted the data better than when based on rainfall intensity solely. Models that assume a positive linear relationship between erosion and slope may overestimate sediment yield.

  • Interrill erosion, runoff and sediment size distribution as affected by slope steepness and Antecedent Moisture content
    Hydrology and Earth System Sciences Discussions, 2010
    Co-Authors: Mengistu B Defersha, S. Quraishi, Assefa M Melesse

    Abstract:

    Abstract. Soil erosion is a two-phase process consisting of the detachment of individual particles and their transport by erosive agents such as flowing water. The rate at which erosion occurs depends upon the individual as well as interactive effects of different parameters responsible for soil erosion. The study discusses results of a laboratory analysis and evaluates the effect of slope steepness and Antecedent Moisture content on sediment yield (wash) and runoff rate. Interrill sediment yield, splash detachment, runoff, and sediment size distribution were measured in laboratory erosion pans under simulated total duration of 90 min. Rainfall intensity at 120 mm/hr, 70 mm/hr, and 55 mm/hr were applied sequentially at 9, 25, and 45% slope steepness for three soils (Alemaya Black soil, Regosols, and Cambisols) varied from clay to sandy clay loam in texture with wet and dry Antecedent water contents. As slope steepness increased from 9 to 25% splash increased for five treatments and decreased for the remaining treatment; washed sediment increased for all treatments. As slope increased from 25 to 45% splash decreased for five treatments but increased for one treatment, and washed sediment increased for three treatments but decreased for the other three treatments. Pre-wetting decreased splash detachment for all soil treatments and rate of reduction was high for the highly aggregated soil, Alemaya Black soil and low for the less aggregated soil Regosols. Splash sediment and sediment yield was not correlated. Change in splash with increase in slope steepness was also not correlated with change in sediment yield. Change in runoff rate with increase in slope steepness was correlated (r=0.66) with change in sediment yield. For Alemaya Black soil and Regosols, splashed sediment size distribution was correlated with washed sediment size distribution. Interrill erosion models that include runoff and rainfall intensity parameters were a better fit for these data than the rainfall intensity based model. The exponent term, b, values in (E=a Ib) model did not approach 2.00 for all treatments. For the same slope steepness factor, both rainfall and rainfall-runoff based models provided different erodibility coefficients at different levels of slope and Moisture contents.

Mengistu B Defersha – One of the best experts on this subject based on the ideXlab platform.

  • effect of rainfall intensity slope and Antecedent Moisture content on sediment concentration and sediment enrichment ratio
    Catena, 2012
    Co-Authors: Mengistu B Defersha, Assefa M Melesse

    Abstract:

    Abstract The transportability of sediment by runoff and its potential for subsequent deposition is dependent on the size and distribution of the transported material and the sediment concentration. The size distribution of the eroded sediment is also expected to vary depending on which erosion process is predominant. This study discusses the results of laboratory analysis to evaluate the effect of rainfall intensity, slope steepness, soil types and Antecedent Moisture content on sediment concentration, runoff coefficients, and sediment enrichment ratios. To achieve the objectives, laboratory experiments were conducted using FEL-3 rainfall simulator and erosion pan. Rainfall intensity of 120, 70, and 55 mm/h was applied sequentially at 9, 25 and 45% slope for three soil types found in Alemaya watershed, eastern Ethiopia Alemaya Black soil (Soil A), Regosols (Soil B), and Cambisols (Soil C) varied from clay to sandy clay loam in texture with wet and dry Antecedent Moisture contents. The rate of sediment concentration significantly varied with Moisture contents. For Soil A, sediment concentrations (24.3 g/l) from an initially air-dry surface and (18.7 g/l) from an initially wet surface were observed. Wetting decreased sediment concentration of Soil A, B and C by 23, 45.7 and 1.3%, respectively. Sediment concentration didn’t show any trend with rainfall intensities. For Soil B, the highest average silt enrichment ratio of 2.15 was observed followed by average clay enrichment ratio of 1.61. For Soil A and Soil C, average clay enrichment ratios of 1.13 and 1.91 were observed, respectively. Initial Moisture contents had effect on the enrichment ratio, however, the actual effect varied with soil type and the size of the particle. Results of the study indicated that the actual effect of slope as well as rainfall intensity on sediment concentration and sediment yield vary with soil types and Moisture contents.

  • The effect of slope steepness and Antecedent Moisture content on interrill erosion, runoff and sediment size distribution in the highlands of Ethiopia
    Hydrology and Earth System Sciences, 2011
    Co-Authors: Mengistu B Defersha, S. Quraishi, Assefa M Melesse

    Abstract:

    Abstract. Soil erosion is a two-phase process consisting of the detachment of individual particles and their transport by the flowing water. This study discusses the results of laboratory experiments in which for three soils, the runoff depth, sediment yield, splash erosion and sediment size were measured. Rainfall intensity, slope and Antecedent Moisture contents were varied in the experiment. The soil types ranged from clay to sandy clay loam (Alemaya Black soil, Regosols and Cambisols). Rainfall was applied for six sequential 15-min periods with rainfall intensities varying between 55 and 120 mm h−1. The three slopes tested were 9, 25, and 45 %. Results show that as slope increased from 9 to 25 %, splash erosion and sediment yield increased. An increase in slope from 25 to 45 % generally decreases in splash erosion. Sediment yield for one soil increased and one soil decreased with slope and for the third soil the trend was different between the two initial Moisture contents. Sediment yield was correlated (r = 0.66) with runoff amounts but not with splash erosion. Interrill erosion models that were based on the flowing water and rainfall intensity fitted the data better than when based on rainfall intensity solely. Models that assume a positive linear relationship between erosion and slope may overestimate sediment yield.

  • Interrill erosion, runoff and sediment size distribution as affected by slope steepness and Antecedent Moisture content
    Hydrology and Earth System Sciences Discussions, 2010
    Co-Authors: Mengistu B Defersha, S. Quraishi, Assefa M Melesse

    Abstract:

    Abstract. Soil erosion is a two-phase process consisting of the detachment of individual particles and their transport by erosive agents such as flowing water. The rate at which erosion occurs depends upon the individual as well as interactive effects of different parameters responsible for soil erosion. The study discusses results of a laboratory analysis and evaluates the effect of slope steepness and Antecedent Moisture content on sediment yield (wash) and runoff rate. Interrill sediment yield, splash detachment, runoff, and sediment size distribution were measured in laboratory erosion pans under simulated total duration of 90 min. Rainfall intensity at 120 mm/hr, 70 mm/hr, and 55 mm/hr were applied sequentially at 9, 25, and 45% slope steepness for three soils (Alemaya Black soil, Regosols, and Cambisols) varied from clay to sandy clay loam in texture with wet and dry Antecedent water contents. As slope steepness increased from 9 to 25% splash increased for five treatments and decreased for the remaining treatment; washed sediment increased for all treatments. As slope increased from 25 to 45% splash decreased for five treatments but increased for one treatment, and washed sediment increased for three treatments but decreased for the other three treatments. Pre-wetting decreased splash detachment for all soil treatments and rate of reduction was high for the highly aggregated soil, Alemaya Black soil and low for the less aggregated soil Regosols. Splash sediment and sediment yield was not correlated. Change in splash with increase in slope steepness was also not correlated with change in sediment yield. Change in runoff rate with increase in slope steepness was correlated (r=0.66) with change in sediment yield. For Alemaya Black soil and Regosols, splashed sediment size distribution was correlated with washed sediment size distribution. Interrill erosion models that include runoff and rainfall intensity parameters were a better fit for these data than the rainfall intensity based model. The exponent term, b, values in (E=a Ib) model did not approach 2.00 for all treatments. For the same slope steepness factor, both rainfall and rainfall-runoff based models provided different erodibility coefficients at different levels of slope and Moisture contents.

Nigel T. Roulet – One of the best experts on this subject based on the ideXlab platform.

  • Antecedent Moisture conditions and catchment morphology as controls on spatial patterns of runoff generation in small forest catchments
    Journal of Hydrology, 2009
    Co-Authors: April L. James, Nigel T. Roulet

    Abstract:

    Summary Although existing empirical studies of runoff generation in headwater catchments have provided evidence of runoff mechanisms, contributing sources and active flowpaths from forest catchments around the world, our understanding of how hydrologic and biological processes vary and aggregate in space within headwater systems remains poor. In this study, we examine the spatial patterns of storm runoff generation from eight small nested forest catchments ranging in size from 7 to 147 ha, as a function of Antecedent Moisture conditions and catchment morphology. The catchments, located in the formerly glaciated terrain of Mont Saint-Hilaire, Quebec, Canada, are complex in both their topographic and subsurface characterization. Hydrologic response from the eight catchments shows a strong nonlinear change with Antecedent Moisture conditions consistent with the hypothesis of different ‘states-of-wetness’. With the transition from wet to dry conditions, local groundwater stores become depleted in some ephemeral catchments, variable source areas shrink and new-water delivered by shallow-subsurface stormflow and the transient development of perched water in valley-bottoms can account for much larger percentages of total runoff (up to 76% of total runoff). For the five storm events, no consistent pattern in percent new-water delivery was observed. However, for the storms observed under dry conditions, larger magnitudes of new water were generated from the three largest catchments attributable to basin morphology, while storms observed under wet conditions exhibited no consistent pattern, with larger variability among the smaller catchments. The results presented here illustrate the complexity of influences of Antecedent Moisture conditions and catchment morphology on spatial patterns of runoff generation in headwater systems.

  • investigating hydrologic connectivity and its association with threshold change in runoff response in a temperate forested watershed
    Hydrological Processes, 2007
    Co-Authors: April L. James, Nigel T. Roulet

    Abstract:

    Hydrological studies across varied climatic and physiographic regions have observed small changes in the ‘states of wetness’; based on average soil Moisture, can lead to dramatic changes in the amount of water delivered to the stream channel. This non-linear behaviour of the storm response has been attributed to a critical switching in spatial organization of shallow soil Moisture and hydrologic connectivity. However, much of the analysis of the role of soil Moisture organization and connectivity has been performed in small rangeland catchments. Therefore, we examined the relationship between hydrologic connectivity and runoff response within a temperate forested watershed of moderate relief. We have undertaken spatial surveys of shallow soil Moisture over a sequence of storms with varying Antecedent Moisture conditions. We analyse each survey for evidence of hydrologic connectivity and we monitor the storm response from the catchment outlet. Our results show evidence of a non-linear response in runoff generation over small changes in measures of Antecedent Moisture conditions; yet, unlike the previous studies of rangeland catchments, in this forested landscape we do not observe a significant change in geostatistical hydrologic connectivity with variations in Antecedent Moisture conditions. These results suggest that a priori spatial patterns in shallow soil Moisture in forested terrains may not always be a good predictor of critical hydrologic connectivity that leads to threshold change in runoff generation, as has been the case in rangeland catchments. Copyright © 2007 John Wiley & Sons, Ltd.