Laboratory Rainfall Simulation Research Articles

Subsurface Sediment Transport in the Shallow Vadose Zone of Fine‐Textured Soils With Heterogenous Preferential Flows

ABSTRACTSubsurface sediment transport in tile‐drained landscapes occurs through macropores; however, little is known regarding how heterogeneous preferential flows influence fluxes. We performed laboratory rainfall simulations on 10 intact core lysimeters from a tile‐drained field in Indiana, USA to study the impacts of surface and subsurface erosion on sediment leachate in heterogeneous preferential flow paths. Seven rainfall simulations were conducted to assess the impact of rainfall intensity on the leachate of surface eroded sediments (three events), and the impact of antecedent conditions on subsurface eroded sediments (four events). Cumulative sediment yield, linear mixed effects modelling, and hysteresis analyses were performed for all events. Results were presented in a series of four case studies. Results showed that surface sediment leachate concentration and yield were tightly linked to the filtration capacity of lysimeters, with more than 2/3rd of sediment originating from a single lysimeter, despite similar flow leachate volumes from each. Rainfall intensity significantly impacted the transport of surface eroded sediment at the highest intensity. Subsurface sediment erosion from undisturbed macropores was low compared to surface soils, but we found contrasting controls on sediment concentrations at low and high antecedent moistures that were equally important to sediment leachate yields. Disturbed macropores produced comparable sediment yields to surface erosion and behaved similarly to soil pipes in terms of erosion mechanics. Hysteresis results generally highlighted contrasting results for surface and subsurface sources but suggest that the prominence of slow flow, low‐concentration leachate sources can alter the interpretation of results in field‐scale applications. Our findings underscore an array of processes and pathways for sediment transport in the shallow vadose zone, and results will be useful for evaluating new model formulations.

Evaluating the Effectiveness of the Biodegradable Superabsorbent Polymer (Fasal Amrit) on Soil Hydrological Properties: A Laboratory Rainfall Simulation Study

Superabsorbent polymers (SAPs) are effective soil amendments that can control soil erosion by improving soil quality. However, many commercial SAPs face challenges including limited biodegradability, high costs, and adverse effects on soil hydrological properties, which can lead to increased water and soil loss. This study examined the potential of lower dosages of biodegradable SAPs to improve the hydrological properties of “Shimajiri-maji” (clay) soil. Three concentrations of biodegradable Fasal Amrit polymer (EFP) (P1: 0, P2: 3 g m−2, and P3: 6 g m−2) were evaluated under three simulated rainfall intensities (I1: 35; I2: 70 and I3: 110 mm h−1) and two gradients (7.5%, and 15%) during consecutive storms. The time to generate runoff, infiltration, runoff, soil loss, and water storage (WS) were quantified over one hour. The results show that runoff generation was delayed in EFP-treated soils compared to the control. Both polymer treatments enhanced infiltration (P2 > P3 > P1) and reduced runoff and soil loss (P2 < P3 < P1). Higher EFP rates improved water storage at surface depths (P3 > P2 > P1). EFP-treated soils exhibited lower interrill erodibility, suggesting greater resistance to soil erosion compared to the control. EFP treatments also significantly improved the soil’s physical properties (bulk density, porosity, organic matter, aggregate stability). EFPs can diminish runoff and soil loss as the EFP-treated plots exhibited greater aggregate stability than the control. It was concluded that low EFP concentrations can improve soil hydrological properties and mitigate soil erosion. Further investigations are needed to optimize the EFP concentrations for different soil types.

Infiltration Characteristics and Hydrodynamic Parameters in Response to Topographic Factors in Bare Soil Surfaces, Laboratory Experiments Based on Cropland Fields of Purple Soil in Southwest China

Topography is an important factor that impacts the hydrological processes on sloping farmlands. Yet, few studies have reported the combined influences of slope gradient and slope position on infiltration characteristics and hydrodynamic parameters on sloping croplands in purple soil regions, an important area for agricultural productivity in Southwest China. Here, laboratory-simulated rainfall experiments were conducted in a steel trough (5 m long, 2 m wide, and 0.45 m deep), and rainfall lasted for 1 h at a rate of 90 mm h−1 to examine the variations in the infiltration rates and hydrodynamic parameters under varying slope gradients (i.e., 3°, 6°, 10°, 15°, 21°, and 27°) and slope positions (i.e., upper, middle, and lower), and explore the relationships between the infiltration rate and the soil detachment rate. The results showed that the infiltration rate decreased gradually with duration rainfall and ultimately approached a steady state in the six slope treatments. Cumulative infiltration ranged from 15.54 to 39.32 mm during rainfall, and gradually reduced with the increase of slope gradient. The Horton’s model outperforms other models for predicting the infiltration rate with an R2 value of 0.86. Factors such as Darcy–Weisbach friction, flow shear force, Manning friction coefficient, unit energy, and runoff depth varied in the following order: upper slope > middle slope > lower slope, whilst the Reynolds number and Froude number gradually increased along the slope transect from the upper to lower slope positions. A significant linear function was fitted between the soil detachment rate and the infiltration rate at the gentle slopes (3°, 6°, 10°), whereas an exponential relationship was observed at the steep slopes (15°, 21°, and 27°). Observation also suggested that 15° was the critical slope gradient of sediment detachment, infiltration characteristics, and hydrodynamic parameters. Our results provide theoretical insight for developing models that predict the impacts of topographic factors on hydrological characteristic and soil erosion in hilly agricultural landscapes of purple soil fields.

Varying particle size selectivity of soil erosion along a cultivated catena

Abstract Sheet erosion is a complex multi-factor-dependent process with high spatial heterogeneity on hillslopes. Although the individual factors have been well studied, their aggregated effect on size-selective erosional processes is highly uncertain. Therefore, this study concentrates on the aggregate size distribution and effective particle size distribution (PSD) of the aggregates in the soil loss, collected from different simulated hillslope positions and surface conditions. These simulated hillslope positions combine moisture content from the extremely dry to the saturated with related slope positions of 2, 5, and 12% steepness and different surface roughness (tilled and crusted surfaces) modelled in a laboratory rainfall simulator. Using hierarchical cluster analysis, the PSD of the aggregates was separated into three groups based on the differences in the 59–116 µm range of the PSD histograms, namely, macro-aggregates, 50–250 µm sized micro-aggregates, and <50 µm sized fractions were classified into distinct groups, although some micro-aggregate samples were classified into the macro-aggregate group. PSDs from the 50–250 µm aggregate size fraction were clustered into a group of macro-aggregates if the PSD changed with time (during the rainfall event), notably on rough surfaces. The role of the specified size range in the classification is believed to be due to the parallel presence of aggregates and single particles in this range. As aggregates have a lower density than mineral particles, they tend to be enriched in soil loss under low-energy runoff conditions. Moreover, all samples in the <50 µm fraction clustered into the macro-aggregate group were eroded from the smooth/crusted surface, probably due to the presence of larger particles. The results indicate that the combined effect of erosional factors is not apparent, and the impact of the crust and extreme moisture content on the selectivity and size distribution of the sediment requires further investigation.

Runoff nitrogen losses under confluence and diverging drainage systems in the sloped plot scale: A comparative study

The drainage system is a key measure for regulating runoff nutrient losses on sloping farmlands. Confluence and diverging drainage systems are two drainage layouts representing natural water network systems and are widely distributed in sloping farmlands; however, the effects of these drainage systems on runoff nutrient losses in the sloped plots remain unclear. This study investigated the effects of different drainage systems on the characteristics of runoff nitrogen (N) losses in sloped plots using laboratory rainfall simulations. Three treatments, including bare slope (without drainage system, CK), confluence drainage system (T1), and diverging drainage system (T2), were used to compare the changes in concentrations and losses of total nitrogen (TN), dissolved nitrogen (DN), and particulate nitrogen (PN), and the DN:TN ratio in runoff under a combination of 1.8 mm min−1 rainfall intensity and three slope gradients (5°, 10°, and 15°). The results showed that the time to runoff was significantly delayed in T2 compared with that in CK and T1 across all slopes (p < 0.05). Accumulated runoff depth was considerably lower in T1 and T2 than in CK across all slopes (p < 0.05). The TN and PN concentrations in T1 were markedly lower than those in T2 on the 10° and 15° slopes (p < 0.05). The DN concentration in T1 was lowest at the 5° slope (p < 0.05). TN loss in T1 was 14.7–33.9% and 17.9–30.3% lower than those in CK and T2 across all slopes, respectively (p < 0.05). The PN loss in T1 was 56.7% and 53.3% lower than that in T2 on the 10° and 15° slopes, respectively (p < 0.05). DN loss in T1 was 39.3–72.5% lower than that in CK for all slopes (p < 0.05). DN:TN in T2 was lower than that in CK and T1 at the 10° and 15° slopes (p < 0.05). Our results confirm the effectiveness of drainage systems in reducing runoff nutrient losses in a sloped plot and demonstrate that the confluence drainage system is better at reducing N losses in runoff than diverging drainage systems.

Effect of Fine Particle Content in Volcanic Ash on Slope Surface Runoff: Laboratory Rainfall Simulation Assuming a Slope with Thin Deposits of Ashfall

Effect of Fine Particle Content in Volcanic Ash on Slope Surface Runoff: Laboratory Rainfall Simulation Assuming a Slope with Thin Deposits of Ashfall

Influence of polycyclic aromatic hydrocarbons on water storage capacity of two lichens species

Abstract The wide variability in functional traits that enable the cosmopolitan distribution of lichens often includes the water storage capacity, S, of their thallus. Lichen S in forest canopies can be large enough to intercept and evaporate significant amounts of rainwater, contributing to the runoff-reduction ecosystem services provided by urban forests; however, S is likely influenced by the presence of air pollutants (polycyclic aromatic hydrocarbons, PAHs) in urban areas. PAHs, being both chemically hydrophobic and damaging to lichen thalli, are expected to reduce lichens’ S and, thereby, limit their contribution to hydrologic ecoservices of urban forests. Hence, the relationship between PAH accumulation and rainwater uptake was examined for two lichen species, common in urban forests around the world – Platismatia glauca and Pseudevernia furfuracea. Samples were collected from an area of low air pollution and another area in a highly urbanized city centre with high air pollution exposure (Kraków, Poland). Lichen S was determined using laboratory-simulated rainfall. PAH bioaccumulation differed between species and among the samples from clean and polluted environments. After exposure to polluted air, the concentration of PAHs was higher in P. glauca than P. furfuracea. Samples from the non-urban setting, however, showed no differences between the two species. In the case of P. glauca, S decreased from 35.8% in samples from clean environment to 8.3% after six months of exposure in the urban setting. The respective S values for P. furfuracea were 25.4% and 12.4%. Results strongly suggest that PAH exposure reduces S in both lichen species. The obtained results are important both in ecohydrology and microclimatology and are part of the research on the condition of urban forests.

Plot-based study to evaluate raindrop detachment capacity on moss-dominated biocrusted slope under simulated rainfall with different intensities

Biocrusts play a critical role in prevention of erosion, but little is known on the relationship between biocrust cover and raindrop detachment capacity at the slope scale. Evaluating the raindrop detachment capacity on biocrusted slope is essential for better knowledge of the rainfall-induced erosion of biocrusted slope. Thus, laboratory simulated rainfall experiments were conducted on plots (140 × 120 cm) representing moss-dominated biocrusted slopes to measure the amounts of raindrop detachment under a complete combination of different biocrust cover (0%, 20%, 40%, 50%, 60%, and 80%) and rainfall intensities (42, 60, 90, 120, and 150 mm h−1) at 26.79% slope gradient with two replications. The results showed that the biocrust cover, rainfall intensity, and their interaction all significantly affected the raindrop detachment modulus (RDm). The RDm in biocrust plot was significantly lower than in bare soil plot under rain events with the same rainfall intensities. The lower biocrust cover or the larger rainfall intensities led to higher RDm. Reduction benefit of raindrop detachment modulus (RB-RDm) were greater than its corresponding biocrust cover values. When the biocrust cover increased from 20% to 40%, the increase range and growth trend of RDm decreased with increasing rainfall intensity. Our results indicated that the development of biocrusts on slope is an effective way of reducing raindrop detachment by protecting soil in their covered area and increasing flow depth. When moss-dominated biocrust cover reached 40%, the raindrop detachment capacity effectively weakened even under rain events with large rainfall intensities. These findings may conducive to offer a scientific guidance for soil erosion control in the Loess Plateau and in other arid and semiarid regions.

Surface‐to‐tile drain connectivity and phosphorus transport: Effect of antecedent soil moisture

AbstractMacropores connecting surface soils to tile drains can alter water and nutrient transport through the subsurface. In this study, laboratory rainfall simulations with artificial macropores combined with edge‐of‐field monitoring were used to evaluate surface‐to‐tile drain connectivity and phosphorus (P) transport as a function of antecedent moisture conditions. Laboratory rainfall simulations using repacked soil boxes with different macropore layouts (i.e., no macropore, surface‐connected macropores, and disconnected macropores) were used to examine changes in water sources and flow pathways to tile drains with varying degrees of connectivity and antecedent wetness. Water, tracer, and P fluxes from a tile‐drained field were also monitored to quantify linkages among water flow pathways, antecedent wetness, and P delivery to tile drains. Both laboratory and field results showed that surface‐to‐tile drain connectivity was important for water transport through the subsurface under both dry and wet antecedent conditions. When soil conditions were dry, discharge was minimal and primarily comprised of event water that bypassed the soil matrix. Increasing wetness resulted in similar event water transport, but greater mobilization of stored pre‐event water and greater discharge; thus, the dominant source of tile water and the magnitude of tile discharge were substantially altered with changing antecedent moisture. Field data revealed that changes in drainage water source and discharge with increasing wetness impacted dissolved P transport. Dissolved P concentration decreased and loading increased with increasing wetness. Findings indicate that greater mobilization of pre‐event water under wet antecedent conditions acted as both a hydrologic and chemical buffer for subsurface dissolved P transport. Comparison of study results to water quality data from a larger edge‐of‐field network suggest that relationships between antecedent moisture conditions, water flow pathways, and P transport from the current study are broadly applicable across tile‐drained fields. Understanding processes controlling P delivery to tile drains has direct applicability for conservation practice implementation and improving process representation in models.

Roles of the stolon and erect grass species in surface–subsurface flow generation and red soil loss

Grass planting is an important and typical measure for soil and water conservation, while the roles of stolon and erect grass species in affecting surface–subsurface flow and soil erosion processes are little understood. Experimental plots (2 m × 0.5 m × 0.5 m) using red soil with two rainfall intensities (60 and 120 mm h−1) and grass coverage (30 % and 70 %) were conducted in laboratory rainfall simulation experiments. Two common and endemic stolon (Eremochloa ophiuroides) and erect (Lolium perenne) grass species were selected due to their fast growth, power adaptability, and wide application in reducing runoff and soil loss; and the bare soil plot was set up as the control. The results showed that bare plot had the highest mean surface flow coefficient (73.5 %), and then were L. perenne (57.1 %) and E. ophiuroides (44.9 %). The mean subsurface flow coefficients were found in order of E. ophiuroides (25.7 %), L. perenne (13.0 %) and bare plot (7.2 %). The role of stolon grass rather than erect grass in reducing surface flow but improving subsurface flow led to lower surface flow hydraulic characteristics such as flow velocity, Reynolds number, Darcy–Weisbach coefficient and Manning friction coefficient, and hydrodynamic characteristics like flow shear stress and stream power. Furthermore, the stolon grass generated the lowest soil loss rate (0.8 g m−2 min−1), while the values on erect grass and bare plot were 2.5 and 10.7 g m−2 min−1, respectively. The indicator of eroded sediment median size also proved the higher efficiency of stolon grass in soil loss control, with values of 31.8, 58.6 and 69.6 μm on stolon grass, erect grass and bare plots, respectively. The lower soil loss on stolon grass cover plot was attributed to the lower surface flow and hydraulic–hydrodynamic characteristics. The stolon grass was more sensitive to the coverage and rainfall intensity, which was explained by the higher flow and sediment trapping capability (2.2 ∼ 4.4 times) under lower coverage and rainfall intensity than erect grass, even under high rainfall intensity (1.1 ∼ 3.2 times). The stolon grass played more beneficial roles in soil and water conservation than erect grass, which was closely related to above-ground and under-ground parts of grass. This study can provide a better understanding of the roles of stolon and erect grass species for soil and water conservation and management practices.

Optimization of a Laboratory Rainfall Simulator to Be Representative of Natural Rainfall

The importance of understanding the effects of rainfall on different materials over time makes it essential to carry out controlled tests to reduce analysis time. Rainfall simulators have been in use for decades and have been implemented as technology and knowledge of the physical behavior of water advanced. There are two main types of rainfall simulators: gravity simulators and pressure simulators. In the former, the drop velocity is normally smaller than the terminal velocity reached by natural droplets; in the latter, the drop size is too small to be representative and has far more speed than the natural speed for those sizes. To solve this problem, a simulator has been developed where the terminal velocity of the raindrops is reached and the drop size can be varied by different nozzles of variable sizes, adapting it to the conditions of a given region. In this study, conditions similar to the rainfall conditions of the city of León have been achieved. This paper presents the design of a rainfall simulator that recreates different rainfall conditions and rainwater composition and its calibration process.

Effect of combining biogeotextile and vegetation cover on the protection of steep slope of highway in northern China: A runoff plot experiment

Biogeotextiles can be used to facilitate the formation of vegetation cover and to reduce soil erosion. Studies have demonstrated that only biogeotextile or vegetation cover can greatly reduce soil erosion. However, information about the effects of the combination of biogeotextile and vegetation cover on soil erosion is still limited, despite that the combination is the commonly practical form for bare road slope protection. Experimental plots, consisting of a relatively loose surface layer and a compacted sublayer, were constructed using movable soil-bin trolleys (200 × 100 × 40 cm3) filled with silt loam sieved through 1-cm screen. The plots were seeded with a seed mixture of tall fescue (Festuca elata Keng ex E. Alexeev) and alfalfa (Medicago sativa L.) as a grass-legume regime. Then the plots were covered with three types of biogeotextile, i.e., coir blanket (CB), mixed coir-straw blanket (MB), and straw blanket (SB). A bare slope (BS) was constructed as the control experiment. Irrigation was applied to ensure vegetation germination and growth. Plant characteristics were measured after every 20 days. Laboratory simulated rainfall of 71 mm/h intensity was applied to each plot for 60 min after 20, 40, and 60 days. Surface runoff and sediment were collected every 5 min during rainfall. The results showed that the MB and SB promoted increased total emergence density of plants by 47% and 23%, respectively, compared to case of vegetation growth without biogeotextiles. The dense structure of the CB impeded the emergence of alfalfa, leading to 9% lower total emergence density. Compared with bare slope (0 day BS), biogeotextiles increased runoff by 3% (MB)–19% (CB) and reduced erosion by 96% (MB and SB)–98% (CB). After 60 days, vegetation cover reduced runoff by 54% and reduced erosion by 81% compared with the BS case. The combination of biogeotextile and vegetation cover reduced runoff by 44% (CB)–62% (SB) and reduced erosion by no less than 99% compared to the BS case. Compared with biogeotextile alone, the combination reduced runoff by 53% (CB)–64% (SB). Compared with vegetation cover alone, the combination reduced erosion by 94% (MB)–99% (CB). The combination takes advantages of vegetation cover for long-term protection and of biogeotextile for facilitating the formation of vegetation cover and immediate erosion control. Thus, the combination is a better choice for road slope protection in northern China. These findings may promote the understanding of how biogeotextiles and vegetation cover work together for runoff and erosion control on bare slopes. In future research more attention should be paid to the selection of plant species and biogeotextile types to avoid impediment by the biogeotextile on the formation of vegetation cover like CB affected alfalfa in this study.

Investigation of Infiltration and Runoff Rate on Agri-Mats Using a Laboratory Rainfall Simulation Study

ABSTRACT Soil erosion by water is a major global environmental concern due to its devastating effect on agricultural sustainability and food security. The aim of the current study was to investigate the effectiveness of agri-mats on water erosion control using a laboratory rainfall simulator. Six agri-mat samples were subjected to a runoff and infiltration rate test for this study: three agri-mats were made with 100% dry sugarcane bagasse and the other three agri-mats were made with 10% dry algae +90% dry sugarcane bagasse. The 100% dry sugarcane bagasse agri-mat treatment had higher runoff rate of 16.8 millimeters per hour (mm.hr−1) compared to a mere 1.7 mm.hr−1 recorded under the BA agri-mat during the first 10 min. In addition, the 100% dry sugarcane bagasse agri-mats released magnesium at a rate of 105.7 mg.hr−1 (milligrams per hour) compared to 6.6 mg.hr−1 lost from the 10% dry algae +90% dry sugarcane bagasse agri-mat treatment. Results shows that when algae was used as an additional organic material during the fabrication process, agri-mat delayed runoff by absorbing more moisture than it loses. Algae has the capacity to increase water-holding capacity while improving nutrient status and electrical conductivity of the agri-mats, making them more beneficial not only as the runoff combating technology but as a soil conditioner.

Exploring the Role of Ash on Pore Clogging and Hydraulic Properties of Ash-Covered Soils under Laboratory Experiments

Fires can alter the hydraulic properties of burned soils through the consumption of organic matter on the ground surface. This study examined the effects of rainfall on the presence of soil pore clogging with varying ash layer thickness using laboratory rainfall simulator experiments. The image analysis with resin impregnation showed that rainfall impact caused plugging of soil pores at 22.2% with soil particles and 14.3% with ash particles on near surface soils (0–5 mm below). High rainfall intensities enhanced soil pore clogging by ash particles, particularly at shallow soil depths (0–10 mm). Ash deposits on the soil surface increased the water-absorbing capacity of ash-covered soils compared with that of bare soils. The rainfall simulation experiments also showed that ash cover led to a reduction in soil hydraulic conductivity, owing to the combined effects of surface crust formation and soil pore clogging. The complementary effects of soil pore clogging and water absorption by ash cover could hamper the accurate understanding of the soil hydrologic processes in burned soils.

SMODERP2D—Sheet and Rill Runoff Routine Validation at Three Scale Levels

Water erosion is the main cause of soil degradation in agricultural areas. Rill erosion can contribute vastly to the overall erosion rate. It is therefore crucial to identify areas prone to rill erosion in order to protect soil quality. Research on rainfall-runoff and subsequent sediment transport processes is often based on observing these processes at several scales, followed by a mathematical description of the observations. This paper presents the use of a combination of data obtained by different approaches at multiple scales to validate the SMODERP2D episodic hydrological-erosion model. This model describes infiltration, surface retention, surface runoff, and rill flow processes. In the model, the surface runoff generation is based on a water balance equation and is described by two separate processes: (a) for sheet flow, the model uses the kinematic wave approximation, which has been parameterized for individual soil textural classes using laboratory rainfall simulations, and (b) for rill flow, the Manning formula is used. Rill flow occurs if the critical water level of sheet flow is exceeded. The concept of model validation presented here uses datasets at different scales to study the surface runoff and erosion processes on the Býkovice agricultural catchment. The first dataset consisted of runoff generated by simulated rainfall on plots with dimensions of 2 × 8 m. The second dataset consisted of the runoff response to natural rainfall events obtained from long-term monitoring of 50 m2 plots. These two datasets were used to validate and calibrate the sheet flow and infiltration parameters. The third dataset consisted of occurrence maps of rills formed during heavy rainfalls obtained using remote sensing methods on a field plot with an area of 36.6 ha. This last dataset was used to validate the threshold critical water level that is responsible in the model for rill flow initiation in the SMODERP2D model. The validation and the calibration of the surface runoff are performed well according to the Nash–Sutcliffe efficiency coefficient. The scale effect was evident in the 50 m2 plots where parameters lower than the mean best fit the measured data. At the field plot scale, pixels with measured rills covered 5% of the total area. The best model solution achieved a similar rill cover for a vegetated soil surface. The model tended to overestimate the occurrence of rills in the case of simulations with bare soil. Although rills occurred both in the model and in the monitored data in many model runs, a spatial mismatch was often observed. This mismatch was caused by flow routing algorithm displacement of the runoff path. The suitability of the validation and calibration process at various spatial scales has been demonstrated. In a future study, data will be obtained from various localities with various land uses and meteorological conditions to confirm the transferability of the procedure.

Erosion and covered zones altered by surface coverage effects on soil nitrogen and carbon loss from an agricultural slope under laboratory-simulated rainfall events

Soil erosion, one of the most serious environmental concerns, might remove topsoil and essential element from terrestrial land. However, few attentions have been given to investigating how soil erosion regimes affect soil carbon and nitrogen loss. Therefore, this study investigated the effects of surface coverage rates (83%, 67%, 50%, 33%, 17% and 0%) and two positions (up- and downslope) on erosion regimes and its associated soil nitrogen and carbon loss under a sequence of six rainfalls (R1-R6). These results showed that the sediment concentrations with 33% (R4) and 17% (R5) coverage downslope were significantly lower than those with coverage upslope, whereas there was no significant difference between the runoff rates of the two slopes. Thus, surface coverage at different positions induced two soil erosion regimes (deposition- and transport-dominated processes). Dynamics of the DON and DIN concentrations indicated different release processes of soil nitrogen into runoff. The DON contributed to a substantial amount of soil nitrogen loss, which accounted approximately 81% of the organic form. The SBOC is significantly correlated with sediment-enriched clay particles from the deposition-dominated processes and is higher than that from the transport-dominated processes. The DOC is significantly correlated with Rr for transport-dominated processes. These results illustrated the critical role of erosion regimes in soil organic carbon loss in dissolved or sediment-bound form. It is concluded that erosion/covered zones altered by surface coverage could produce transport- and deposition-dominated erosion regimes and consequently affect soil carbon and nitrogen loss. In addition, these results demonstrated that surface coverage pattern may efficiently control soil erosion and soil carbon and nitrogen loss.

Physical and numerical modelling of infiltration and runoff in unsaturated exposed soil using a rainfall simulator

Context Tropical soils have complex hydromechanical behaviour compared to ordinary soils and are often found in regions with well-defined wet and dry seasons. The analysis of the interaction between the soil and the atmosphere comprises understanding of multiple phenomena, such as infiltration and runoff. Unfortunately, the dynamics of soil–atmosphere interaction are commonly modelled at the watershed scale, using average parameters that do not allow an in depth understanding of the soil–water phenomena involved. Aims This paper presents an investigation of the soil–atmosphere interaction at the local scale, using numerical and physical modelling of the infiltration and runoff of an exposed tropical soil in a laboratory rainfall simulator. Methods The effect of rainfall with two different intensities of 86.0 and 200.0 mm h−1 was used to physically and numerically evaluate infiltration parameters, runoff, volumetric water content, and degree of saturation at five locations in the soil specimen. Key results Calibration of the numerical model showed a maximum root-mean-square error of 0.17. In addition, the modelling exercises indicated the need for an equilibrium time of 48 h for the sample studied under the imposed conditions. Conclusions Results of numerical simulation showed that the representation of the physical model by the numerical model was satisfactory and promising. Thus, the numerical model showed applicability for validating the boundary conditions of physical tests using rainfall simulators.

Lab simulation of soil erosion on cultivated soil slopes with wheat straw incorporation

Sloping lands are prone to heavy soil erosion, resulting in farmland degradation in most mountainous regions. Straw incorporation can enhance resistance to erosion by improving the soil structure and reducing runoff. Laboratory-simulated rainfall experiments were conducted on silt loam soil to measure the effect of wheat straw incorporation on soil erosion and sediment delivery processes. Different wheat straw incorporation rates (WSIRs: 0, 2, 4, and 8 t ha−1), slope gradients (SGs: 10°, 15°, and 20°), and rainfall intensities (RIs: 80, 120, and 160 mm h−1) were evaluated. The results showed that the total rainfall amount was set at 100 mm. The sediment delivery processes were measured using oven-dried runoff samples collected during the specific intervals. The experimental data revealed that sediment delivery processes increased with increasing rainfall duration and decreased with increasing straw incorporation rates, respectively. Straw incorporation significantly reduced soil losses under steep slopes (20°) and during high-intensity rainfall (160 mm h−1). Compared with the control plot, the sediment concentrations, sediment delivery rates, and soil losses of slopes with straw incorporation were reduced by 52%, 62%, and 63%, respectively. The reduced sediment delivery rate and soil loss be attributed to increased soil surface roughness (exposed straw) that intercepted runoff, increased soil infiltration, and reduced flow velocity. The incorporated straw indirectly reduced sediment concentration by improving soil structure, which resulted in a smaller reduction in sediment concentration. The sediment reduction was maximized at 80 mm h−1 or 10°, while the benefits of straw incorporation on soil and water conservation were reduced with higher rainfall intensities and slope gradients. This study can serve as a basis for guiding soil and water conservation on sloping farmlands.

Sand cover enhances rill formation under laboratory rainfall simulation

Soil erosion was severe in the wind-water erosion crisscross region of the northern Loess Plateau of China, where aeolian sand cover may enhance water erosion. In order to evaluate the effects of sand cover on runoff, soil loss and rill information, a series of rainfall simulation experiments was conducted in two contrasting treatments. One is a bare loess soil slope serving as a control, and another is a sand-covered loess slope (namely sand cover treatment). Nine simulated rains, 60 min each, were applied to each treatment in duplicates. The results showed that the runoff generation mechanism changed from infiltration-excess runoff in the control to subsurface saturation-excess runoff in the sand cover treatment. The runoff initiation was delayed about three folds in the sand cover treatment as compared to the control. Total runoff volumes and total soil loss in the sand cover treatment in each event were 1.1–1.2 times and 1.9–9.3 times those in the control treatment. A paired t-test showed that the runoff rates were greater in sand cover than in control in all events except for the last run at P < 0.05. The soil loss rates were greater in sand cover than in control for all events except the first rain at P < 0.05. Measured total rill length, width, and degree of rill dissection in the sand cover treatment were much greater than those in the control. The rill erosion of sand cover slope was dominated by the shallow and wider rills, while loess slope was dominated by the narrow and deeper rills. The results clearly indicated that a sand layer overlying a less pervious loess soil facilitated return flow and thus accelerated rill formation and development. This finding is in line with the common understanding that return flow in layered soil can promote rill formation and cause severe rill erosion, which is also important to understand the effect of wind-blown sand cover on water erosion in the wind-water erosion crisscross region.

Chinese zokor (Myospalax fontanierii) excavating activities lessen runoff but facilitate soil erosion – A simulation experiment

The Chinese zokor (Myospalax fontanierii) affects the physical and chemical properties of soil and the evolution of vegetation. However, few studies have evaluated the effects of zokors on soil erosion. In this study, alfalfa (Medicago sativa L.) was planted in tanks to quantify the effects of zokor excavation activities on runoff and soil erosion rate. Three slope gradients of 5°, 10°, and 15° were set, two soil tanks (with and without a zokor) were used for each gradient, and six soil tanks were prepared in total. Laboratory simulated rainfall was applied using a side-spray simulation system, and the rainfall intensity was set to 80 mm h−1 with the rainfall duration of 60 min after runoff generation. Results showed that the soil bulk density of fresh zokor mounds was 0.82 ± 0.02 g cm−3, which was 39% lower than that of the soil matrix (1.35 g cm−3). The vegetation coverage decreased to 32%, 45%, and 43% respectively, after 3 days of disturbance by zokor, compared with 87%, 90%, and 92% in the tanks without zokor. The presence of zokor reduced the runoff rate by 88% on the lowest gradient to 21% on the steepest gradient, and increased the water infiltration and soil water storage within 90 cm depth. The soil mounds and herbivorous tunnel changed the microtopography and consequently the runoff pathway, increased the sediment yield, and intensified soil erosion, especially at a steep slope gradient. Although the activity of zokors does not directly increase soil erosion, and the tunnel system can facilitate water infiltration and lessen runoff, the mounds they create provide loose and erodible materials. The destruction of vegetation by zokors would facilitate soil erosion and reduce the benefit of vegetation restoration. This study provides insights into the effects of subterranean rodents on soil erosion in the Loess Plateau.