Recent research has indicated the large spatial and temporal variation in soil erosion resistance against concentrated flow (SER). This study analyzes this variability in relation to rill and gully initiation locations on slopes and the downslope eroded volumes. The soil erodibility (Kc) and critical flow shear stress (τcr), were estimated from topsoil properties and correlated to eroded rill and gully volumes and their initiation points on slopes in the Belgian loess belt. Therefore, concentrated flow paths and topsoil properties were measured in their vicinity. The results show that rill and gully initiation points, and hence the lengths of concentrated flow paths, depend on τcr, which is controlled by soil surface conditions and can be predicted from saturated soil shear strength. Soil erosion control measures that increase soil shear strength (e.g. thalweg compaction), can therefore decrease rill and gully lengths. Once a rill or an ephemeral gully is initiated, its cross‐section was found to depend on Kc, which can be estimated from the soil water content, dry bulk density, and the dry density of roots and crop residues incorporated in the topsoil. 74% of the variation in the channel cross‐sectional area measured in the study area could be predicted from the combined effect of flow intensity and these three soil properties, whereas flow intensity alone could only account for 31% of the variation. Soil conservation measures affecting one of the soil properties that control Kc (e.g. double drilling of the thalweg, conservation tillage) can therefore decrease the cross‐sections of the concentrated flow paths. These findings also indicate that rill and gully initiation points are not only topographically controlled but also depend on the SER, which in turn determines the dimensions of these concentrated flow paths. Hence, knowledge of the variability in SER is indispensable. Copyright © 2009 John Wiley & Sons, Ltd.

Soil erosion by concentrated flow can cause serious environmental damage. Erosion-control geotextiles have considerable potential for reducing concentrated flow erosion. However, limited data are available on the erosion-reducing potential of geotextiles. In this study, the effectiveness of three biological geotextiles in reducing soil losses during concentrated flow is investigated. Hereto, runoff was simulated in a concentrated flow flume, filled with an erodible sandy loam on three slope gradients (13.5, 27.0 and 41.5%). Treatments included three biological geotextiles (borassus, buriti and bamboo) and one bare soil surface. Darcy-Weisbach friction coefficients ranged from 0-01 to 2.84. The highest values are observed for borassus covered soil surfaces, followed by buriti, bamboo and bare soil, respectively. The friction coefficients are linearly correlated with geotextile thickness. For the specific experimental conditions of this study, borassus geotextiles reduced soil detachment rate on average to 56%, buriti geotextiles to 59% and bamboo geotextiles to 66% of the soil detachment rate for bare soil surfaces. Total flow shear stress was the hydraulic parameter best predicting soil detachment rate for bare and geotextile covered surfaces (R-2 = 0.75-0-84, p < 0.001, n = 12-15). The highest resistance against soil detachment was observed for the borassus covered soil surfaces, followed by buriti, bamboo and bare soil surfaces, respectively. Overall, biological geotextiles are less effective in controlling concentrated flow erosion compared with interrill erosion. Copyright (C) 2009 John Wiley & Sons, Ltd.

Roughness elements at the soil surface (e.g. crop residues, rock fragments, vegetation, geotextiles) strongly reduce the erosivity of overland flow (both interrill and concentrated overland flow) and hence soil detachment rates. Common methods for shear stress partitioning that allow the calculation of effective flow shear stress in the presence of roughness elements originate from river hydraulics but seem invalid for overland flow. An alternative method to estimate the effective flow shear stress in the presence of a soil surface cover has been presented and tested for soil detachment by concentrated runoff on soil surfaces covered by crop residues by Knapen et al., (2008). In this method, the estimation of effective flow shear stress is based on the recalculation of the hydraulic radius for covered soil surfaces using flow hydraulics on uncovered surfaces. However, the applicability of this method for roughness elements different from crop residues and under field conditions needs to be tested to confirm its validity. Therefore, runoff data from three experimental studies (conducted on geotextile and grassed‐covered soil surfaces) are analysed in this study. The results show that effective flow shear stress, calculated using this method, is not only a good soil detachment predictor for soil surfaces covered with crop residues but also for the tested soil surfaces with a vegetation or geotextile cover. However, the geotextile experiments point to one of the shortcomings of the method. At high flow shear stress levels, vortex erosion due to flow turbulence is reported for the thickest geotextiles. These flow turbulences are not accounted for since the method is based on average flow characteristics. Copyright © 2009 John Wiley & Sons, Ltd.

Flume experiments simulating concentrated runoff were carried out on remolded silt loam soil samples (0.36 x 0.09 x 0.09 m(3)) to measure the effect of rainfall-induced soil consolidation and soil surface sealing on soil erosion by concentrated now for loess-derived soils and to establish a relationship between soil erodibility and soil bulk density. Soil consolidation and sealing were simulated by successive simulated rainfall events (0-600 mm of cumulative rainfall) alternated by periods of drying. Soil detachment measurements were repeated for four different soil moisture contents (0.04, 0.14, 0.20 and 0.31 g g(-1)). Whereas no effect of soil consolidation and sealing is observed for critical flow shear stress (tau(cr)), soil erodibility (Kc) decreases exponentially with increasing cumulative rainfall depth. The erosion-reducing effect of soil consolidation and sealing decreases with a decreasing soil moisture content prior to erosion due to slaking effects occurring during rapid wetting of the dry topsoil. After about 100 mm of rainfall, Kc attains its minimum value for all moisture conditions, corresponding to a reduction of about 70% compared with the initial Kc value for the moist soil samples and only a 10% reduction for the driest soil samples. The relationship estimating relative Kc values from soil moisture content and cumulative rainfall depth predicts Kc values measured on a gradually consolidating cropland field in the Belgian Loess Belt reasonably well (MEF = 0.54). Kc is also shown to decrease linearly with increasing soil bulk density for all moisture treatments, suggesting that the compaction of thalwegs where concentrated flow erosion often occurs might be an alternative soil erosion control measure in addition to grassed waterways and double drilling. Copyright (C) 2007 John Wiley & Sons, Ltd.

Crop residues in conservation tillage systems are known to cause both a reduction in the erosive runoff power and an increase in the topsoil erosion resistance. In this study, the relative importance of both mechanisms in reducing soil loss by concentrated flow erosion is examined. Therefore, a method to calculate the effective flow shear stress responsible for soil detachment in the presence of a residue cover is applied. The determination of effective flow shear stress is based on the recalculation of the hydraulic radius for residue treatments. The method was tested in a laboratory flume by comparing soil detachment rates of identical pairs of soil samples that only differ in the presence or absence of crop residues. This shear stress partitioning approach and a soil detachment correction were then applied to a dataset of soil detachment measurements on undisturbed topsoil samples from a no-till field plot on a loess-derived soil, sampled during one growing season. Results indicate that only a small fraction (10% on average) of the difference in soil detachment rate between conventional and conservation tillage can be attributed to the dissipation of shear forces on the residues. The remaining decrease in soil detachment during concentrated runoff after a two-year application of conservation tillage can be explained by the increased dry bulk density and root and crop residue content in the topsoil that reduces soil erodibility. After correcting for the presence of residues, the temporal variability in soil detachment rates (D-r) during concentrated flow for a given flow shear stress (tau) for both treatments can be predicted fairly well (R-2 = 0.87) from dry soil bulk density (DBD, representing consolidation effects), soil moisture content (SMC, representing antecedent rainfall conditions), the dry mass of organic material (OM, representing root growth and residue decomposition) and saturated soil shear strength using an equation of the form:D-r = [a exp(-b x OM) - c x SMC -d x DBD] [tau - (e x sigma(s,sat) + f]This study is the first to show that the effect of conservation tillage on soil detachment rates is a result of soil property modifications affecting soil erodibility, rather than a result of the surface residue decreasing flow erosivity. Copyright (c) 2007 John Wiley & Sons, Ltd.

Experimental research revealed that mulching the soil surface is an effective soil conservation practice. However, reported effectiveness of mulch covers varies widely and there are indications that spatial measurement scale (i.e. plot length) explains part of this variability. The objective of this study is therefore to analyse the impacts of plot length at which field and laboratory experiments were conducted on the effectiveness of mulch covers in reducing soil loss by water erosion. In this review, 41 studies investigating the impacts of mulch cover on soil erosion by water are analysed (plot length ranges between 0.1 and 30.5 m). Calculated mulch effectiveness factors, i.e. b-values from the mulch factor equation, range between 0.0097 and 0.1320 and increase linearly with plot length for the reviewed experiments: b=0.022+0.0017⁎plot_length (m); R2=0.37; n=41. However, care should be taken when using this relationship for extrapolations to longer plots. Furthermore, slope gradient, soil type and mulch type determine the variability of the effectiveness of mulch covers in reducing soil erosion rates by water. Depending on the dominant soil erosion process (i.e. splash, interrill, rill and interrill or rill erosion), these variables also partly control the effectiveness of a mulch cover in reducing soil erosion by water.

Many studies focus on the effects of vegetation cover on water erosion rates, whereas little attention has been paid to the effects of the below ground biomass. Recent research indicates that roots can reduce concentrated flow erosion rates significantly. In order to predict this root effect more accurately, this experimental study aims at gaining more insight into the importance of root architecture, soil and flow characteristics to the erosion‐reducing potential of roots during concentrated flow. Treatments were (1) bare, (2) grass (representing a fine‐branched root system), (3) carrots (representing a tap root system) and (4) carrots and fine‐branched weeds (representing both tap and fine‐branched roots). The soil types tested were a sandy loam and a silt loam. For each treatment, root density, root length density and mean root diameter ( D ) were assessed. Relative soil detachment rates and mean bottom flow shear stress were calculated. The results indicate that tap roots reduce the erosion rates to a lesser extent compared with fine‐branched roots. Different relationships linking relative soil detachment rate with root density could be established for different root diameter classes. Carrots with very fine roots ( D < 5 mm) show a similar negative exponential relationship between root density and relative soil detachment rate to grass roots. With increasing root diameter (5 < D < 15 mm) the erosion‐reducing effect of carrot type roots becomes less pronounced. Additionally, an equation estimating the erosion‐reducing potential of root systems containing both tap roots and fine‐branched roots could be established. Moreover, the erosion‐reducing potential of grass roots is less pronounced for a sandy loam soil compared with a silt loam soil and a larger erosion‐reducing potential for both grass and carrot roots was found for initially wet soils. For carrots grown on a sandy loam soil, the erosion‐reducing effect of roots decreases with increasing flow shear stress. For grasses, grown on both soil types, no significant differences could be found according to flow shear stress. The erosion‐reducing effect of roots during concentrated flow is much more pronounced than suggested in previous studies dealing with interrill and rill erosion. Root density and root diameter explain the observed erosion rates during concentrated flow well for the different soil types tested. Copyright © 2007 John Wiley & Sons, Ltd.

Soil erodibilty during concentrated flow (Kc) and critical flow shear stress (τcr), both reflecting the soil's resistance to erosion by concentrated runoff, are important input parameters in many physically-based soil erosion models. Field data on the spatial and temporal variability of these parameters is limited but crucial for accurate prediction of soil loss by rill or gully erosion. In this study, the temporal variations in Kc and τcr for a winter wheat field on a silt loam soil under three different tillage practices (conventional ploughing, CP; shallow non-inversion tillage, ST; deep non-inversion tillage, DT) in the Belgian Loess Belt were monitored during one growing season. Undisturbed topsoil samples (0.003m3) were taken every three weeks and subjected to five different flow shear stresses (τ=4–45Pa) in a laboratory flume to simulate soil detachment by concentrated flow. To explain the observed variation, relevant soil and environmental parameters were measured at the time of sampling. Results indicated that after two years of conservation tillage, Kc(CP)>Kc(DT)>Kc(ST). Kc values can be up to 10 times smaller for ST compared to CP but differences strongly vary over time, with an increasing difference with decreasing soil moisture content. The beneficial effects of no-tillage are not reflected in τcr. Kc values vary from 0.006 to 0.05sm−1 for CP and from 0.0008 to 0.01sm−1 for ST over time. Temporal variations in Kc can be mainly explained by variations in soil moisture content but consolidation effects, root growth, residue decomposition and the presence of microbiotic soil crusts as well play a role. τcr values increase with increasing soil shear strength but Kc seems more appropriate to represent the temporal variability in soil erosion resistance during concentrated flow. The large intra-seasonal variations in Kc, which are shown to be at least equally important as differences between different soil types reported in literature, demonstrate the importance of incorporating temporal variability in soil erosion resistance when modelling soil erosion by concentrated flow.

Gully erosion is an important soil degradation process in Mediterranean environments. Revegetation strategies for erosion control rely in most cases on the effects of the above-ground biomass on reducing water erosion rates, whereas the role of the below-ground biomass is often neglected. In a Mediterranean context, the above-ground biomass can temporally disappear because of fire or overgrazing and when concentrated flow erosion occurs, roots can play an important role in controlling soil erosion rates. Unfortunately, information on root characteristics of Mediterranean plants, growing on semi-natural lands, and their effects on the topsoil resistance to concentrated flow erosion is lacking. Therefore, typical Mediterranean grass, herb, reed, shrub and tree root systems of plants growing in habitats that are prone to concentrated flow erosion (i.e. in ephemeral channels, abandoned fields and steep badland slopes) are examined and their erosion-reducing potential was evaluated. Root density (RD), root length density (RLD) and root diameters are measured for 26 typical Mediterranean plant species. RD values and root diameter distribution within the upper 0.10–0.90 m of the soil profile are then transformed into relative soil detachment rates using an empirical relationship in order to predict the erosion-reducing effect of root systems during concentrated runoff. Comparing the erosion-reducing potential of different plant species allows ranking them according to their effectiveness in preventing or reducing soil erosion rates by concentrated flow. RD in the 0.10 m thick topsoil ranges between 0.13 kg m−3 for Bromus rubens (L.) and 19.77 kg m−3 for Lygeum spartum (L.), whereas RLD ranges between 0.01 km m−3 for Nerium oleander (L.) and 120.43 km m−3 for Avenula bromoides ((Gouan) H. Scholz.) Relative soil detachment rates, compared to bare soils, range between 0.3 × 10-12 and 0.7 for the 0.10 m thick topsoil. The results show that grasses such as Helictotrichon filifolium ((Lag.) Henrard), Piptatherum miliaceum ((L.) Coss.), Juncus acutus (L.), Avenula bromoides ((Gouan) H. Scholz), Lygeum spartum (L.) and Brachypodium retusum ((Pers.) Beauv.) have the highest potential to reduce soil erosion rates by concentrated flow in the 0–0.1 m topsoil. But also shrubs such as Anthyllis cytisoides (L.) and Tamarix canariensis (Willd.), having high root densities in the topsoil, can reduce erosion rates drastically. Among the species growing in channels, Juncus acutus (L.) has the highest erosion reducing potential, whereas Phragmites australis (Cav.) is the least effective. On abandoned fields, Avenula bromoides ((Gouan) H. Scholz) and Plantago albicans (L.) are the most effective species in reducing concentrated flow erosion rates, while Thymelaea hirsuta (L. (Endl.)) and Bromus rubens (L.) perform the worst. On steep badland slopes, Helictotrichon filifolium ((Lag.) Henrard) and Anthyllis cytisoides (L.) perform the best in the analysis of erosion reducing potential, while Ononis tridentata (L.) is the least effective species. These findings have implications for ecological restoration and management of erosion-prone slopes.

In this study, the LAPSUS-LS landslide model, together with a digital terrain analysis of topographic attributes, is used as a spatially explicit tool to simulate recent shallow landslides in Manjiya County on the Ugandan slopes of Mount Elgon. Manjiya County is a densely populated mountainous area where landslides have been reported since the beginning of the twentieth century. To better understand the causal factors of landsliding, 81 recent landslides have been mapped and investigated. Through statistical analysis it was shown that steep concave slopes, high rainfall, soil properties and layering as well as human interference were the main factors responsible for landslides in the study area. LAPSUS-LS is used to construct a landslide hazard map, and to confirm or reject the main factors for landsliding in the area. The model is specifically designed for the analysis of shallow landslide hazard by combining a steady state hydrologic model with a deterministic infinite slope stability model. In addition, soil redistribution algorithms can be applied, whereby erosion and sedimentation by landsliding can be visualized and quantified by applying a threshold critical rainfall scenario. The model is tested in the Manjiya study area for its ability to delineate zones that are prone to shallow landsliding in general and to group the recent landslides into a specific landslide hazard category. The digital terrain analysis confirms most of the causal topographic factors for shallow landsliding in the study area. In general, shallow landslides occur at a relatively large distance from the water divide, on the transition between steep concave and more gentle convex slope positions, which points to concentration of (sub)surface flow as the main hydrological triggering mechanism. In addition, LAPSUS-LS is capable to group the recent shallow landslides in a specific landslide hazard class (critical rainfall values of 0.03-0.05 in day(-1)). By constructing a landslide hazard map and simulating future landslide scenarios with the model, slopes in Manjiya County can be identified as inherently unstable and volumes of soil redistribution can yield four times higher than currently observed. More than half of this quantity can end up in the stream network, possibly damming rivers and causing major damage to infrastructure or siltation and pollution of streams. The combination of a high population density, land shortage and a high vulnerability to landslides will likely continue to create a major sustainability problem. (c) 2007 Elsevier B.V. All rights reserved.

Several studies illustrate the wind and water erosion‐reducing potential of semi‐permanent microbiotic soil crusts in arid and semi‐arid desert environments. In contrast, little is hitherto known on these biological crusts on cropland soils in temperate environments where they are annually destroyed by tillage and quickly regenerate thereafter. This study attempts to fill the research gap through (a) a field survey assessing the occurrence of biological soil crusts on loess‐derived soils in central Belgium in space and time and (b) laboratory flume (2 m long) experiments simulating concentrated runoff on undisturbed topsoil samples (0.4 × 0.1 m2) quantifying the microbiotic crust effect on soil erosion rates. Three stages of microbiotic crust development on cropland soils are distinguished: (1) development of a non‐biological surface seal by raindrop impact, (2) colonization of the soil by algae and gradual development of a continuous algal mat and (3) establishment of a well‐developed microbiotic crust with moss plants as the dominant life‐form. As the silt loam soils in the study area seal quickly after tillage, microbiotic soil crusts are more or less present during a large part of the year under maize, sugar beet and wheat, representing the main cropland area. On average, the early‐successional algae‐dominated crusts of stage 2 reduce soil detachment rates by 37%, whereas the well‐developed moss mat of stage 3 causes an average reduction of 79%. Relative soil detachment rates of soil surfaces with microbiotic crusts compared with bare sealed soil surfaces are shown to decrease exponentially with increasing microbiotic cover (b = 0·024 for moss‐dominated and b = 0·006 for algae‐dominated crusts). In addition to ground surface cover by vegetation and crop residues, microbiotic crust occurrence can therefore not be neglected when modelling small‐scale spatial and temporal variations in soil loss by concentrated flow erosion on cropland soils in temperate environments. Copyright © 2007 John Wiley & Sons, Ltd.

The soil's resistance to concentrated flow erosion is an important factor for predicting rill and (ephemeral) gully erosion rates. While it is often treated as a calibration parameter in process-based soil erosion models, global change studies require the estimation of erosion resistance from measurable soil properties. Several laboratory and field experiments have been conducted to determine the erosion resistance of various types of soils, but no attempts have been made hitherto to summarize all these data and to explore them for general trends. In this study, all available data on the resistance of topsoils to concentrated flow erosion in terms of channel erodibility (Kc) and critical shear stress (tau(cr)) has been collected together with all soil and environmental properties reported in literature to affect the soil erosion resistance. Reported Kc values for cropland topsoils range between 0.002 10(-3) s m(-1) and 250 10(-3) s m(-1) (n=470), whereas tau(cr) values range between 0 and 15 Pa (n=522). It is demonstrated that so far, the heterogeneity of measurement methods, the lack of standardized definitions and the shortcomings of the flow shear stress model hamper the comparability of soil erosion resistance values from different datasets. Nevertheless, combining Kc and tau(cr) data from different datasets, a general soil erosion resistance ranking for different soil textures can be proposed. The compiled dataset also reveals that tillage practices clearly affect Kc (Kc for conventional tillage > Kc for reduced tillage > Kc for no tillage) but not tau(cr).It was concluded that Kc and tau(cr) are not related to each other and that soil and macro-environmental properties affecting the foremost do not necessarily affect the latter as well and vise versa. Often Kc seems to be a more appropriate parameter than tau(cr) to represent the differences in soil erosion resistance under various soil and environmental conditions (e.g. bulk density, moisture content, consolidation, tillage). The two parameters represent different quantities and are therefore both needed to characterize the soil's resistance to concentrated flow erosion. (c) 2006 Elsevier B.V. All rights reserved.

Research conducted during the first half of the last century has shown that a strong power relationship exists between channel width and total flow discharge in streams. Recent studies have shown that this power relationship can be theoretically derived for bankfull discharge in channels. The relationship has been extended empirically to rills and gullies, revealing that the discharge exponent for rills and gullies is significantly smaller than that for rivers. However, water flow in rills and gullies is only rarely bankfull, indicating that the theoretical explanation for the power relationship found for rivers does not apply to rills and gullies. In order to investigate the width−discharge relationships for rills and gullies, a new method is proposed based on field measurements of widths of concentrated-flow erosion channels both upstream and downstream of channel junctions. Although the method only allows the determination of the exponent of the power relationship, it is easy and inexpensive to apply. A total of 322 rill and gully channel junctions with various soils and land use types were investigated in Belgium, Italy and Spain. The obtained data confirmed the existence of the power relationship for rills and gullies, with the exponent varying from 0.43 for small rills (about 3 cm in width) to 0.5 for gullies (about 50 to 100 cm in width). The data did not allow deciding whether the exponent varies consistently with channel width or in a step-wise fashion. The exponent values obtained in this study are larger than those reported in previous studies, but this may result from differences in the definition of the discharge that eroded the channel to its current width.

Soil erosion on agricultural land and its detrimental environmental and economical effects has aroused increased interest among both the research and policy-making communities. The call for erosion control measures adapted to local farming practices is high, especially in Europe where farmers are reluctant to adopt soil conservation techniques. This study investigates a new technique for controlling concentrated flow erosion rates in the loess belt of central Belgium: i.e. double drilling of cereals in zones of concentrated flow. Cross-sectional areas of erosion channels as well as crop yield parameters in single- and double-drilled zones were compared. The technique is based on the combined effect of the increased density of plant shoots and roots for reducing soil loss. Results indicate that double drilling can reduce soil loss through concentrated flow by 25% on average and by up to 40% under optimal conditions. No net change in wheat grain yield was observed, and farmers who participated in the experiments were satisfied with the results and the easy application of the technique. Globally, benefits were larger than costs. However, the effectiveness of the technique in reducing soil loss by concentrated flow erosion seems to be topographically restricted. For positions in the landscape with a contributing drainage area larger than ca. 0.75ha, the effectiveness of double drilling can be doubted. Double drilling should therefore be regarded as one possibility amongst others to reduce concentrated flow erosion rates in farmers’ fields.

Conservation tillage is known to decrease soil loss due to concentrated flow erosion. Whereas the effect of crop residue cover on the erosivity of the runoff water has been extensively studied, less attention has been paid to the effect of conservation tillage on topsoil resistance to concentrated flow erosion. Therefore, this study compares the soil's erosion resistance, in terms of channel erodibility (Kc) and critical flow shear stress (tau(cr)), of a winter wheat field on a silt loam soil in Belgium for three tillage treatments; shallow non-inversion tillage (ST), deep non-inversion tillage (DT) and conventional ploughing (CP). To quantify the tillage effect over time, undisturbed topsoil samples (35X9X9cm(3)) for ST, DT and CP were taken 12 times during one growing season. By subjecting the soil samples to concentrated flow (tau = 0-40 Pa) in a laboratory flume, Kc and Tcr were measured. Results showed that Kc(CP) > Kc(DT) > Kc(ST) but differences vary strongly over time. The difference between Kc(CP) and Kc(ST) decreases with increasing soil moisture content. Therefore, non-inversion tillage can reduce soil erodibility significantly in dry conditions, when conventionally ploughed topsoils would be very erodible. Nevertheless, in dry conditions, Kc(CP) can be lower than Kc(ST) as a result of the presence of cryptogamic crusts on the scaled surface of the CP field. The beneficial effect of non-inversion tillage is not reflected in tau(cr), which is in contrast to assumptions made in some soil erosion models.

Summary Farmers in Europe want to control soil erosion in ways that are easily incorporated in their normal practices. We have investigated the possibility of reducing soil erosion by concentrated flow (i.e. rill and gully erosion) through increasing the root density of cereal crops. In situ root density measurements on cereal fields were combined with laboratory flume experiments on samples, taken in single‐ and double‐drilled fields, of which the above‐ground biomass was clipped. During the laboratory experiments no significant effect of root densities on critical shear stress or channel erodibility was observed because of interactions with other changing parameters (e.g. ageing effects). Therefore, the expected relative detachment rates as a function of plant root density were calculated using an empirical equation. During the first 75 days of the crop growth season relative soil detachment rates for single‐drilled field parcels can be reduced up to 50% compared with a rootless field, whereas relative soil detachment rates in double‐drilled field parcels can be reduced up to 60% in this period. Thereafter, plant roots in double‐drilled field parcels reduce relative soil detachment rates on average by 9% compared with single‐drilled field parcels (up to an absolute maximum of 90% compared with rootless soils). During the growing season, not only root density increases but also the vegetation cover changes, which enhances soil protection from erosion. Therefore, cereal roots will help to conserve the soil when seed is drilled at double rates, especially during the early growth stages and in fields with medium risk of concentrated flow.

Traditional vegetative techniques to control gully development rely mainly on the effects of above ground biomass, whereas little attention has been given to the role of below ground biomass. Yet, in a context where above ground biomass may temporarily or spatially disappear (e.g. due to fire or grazing), roots can play an important role in protecting soil against erosion. Few studies have investigated the impacts of roots of natural vegetation (such as grass) on the resistance of topsoils in concentrated flow erosion zones, although grasses grow in many environments. Therefore, the objective of this study is to investigate the impact of root density and root length density of grass on the erodibility of root-permeated saturated topsoils. Three plots were established on a sandy loam. Their treatments were (1) bare, (2) low density drilled grass and (3) high density drilled grass, simulating different root densities. After one month, topsoil samples were taken and subjected to concentrated flow using a hydraulic flume in the laboratory. Slope, flow discharge, mean velocity, water temperature and sediment concentration were measured. Root density and root length density values were assessed. Relative soil detachment rates and mean flow shear stresses were calculated. The results indicate a negative exponential relation between the relative soil detachment rate and root density as well as root length density, independent of the applied flow shear stresses. However, the best relationship fitting the data is the Hill curve, indicating that relative soil detachment rates decrease to very low values (0.05) with an increase in root density from 0 to 4 kg m−3 or root length density from 0 to 400 km m−3. A comparison between the effects of vegetation cover on sheet and rill erosion rates and those of the root area ratio of grass roots on relative soil detachment rates reveals that grass roots are very effective in reducing soil detachment rates. The equations obtained can be used to predict the effect of grass roots on soil erosion rates during concentrated runoff and to evaluate the ability of roots to increase topsoil resistance against erosion by concentrated flow. Calculations of relative erosion rates using the equations from the RUSLE and WEPP models indicate that the observed trend is better predicted with the RUSLE model and the WEPP model for croplands than with the WEPP model for rangelands.

Manjiya County on the Ugandan slopes of Mount Elgon is a densely populated mountainous area where landslides have been reported since the beginning of the twentieth century. The numerous fatalities and the damage done during the extreme rainfall events of 1997 to 1999 drew attention to this phenomenon. In order to better understand the causal factors of these landslides, 98 recent landslides in the study area, mostly debris slumps, were mapped and investigated. Together, they displaced 11 millions m(3) of slope material. Statistical analysis shows that landslides dominate on steep concave slope segments that are oriented to the dominant rainfall direction (northeast) and at a relatively large distance from the water divide. Based on landslide occurrence and impact, four different zones can be distinguished within the study area. Causal factors as well as landslide characteristics differ greatly between the four zones.Besides the fact that steep slopes, high rainfall and typical soil properties and stratification turn Manjiya into an inherently unstable area, human interference cannot be neglected. Whereas deforestation has reduced the stability of the shallow soils on the eastern slopes of the study area, the excavation of slopes, mainly for house building, is an important destabilizing factor for the western slopes. The growing population density not only increases the risk on damage, but hampers the search for solutions for the landslide problem as well. (c) 2005 Elsevier B.V. All rights reserved.