Effects of rainfall and slope on runoff,soil erosion and rill development: an experimental study using two loess soils

Runoff generation and soil loss from slopes have been studied for decades, but the relationships among runoff, soil loss and rill development are still not well understood. In this paper, rainfall simulation experiments were conducted in two neighbouring plots (scale: 1 m by 5 m) with four varying slopes (17.6%, 26.8%, 36.4% and 46.6%) and two rainfall intensities (90 and 120 mm h^?1) using two loess soils. Data on rill development were extracted from the digital elevation models by means of photogrammetry. The effects of rainfall intensity and slope gradient on runoff, soil loss and rill development were different for the two soils. The runoff and soil loss from the Anthrosol surface were generally higher than those from the Calcaric Cambisol surface. Higher rainfall intensity produced less runoff and more sediment for almost each treatment. With increasing slope gradient, the values of cumulative runoff and soil loss peaked, except for the treatments with 90 mm h^?1 rainfall on the slopes with Anthrosol. With rainfall duration, runoff discharge decreased for Anthrosol and increased for Calcaric Cambisol for almost all the treatments. For both soils, sediment concentration was very high at the onset of rainfall and decreased quickly. Almost all the sediment concentrations increased on the 17.6% and 26.8% slopes and peaked on the 36.4% and 46.6% slopes. Sediment concentrations were higher on the Anthrosol slopes than on the Calcaric Cambisol slopes. At 90 mm h^?1 rainfall intensity, increasingly denser rills appeared on the Anthrosol slope as the slope gradient increased, while only steep slopes (36.4% and 46.6%) developed rills for the Calcaric Cambisol soil. The contributions of rill erosion ranged from 36% to 62% of the cumulative soil losses for Anthrosol, while the maximum contribution of rill erosion to the cumulative soil loss was only 37.9% for Calcaric Cambisol. Copyright © 2014 John Wiley & Sons, Ltd.     

Runoff and sediment loss responses to rainfall and land use in two agricultural catchments on the Loess Plateau of China

Soil erosion is a severe problem hindering sustainable agriculture on the Loess Plateau of China. Plot experiments were conducted under the natural rainfall condition during 1995–1997 at Wangdongguo and Aobao catchments in this region to evaluate the effects of various land use, cropping systems, land slopes and rainfall on runoff and sediment losses, as well as the differences in catchment responses. The experiments included various surface conditions ranging from bare soil to vegetated surfaces (maize, wheat residue, Robinia pseudoacacia L., Amorpha fruticosa L., Stipa capillata L., buckwheat and Astragarus adsurgens L.). The measurements were carried out on hill slopes with different gradients (i.e. 0 ° to 36 °). These plots varied from 20 to 60 m in length. Results indicated that runoff and erosion in this region occurred mainly during summer storms. Summer runoff and sediment losses under cropping and other vegetation were significantly less than those from ploughed bare soil (i.e. without crop/plant or crop residue). There were fewer runoff and sediment losses with increasing canopy cover. Land slope had a major effect on runoff and sediment losses and this effect was markedly larger in the tillage plots than that in the natural grass and forest plots, although this effect was very small when the maximum rainfall intensity was larger than 58·8 mm/h or smaller than 2·4 mm/h. Sediment losses per unit area rose with increasing slope length for the same land slope and same land use. The effect of slope length on sediment losses was stronger on a bare soil plot than on a crop/plant plot. The runoff volume and sediment losses were both closely related to rainfall volume and maximum intensity, while runoff coefficient was mainly controlled by maximum rainfall intensity. Hortonian overland flow is the dominant runoff process in the region. The differences in runoff volume, runoff coefficient and sediment losses between the catchments are mainly controlled by the maximum rainfall intensity and infiltration characteristics. The Aobao catchment yielded much larger runoff volume, runoff coefficient and sediment than the Wangdongguo catchment. Copyright © 2001 John Wiley & Sons, Ltd.     

Sediment and solute transport on soil slope under simultaneous influence of rainfall impact and scouring flow

Soil erosion and nutrient losses with surface runoff in the loess plateau in China cause severe soil quality degradation and water pollution. It is driven by both rainfall impact and runoff flow that usually take place simultaneously during a rainfall event. However, the interactive effect of these two processes on soil erosion has received limited attention. The objectives of this study were to better understand the mechanism of soil erosion, solute transport in runoff, and hydraulic characteristics of flow under the simultaneous influence of rainfall and shallow clear‐water flow scouring. Laboratory flume experiments with three rainfall intensities (0, 60, and 120 mm h⁻¹) and four scouring inflow rates (10, 20, 30, and 40 l min⁻¹) were conducted to evaluate their interactive effect on runoff. Results indicate that both rainfall intensity and scouring inflow rate play important roles on runoff formation, soil erosion, and solute transport in the surface runoff. A rainfall splash and water scouring interactive effect on the transport of sediment and solute in runoff were observed at the rainfall intensity of 60 mm h⁻¹ and scouring inflow rates of 20 l min⁻¹. Cumulative sediment mass loss (Ms) was found to be a linear function of cumulative runoff volume (Wr) for each treatment. Solute transport was also affected by both rainfall intensity and scouring inflow rate, and the decrease in bromide concentration in the runoff with time fitted to a power function well. Reynolds number (Re) was a key hydraulic parameter to determine erodability on loess slopes. The Darcy–Weisbach friction coefficients (f) decreased with the Reynolds numbers (Re), and the average soil and water loss rate (M_l) increased with the Reynolds numbers (Re) on loess slope for both scenarios with or without rainfall impact. Copyright © 2010 John Wiley & Sons, Ltd.     

Compaction and cover effects on runoff and erosion in post-fire salvage logged areas in the Valley Fire,California

Runoff and erosion processes can increase after wildfire and post-fire salvage logging, but little is known about the specific effects of soil compaction and surface cover after post-fire salvage logging activities on these processes. We carried out rainfall simulations after a high-severity wildfire and post-fire salvage logging to assess the effect of compaction (uncompacted or compacted by skid traffic during post-fire salvage logging) and surface cover (bare or covered with logging slash). Runoff after 71 mm of rainfall across two 30-min simulations was similar for the bare plots regardless of the compaction status (mean 33 mm). In comparison, runoff in the slash-covered plots averaged only 22 mm. Rainsplash in the downslope direction averaged 30 g for the bare plots across compaction levels and decreased significantly by 70% on the slash-covered plots. Sediment yield totalled 460 and 818 g m⁻² for the uncompacted and compacted bare plots, respectively, and slash significantly reduced these amounts by an average rate of 71%. Our results showed that soil erosion was still high two years after the high severity burning and the effect of soil compaction nearly doubled soil erosion via nonsignificant increases in runoff and sediment concentration. Antecedent soil moisture (dry or wet) was the dominant factor controlling runoff, while surface cover was the dominant factor for rainsplash and sediment yield. Saturated hydraulic conductivity and interrill erodibility calculated from these rainfall simulations confirmed previous laboratory research and will support hydrologic and erosion modelling efforts related to wildfire and post-fire salvage logging. Covering the soil with slash mitigated runoff and significantly reduced soil erosion, demonstrating the potential of this practise to reduce sediment yield and soil degradation from burned and logged areas.     

Influence of thawed soil depth on rainfall erosion of frozen bare meadow soil in the Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau has a vast area of approximately 70×10⁴ km² of alpine meadow under the impacts of soil freezing and thawing, thereby inducing intensive water erosion. Quantifying the rainfall erosion process of partially thawed soil provides the basis for model simulation of soil erosion on cold-region hillslopes. In this study, we conducted a laboratory experiment on rainfall-induced erosion of partially thawed soil slope under four slope gradients (5, 10, 15, and 20°), three rainfall intensities (30, 60, and 90 mm h⁻¹), and three thawed soil depths (1, 2, and 10 cm). The results indicated that shallow thawed soil depth aggravated soil erosion of partially thawed soil slopes under low hydrodynamic conditions (rainfall intensity of 30 mm h⁻¹ and slope gradient ≤ 15°), whereas it inhibited erosion under high hydrodynamic conditions (rainfall intensity ≥ 60 mm h⁻¹ or slope gradient > 15°). Soil erosion was controlled by the thawed soil depth and runoff hydrodynamic conditions. When the sediment supply was sufficient, the shallow thawed soil depth had a higher erosion potential and a larger sediment concentration. On the contrary, when the sediment supply was insufficient, the shallow thawed soil depth resulted in lower sediment erosion and a smaller sediment concentration. The hydrodynamic runoff conditions determined whether the sediment supply was sufficient. We propose a model to predict sediment delivery under different slope gradients, rainfall intensities, and thawed soil depths. The model, with a Nash–Sutcliffe efficiency of 0.95, accurately predicted the sediment delivery under different conditions, which was helpful for quantification of the complex feedback of sediment delivery to the factors influencing rainfall erosion of partially thawed soil. This study provides valuable insights into the rainfall erosion mechanism of partially thawed soil slopes in the Qinghai–Tibet Plateau and provides a basis for further studies on soil erosion under different hydrodynamic conditions.     

Effects of the undecomposed layer and semi-decomposed layer of Quercus variabilis litter on the soil erosion process and the eroded sediment particle size distribution

Simulated rainfall experiments were performed on bare, undecomposed litter layer and semi-decomposed litter layer slopes with litter biomasses of 0, 50, 100 and 150 g m⁻², respectively, to evaluate the effect of the undecomposed layer and semi-decomposed layer of Quercus variabilis litter on the soil erosion process and the particle size distribution of eroded sediment. The undecomposed layer and semi-decomposed layer of litter reduced the runoff rate by 10.91–27.04% and 12.91–36.05%, respectively, and the erosion rate by 13.35–40.98% and 17.16–59.46%, respectively. The percentage of smaller particles (clay and fine silt particles) decreased and the percentage of larger particles (coarse silt and sand particles) increased with an increased rainfall duration on all treated slopes, while the extent of the eroded sediment particle content varied among the treated slopes with the rainfall duration, with bare slopes exhibiting the largest variability, followed by undecomposed litter layer slopes and finally semi-decomposed litter layer slopes. The clay and sand particles were transported as aggregates, and fine silt and coarse silt particles were transported as primary particles. Compared with the original soil, sediment eroded from all treated slopes was mainly enriched in smaller particles. Furthermore, the loss of the smaller particles from the undecomposed litter layer slopes was lower than that from the semi-decomposed litter layer slopes, indicating that the undecomposed litter layer alleviated soil coarsening to some extent. The findings from this study improve our understanding of how litter regulates slope erosion and provide a reference for effectively controlling soil erosion.     

Characteristics of runoff and sediment yield for two typical erodible soils in southern China

Granite red soil (GRS) and Quaternary red clay (QRC) are two typical erodible soils in the red-soil region of southern China. Analytical and comparative studies of the characteristics of runoff and sediment yield for the two soils at various slopes are currently needed. The purpose of the current study was to clarify the characteristics of runoff and sediment yield for GRS and QRC at different slopes and to establish models for estimating sediment yield for the two soils. Forty-eight runoff microplots with four slopes (5°, 15°, 25°, and 35°) and two soils (GRS and QRC) were established and exposed to natural rainfall. Runoff and sediment yield were measured 10 times during the study period. Runoff and sediment yield for the two soils under the various slopes had similar temporal variations, and both increased with prior cumulative erosive rainfall. Runoff for GRS and QRC was moderately temporally variable, with coefficients of variation (CVs) from 46.2% to 60.6%, and sediment yield for QRC was strongly temporally variable, with CVs from 114.8% to 145.8%. Sediment yield for GRS increased with slope, but sediment yield for QRC first increased and then decreased, with a calculated inflection point of 18°, but runoff for both soils decreased with slope. The CVs of both runoff and sediment yield with slope for the two soils ranged from 3.6% to 88.0%, lower than the temporal variabilities, indicating that rainfall may have a larger impact than slope on runoff and sediment yield for QRC and GRS. Under the various slopes, runoff and sediment yield for both soils increased with rainfall and sediment yield increased with runoff, but the proportions of effective rainfall and runoff differed. Pedotransfer-function models based on rainfall, runoff, and slope accurately estimated sediment yield for the two soils, with the model fit coefficient of determination (R²) > 0.81 and the R² for verification >0.79. These results improve the understanding of the laws governing erosion for different soil types in the red-soil region of southern China and are important for managing the erosion of collapsing gullies and sloping farmland in the region.     

Rainfall runoff and erosion in Napa Valley vineyards: effects of slope,cover and surface roughness

The effects of slope, cover and surface roughness on rainfall runoff, infiltration and erosion were determined at two sites on a hillside vineyard in Napa County, California, using a portable rainfall simulator. Rainfall simulation experiments were carried out at two sites, with five replications of three slope treatments (5%, 10% and 15%) in a randomized block design at each site (0%bsol;64 m² plots). Prior to initiation of the rainfall simulations, detailed assessments, not considered in previous vineyard studies, of soil slope, cover and surface roughness were conducted. Significant correlations (at the 95% confidence level) between the physical characteristics of slope, cover and surface roughness, with total infiltration, runoff, sediment discharge and average sediment concentration were obtained. The extent of soil cracking, a physical characteristic not directly measured, also affected analysis of the rainfall–runoff–erosion process. Average cumulative runoff and cumulative sediment discharge from site A was 87% and 242% greater, respectively, than at site B. This difference was linked to the greater cover, extent of soil cracking and bulk density at site B than at site A. The extent of soil cover was the dominant factor limiting soil loss when soil cracking was not present. Field slopes within the range of 4–16%, although a statistically significant factor affecting soil losses, had only a minor impact on the amount of soil loss. The Horton infiltration equation fit field data better than the modified Philip's equation. Owing to the variability in the ‘treatment’ parameters affecting the rainfall–runoff–erosion process, use of ANOVA methods were found to be inappropriate; multiple‐factor regression analysis was more useful for identifying significant parameters. Overall, we obtained similar values for soil erosion parameters as those obtained from vineyard erosion studies in Europe. In addition, it appears that results from the small plot studies may be adequately scaled up one to two orders of magnitude in terms of land areas considered. Copyright © 2000 John Wiley & Sons, Ltd.     

Loess erosion change modeling during heavy rainfall

A typical gully sub-basin with a complex geomorphological form is used to do a model test of gravity erosion of loess by considering the sequence of slopes in a prototype gully creating a sequence of underlying surface forms in the upper reaches. The results show that the runoff from heavy rainfall is the main external force for the erosion of loess, and also is an important influencing factor to stimulate and intensify the development of gravity erosion. The soil structure and the height of the...     

Runoff generation and routing on artificial slopes in a Mediterranean–continental environment: the Teruel coalfield,Spain

The aim of this study was to identify the mechanisms of runoff generation and routing and their controlling factors at the hillslope scale, on artificial slopes derived from surface coal mining reclamation in a Mediterranean–continental area. Rainfall and runoff at interrill and microcatchment scales were recorded for a year on two slopes with different substrata: topsoil cover and overburden cover. Runoff coefficient and runoff routing from interrill areas to microcatchment outlets were higher in the overburden substratum than in topsoil, and greater in the most developed rill network. Rainfall volume is the major parameter responsible for runoff response on overburden, suggesting that this substratum is very impermeable—at least during the main rainfall periods of the year (late spring and autumn) when the soil surface is sealed. In such conditions, most rainfall input is converted into runoff, regardless of its intensity. Results from artificial rainfall experiments, conducted 3 and 7 years after seeding, confirm the low infiltration capacity of overburden when sealed. The hydrological response shows great seasonal variability on the overburden slope in accordance with soil surface changes over the year. Rainfall volume and intensities (I₃₀, I₆₀) explain runoff at the interrill scale on the topsoil slope, where rainfall experiments demonstrated a typical Hortonian infiltration curve. However, no correlation was found at the microcatchment level, probably because of the loss of functionality of the only rill as ecological succession proceeded. The runoff generation mechanism on the topsoil slope is more homogeneous throughout the year. Runoff connectivity, defined as the ratio between runoff rates recorded at the rill network scale and those recorded at the interrill area scale in every rainfall event, was also greater on the rilled overburden slope, and in the most developed rill network. The dense rill networks of the overburden slope guarantee very effective runoff drainage, regardless of rainfall magnitude. Rills drain overland flow from interrill‐sealed areas, reducing the opportunity of reinfiltration in areas not affected by siltation. Runoff generation and routing on topsoil slopes are controlled by grass cover and soil moisture content, whereas on overburden slopes rill network density and soil moisture content are the main controlling factors. Copyright © 2002 John Wiley & Sons, Ltd.     

Surface microrelief changes affect the soil and water conservation benefits of rainwater harvesting tillage operation during rainfall events

Reservoir tillage (RT) improves the soil rainwater harvesting capacity and reduces soil erosion on cropland, but there is some debate regarding its effectiveness. The objective of this study was to further verify the effect of RT on soil erosion and explore the reasons for this effect by analysing microrelief changes during rainfall. Rainfall intensities of 60, 90, and 120 mm/hr and three slope degrees (5, 15, and 25°, representing gentle, medium, and steep slopes) were considered. A smooth surface (SS) served as the control. The microrelief changes were determined based on digital elevation models, which were measured using a laser scanner with a 2‐cm grid before and after rainfall events. The results showed that compared with the values for the SS, RT reduced both the runoff and sediment by approximately 10‐20% on the gentle slope; on the medium slope, although RT also reduced the runoff in the 90‐ and 120‐mm/hr intensity rainfall events, the sediment increased by 158.90% and 246.08%; on the steep slope, the sediment increased by 92.33 to 296.47%. Overall, when the runoff control benefit of RT was lower than 5%, there was no sediment control benefit. RT was effective at controlling soil loss on the gentle slopes but was not effective on the medium and steep slopes. This is because the surface depressions created by RT were filled in with sediment that eroded from the upslopes, and the surface microrelief became smoother, which then caused greater soil and water loss than that on an SS at the later rainfall stage.     

Soil and water losses from new citrus orchards growing on sloped soils in the western Mediterranean basin

Ten representative research sites were selected in eastern Spain to assess soil erosion rates and processes in new citrus orchards on sloping soils. The experimental plots were located at representatives sites on limestone, in areas with 498 to 715 mm year^?1 mean annual rainfall, north‐facing slopes, herbicide treated, and new (less than 3 years old) plantations. Ten rainfall simulation experiments (1 h at 55 mm h^?1 on 0·25 m² plots) were carried out at each of the 10 selected study sites to determine the interill soil erosion and runoff rates. The 100 rainfall simulation tests (10 × 10 m) showed that ponding and runoff occurred in all the plots, and quickly: 121 and 195 s, respectively, following rainfall initiation. Runoff discharge was one third of the rainfall, and sediment concentration reached 10·4 g L^?1. The soil erosion rates were 2·4 Mg ha^?1 h^?1 under 5‐year return period rainfall thunderstorms. These are among the highest soil erosion rates measured in the western Mediterranean basin, similar to badland, mine spoil and road embankment land surfaces. The positive relationship between runoff discharge and sediment concentration (r² = 0·83) shows that the sediment availability is very high. Soil erosion rates on new citrus orchards growing on sloped soils are neither tolerable nor sustainable. Copyright © 2009 John Wiley & Sons, Ltd.     

Evaluating and modelling the hydrological and erosive behaviour of an olive orchard microcatchment under no‐tillage with bare soil in Spain

Despite the high risk of erosion in olive orchards located in mountainous areas in Spain, little research has been carried out to account for the complexity and interaction of the natural processes of runoff and soil erosion on the catchment scale or small catchment scale. In this study, a microcatchment of 6·7 ha in a mountainous area under no‐tillage farming with bare soil was set up to record runoff and sediment. Soil erosion and runoff patterns were monitored over a two‐year period. Totally, 22 events were observed. The data were analysed, and then used to calibrate the AnnAGNPS model, which allowed us to complete the data period and describe the hydrological and erosive behaviour on a monthly and annual basis. A high variability in catchment responses was observed, due to differences in the storms and to the effect of the surface soil moisture content. Maximum intensities of 10 and 30 min determined the final runoff values while the total sediment loads were dependent on the rainfall depth. The impact of management on the reduction of porosity can explain the relationship between runoff and intensity in the microcatchment. However, the impact of the spatial scale meant that the transport of sediment required substantial rainfall depths to ensure a continuous flow from the hillslopes. The results of the calibration (E >0·60 and r >0·75) on the event and monthly scale confirmed the applicability of AnnAGNPS to predict runoff and erosion in the microcatchment. The predicted average runoff coefficient was 3·3% for the study period and the total average sediment loads, 1·3 Mg/ha/yr. Despite these low values, the model simulation showed that much larger runoff coefficients and soil losses can be expected for periods with several consecutive years in which the annual rainfall depth was over 500 mm. The use of cover is recommended to prevent the high levels of erosion associated with these conditions. Copyright © 2009 John Wiley & Sons, Ltd.     

Prescribed‐fire effects on rill and interrill runoff and erosion in a mountainous sagebrush landscape

Changing fire regimes and prescribed‐fire use in invasive species management on rangelands require improved understanding of fire effects on runoff and erosion from steeply sloping sagebrush‐steppe. Small (0·5 m²) and large (32·5 m²) plot rainfall simulations (85 mm h^–1, 1 h) and concentrated flow methodologies were employed immediately following burning and 1 and 2 years post‐fire to investigate infiltration, runoff and erosion from interrill (rainsplash, sheetwash) and rill (concentrated flow) processes on unburned and burned areas of a steeply sloped sagebrush site on coarse‐textured soils. Soil water repellency and vegetation were assessed to infer relationships in soil and vegetation factors that influence runoff and erosion. Runoff and erosion from rainfall simulations and concentrated flow experiments increased immediately following burning. Runoff returned to near pre‐burn levels and sediment yield was greatly reduced with ground cover recovery to 40 per cent 1 year post‐fire. Erosion remained above pre‐burn levels on large rainfall simulation and concentrated flow plots until ground cover reached 60 per cent two growing seasons post‐fire. The greatest impact of the fire was the threefold reduction of ground cover. Removal of vegetation and ground cover and the influence of pre‐existing strong soil‐water repellency increased the spatial continuity of overland flow, reduced runoff and sediment filtering effects of vegetation and ground cover, and facilitated increased velocity and transport capacity of overland flow. Small plot rainfall simulations suggest ground cover recovery to 40 per cent probably protected the site from low‐return‐interval storms, large plot rainfall and concentrated flow experiments indicate the site remained susceptible to elevated erosion rates during high‐intensity or long duration events until ground cover levels reached 60 per cent. The data demonstrate that the persistence of fire effects on steeply‐sloped, sandy sagebrush sites depends on the time period required for ground cover to recover to near 60 per cent and on the strength and persistence of ‘background’ or fire‐induced soil water repellency. Published in 2009 by John Wiley & Sons, Ltd.     

Land use change effects on runoff and erosion from plot to catchment scale on the basaltic plateau of Southern Brazil

In the region of the basaltic plateau in Southern Brazil, problems of runoff and erosion on the deep ferrallitic soils are becoming increasingly recognized. Land use change from conventional tillage using disk plough to no‐tillage on residues without terracing occurred at the beginning of the 1990s and it spread very quickly. Measurements of runoff and sediment concentrations on 1 m² plots receiving natural rainfall and simulated rainfall under different crops with different stages of growth and different tillage systems, field surveys and measurements of rills and gullies in nested experimental catchments indicate a relative decrease of runoff on slopes but an increase of subsurface flow, and a marked decrease of sheet and rill erosion and soil loss from plot to catchment scales. Nevertheless, the extension of parts of the gully system is still continuing, strongly influenced by extreme rainfall. Copyright © 1999 John Wiley & Sons, Ltd.     

Runoff and erosion processes and rates in the Zin valley badlands,northern Negev,Israel

Badland areas are usually regarded as impermeable zones which generate high runoff and are very vulnerable to sheetwash and rainsplash. To test those considerations sprinkling experiments using two rainfall simulators were carried out on slopes of varying aspect in the northern Negev (Israel). For one unit 1·5 m² plots were used with rainfall of natural characteristics at 36 mm/hr intensity and 43–48 minute duration, runoff being recorded and water/sediment samples taken every 5 minutes. The second unit was used on 30–50 m² plots but rainfall energy production was below that of natural rainfall. Results show that due to the high stability and strong flocculation of clay-rich aggregates rainsplash is ineffective in surface sealing so that infiltration capacities remain high despite intense, prolonged rainfall. Aspect differences are reflected in variation of surface properties despite homogenous bed-rock, which cause marked differences in hydrological response. North-facing slopes respond more quickly, more frequently and produce more runoff than south-facing slopes. Non-uniform runoff generation is also seen within plots of one aspect reflecting subtle variations in surface properties. Comparison of rainfall intensity and duration used during the experiments with those prevailing under natural conditions shows that under present day conditions surface flow in the Zin valley badlands must be extremely infrequent and denudation rates very low.     

The influence of leaf litter diversity and soil fauna on initial soil erosion in subtropical forests

Although the protective role of leaf litter cover against soil erosion is known for a long time, little research has been conducted on the processes involved. Moreover, the impact of soil meso‐ and macrofauna within the litter layer on erosion control is not clear. To investigate how leaf litter cover and diversity as well as meso‐ and macrofauna influence sediment discharge in subtropical forest ecosystems, a field experiment has been carried out in Southeast China. A full‐factorial random design with 96 micro‐scale runoff plots and 7 domestic leaf species was established and erosion was triggered by a rainfall simulator. Our results demonstrate that leaf litter cover protects soil from erosion (?82 % sediment discharge on leaf covered plots) by rainfall and this protection is removed as litter decomposes. The protective effect is influenced by the presence or absence of soil meso‐ and macrofauna. Fauna presence increases soil erosion rates significantly by 58 %, while leaf species diversity shows a non‐significant negative trend. We assume that the faunal effect arises from arthropods slackening and processing the soil surface as well as fragmenting and decomposing the protecting leaf litter covers. Even though the diversity level did not show a significant influence, single leaf species in monocultures show rather different impacts on sediment discharge and thus, erosion control. In our experiment, runoff plots with leaf litter from Machilus thunbergii showed the highest sediment discharge (68.0 g m^?2) whereas plots with Cyclobalanopsis glauca showed the smallest rates (7.9 g m^?2). Copyright © 2015 John Wiley & Sons, Ltd.     

Sediment transport capacity and erosion processes: model concepts and reality

Sediment transport capacity, T_c, defined as the maximum amount of sediment that a flow can carry, is the basic concept in determining detachment and deposition processes in current process-based erosion models. Although defined conceptually and used extensively in modelling erosion, T_c was rarely measured. Recently, a series of laboratory studies designed to quantify effects of surface hydrologic conditions on erosion processes produced data sets feasible to evaluate the concept of T_c. A dual-box system, consisting of 1·8 m long sediment feeder box and a 5 m long test box, was used. Depending on the relative magnitudes of sediment delivery from feeder and test boxes, five scenarios are proposed ranging from deposition-dominated to transport-dominated sediment regimes. Results showed that at 5 per cent slope under seepage or 10 per cent slope under drainage conditions, the runoff from the feeder box caused in the additional sediment transport in the test box, indicating a transport-dominated sediment regime. At 5 per cent slope under drainage conditions, deposition occurred at low rainfall intensities. Increases in slope steepness, rainfall intensity and soil erodibility shifted the dominant erosion process from deposition to transport. Erosion process concepts from the Meyer–Wishmeier, Foster–Meyer and Rose models were compared with the experimental data, and the Rose model was found to best describe processes occurring during rain. A process-based erosion model needs to have components that can represent surface conditions and physical processes and their dynamic interactions. Copyright © 1999 John Wiley & Sons, Ltd.     

Field studies of sediment transport by overland flow

Field studies on sandy soils of the Cottenham Series in mid-Bedfordshire show that the mean annual rate of sediment transport by overland flow on an 11° mid-slope is 98 g cm¹. The feasibility of using sediment transport equations to predict erosion by overland flow on a storm basis is examined by comparing the observed values of sediment yield with values predicted by four sediment transport equations and a regression equation which relates soil loss to runoff energy and rainfall energy. An expression combining Engelund's sediment transport capacity equation and the Manning equation for flow velocity, as modified by Savat for disturbed flow, best reflects field conditions. Although there is a significant correlation (r = 0.69; n 30) between the observed and predicted values using this expression, the coefficient of determination is too low for predictive purposes. Reasons for this are presented.     

Surface runoff and soil erosion under eucalyptus and oak canopy

To assess potential differences in stormwater runoff and sediment yield between plots of blue gum eucalyptus (Eucalyptus globulus) and coast live oak (Quercus agrifolia), we measured runoff, sediment yield, water repellency and soil moisture at eight paired sites. Eucalyptus has been associated in many studies worldwide with elevated soil water repellency and increased runoff, a likely contributor to soil erosion. To better understand these connections and their relationship to land cover, there is a need for studies employing either rainfall simulators or natural rainfall. Our research employs the latter, and was subject to contrasting hydrologic conditions in the two years of the study. Fieldwork was conducted from October 2006 to February 2008 in the San Francisco Bay Area of central California. During the 2006–2007 winter wet season, runoff was significantly higher under eucalypts than at paired oak sites, and in the early phases of the season was connected with elevated water repellency. However, sediment yield at all sites during the 2006–2007 hydrologic year was below the detection limit of the Gerlach sediment collection traps, possibly due to a limited wet season, and only appeared as suspended sediment captured in overflow buckets. Intensive rainfall events in January 2008 however created substantial runoff of sediment and litter with significantly greater yield at oak sites compared to paired eucalyptus sites. Water repellency likely had little effect on runoff during these events, and the primary cause of greater erosion under oaks is the thinner cover of leaf litter in comparison to eucalyptus. Our study is limited to undisturbed sites with intact litter cover that have not experienced recent wildfires; if disturbed, we would expect a different picture given the propensity for crown fires of eucalypts, enhancement of rainsplash erosion, and the likely greater potential for stream‐connected sediment yield from post‐disturbance soil erosion events.