Hillslope Soil Erosion Research Articles

Impacts of straw mulching in longitudinal furrows on hillslope soil erosion and deposition in the Chinese Mollisol region

In the Northeast Mollisol region of China, straw mulching has a great effect on preventing soil loss from ridge tillage, but how furrow straw mulching (FSM) impacts the spatial distribution of erosion-deposition and sediment sources in longitudinal ridge tillage systems is still unclear. Therefore, this study was to quantify the effects of the FSM on the spatial distribution of soil erosion-deposition and sediment sources in longitudinal ridge tillage systems by using the magnetism tracing methodology and indoor simulation rainfall experiments. The experimental treatments included two surface treatments: without the FSM (NFSM) and with the FSM of 0.30 kg·m−2 amount, two rainfall intensities of 50 and 100 mm h−1 were included; each experiment was run for 60 min at a soil pan with a 5° slope gradient, 6.2 m length, 1.3 m width and 0.7 m depth. The results indicated that, compared with the NFSM treatment, the total slope runoff and soil loss in the FSM treatment at two rainfall intensities were decreased by 41.6 % and 99.5 % and by 37.9 % and 97.7 %, respectively, and the reduction in slope runoff and soil loss decreased with an increase of rainfall intensity. The total mass of sediments intercepted by straw mulching on the entire slope accounted for 36.9 % and 10.4 % of the total soil loss in the NFSM treatment, respectively. Erosion regimes dominated on the hillslope of the NFSM treatment with strong-weak alternation at two rainfall intensities, while for the FSM treatment, only deposition regime was observed at 50 mm h−1 rainfall intensity; a slight change with erosion-deposition on the ridge tops was observed and deposition in the furrows was observed at 100 mm h−1 rainfall intensity. Furthermore, the FSM treatment changed the main sediment source from ridge tops to furrows on the hillslope compared to the NFSM treatment, whose sediment contributions from the ridge tops and the furrows at the two rainfall intensities were approximately 16.4 % and 83.6 %, respectively. In conclusion, FSM should be promoted due to its high efficiency in decreasing slope runoff and intercepting sediments. The findings of this study served as a scientific reference for effectively controlling soil erosion along slopes in the Chinese Mollisol region.

Assessing the hillslope-channel contributions to the catchment sediment balance under climate change

To get a full understanding of the impacts of global change on the catchment-scale sediment balance, models are needed that combine hillslope soil erosion processes with channel morphodynamics. Here we present a modification to the SPHY-MMF model that includes a novel channel morphodynamics module, which determines erosion and deposition in rills and channels. We applied the model to a Mediterranean study area in southeast Spain, in which we show that channel erosion contributes substantially (35%–40%) to the total sediment yield, highlighting the importance of accounting for channel erosion in catchment-scale sediment budget estimations. The climate change scenarios show that the different erosional processes (i.e. sheet, rill, channel) are projected to decrease or increase, depending on the projected change in annual and extreme precipitation. From this we conclude that interactions between different erosional and depositional processes should be considered when studying the impact of global change on the catchment-scale sediment balance.

Effects of upslope inflow rate, tillage depth, and slope gradients on hillslope erosion processes and hydrodynamic mechanisms

Inflow rate, tillage depth and slope gradient play vital roles in hillslope soil erosion processes. However, the effects of these factors and their interactions on hillslope soil erosion processes, particularly on long slopes in the Chinese Mollisol region, are still lacking to date. Thus, a set of laboratory experiments, including six upslope inflow rates (0, 50, 100, 150, 200 and 300 L min−1), two tillage depths (5 and 20 cm) and two slope gradients (5° and 10°), were conducted to quantify the impacts of these factors and their interactions on hillslope soil erosion processes. The results showed that soil erosion rates increased as a power function with increase of inflow rate and slope gradient, while they decreased as a power function with an increase in tillage depth. The slope gradient had a greater impact on soil erosion rates than inflow rate and tillage depth. Moreover, inflow rate, slope gradient, and their pairwise interaction mainly governed hillslope soil erosion. Compared with hillslope soil erosion, hillslope runoff rate was dominantly affected by inflow rate. The inflow rate and slope gradient affected the hydrodynamic characteristics and erosion rates by shifting erosion patterns; while tillage depth mainly influenced them by converting surface flow to seepage flow. In addition, the dimensionless effective stream power was the optimal index (R2 = 0.884, NSE = 0.861) affecting soil erosion rates, followed by the dimensionless stream power (R2 = 0.879, NSE = 0.848). The dimensionless soil erosion rates were correlated with the dimensionless hydrodynamic parameter following a power function. Therefore, soil erosion in the Chinese Mollisol region could be effectively prevented by dispersing runoff, increasing deep tillage and reducing the impact of slope gradient through soil conservation measures.

Climate and Vegetation Change, Hillslope Soil Erosion, and the Complex Nature of Late Quaternary Environmental Transitions, Eastern Mojave Desert, USA

In what are now the warm deserts of the American Southwest, direct effects of changing climate on plant distributions are typically viewed as the principal driver of vegetation changes that followed the late Pleistocene–Holocene transition (LPH). However, at a semi-arid site in the eastern Mojave Desert, the transition to modern, shrub-dominated desert scrub on xeric, south-aspect hillslopes occurred only after the erosion of relatively thick soils toward the end of the mid-Holocene. Soils with well-developed Bt horizons began to form in the late Pleistocene on both north- and south-aspect hillslopes through the entrapment and accumulation of aeolian sediments in coarse colluvium. Those soils are capable of absorbing and retaining substantial moisture and support relatively dense stands of perennial C4 grasses that have diffuse, fibrous root systems. The age of alluvial deposits on the basin floor indicates a surge in sediment production through the erosion of some of those hillslope soils toward the end of the mid-Holocene. However, that erosion was largely limited to the more xeric, more sparsely vegetated, south-aspect hillslopes. The soils formed on mesic north-aspect hillslopes remain largely non-eroded to the present day, demonstrating the central role of vegetation in modulating erosion and sediment supply. The loss of soils from south-aspect hillslopes fundamentally changed the capacity of those environments to absorb and store moisture, and altered the depth and temporal durations of plant-available moisture. Those hydrological changes drove a loss of perennial C4 grasses and a transition to dominance by xerophytic plants—shrubs with deeper taproots capable of extracting moisture stored within bedrock joints and fractures, and shallow-rooted succulent plants that store moisture internally. Following the LPH, vegetation change at the site apparently occurred in two distinct phases separated in time: (1) initial vegetation changes driven directly by increasing climatic aridity and (2) subsequent changes linked to the later episode of soil erosion. Although climate shifts ultimately generate vegetation changes, the proximate mechanisms to which plants directly respond can lag far behind climatic transitions and involve complex relationships of vegetation, soils, and changing soil hydrologic conditions.

A new large-scale suspended sediment model and its application over the United States

Abstract. Suspended sediment plays a vital role in the regional and global cycling of carbon and nutrients by carrying carbon and nutrients from headwaters into lowland rivers and the oceans. Sediment transport through river systems is often fundamentally modified by human activities such as reservoir management. However, a physically based representation of sediment transport is still missing in most existing Earth system models (ESMs), which are essential tools for modeling and predicting Earth system changes. Here, we introduce a multi-process river sediment module for ESMs, which includes (1) hillslope soil erosion and sediment discharge into streams, (2) sediment transport processes through river networks, (3) reservoir operation based on the inflows from upstream areas and water demand from downstream areas, and (4) sediment trapping by reservoirs. All model parameters are estimated a priori without calibration. We apply this new sediment modeling framework to the contiguous United States and validate it against historical observations of monthly streamflow and sediment discharges at 35 river gauges. The model reasonably well captures the long-term balance and seasonal variations of suspended sediment in large river systems. Furthermore, our model results show that suspended sediment discharge in managed rivers is affected more by reservoirs' direct trapping of sediment particles than by their flow regulation. This new sediment module enables future modeling of the transportation and transformation of carbon and nutrients carried by the fine sediment along the river–ocean continuum to close the global carbon and nutrient cycles.

Climate and Vegetation Change, Hillslope Soil Erosion, and the Complex Nature of Late Quaternary Environmental Transitions, Eastern Mojave Desert, USA

Climate and Vegetation Change, Hillslope Soil Erosion, and the Complex Nature of Late Quaternary Environmental Transitions, Eastern Mojave Desert, USA

Identifying the provenance of bottom sediments in the Three Gorges Reservoir using stable Pb isotopes

Sediment deposition is considered a potential risk to the operation of the Three Gorges Reservoir (TGR). In this study, the bottom sediments of the TGR and its main tributaries were sampled during the flood, dry, and even periods, for 2017–2018, together with the surrounding hillslope soils. These samples were analyzed for Pb isotopes, and major and trace elements. Through the Pb isotopic comparison, it was understood that the TGR sediments originated from the weathering of rocks in the Qiangtang, Songpan–Ganzi, and Regional Yangtze tectonic blocks. Combined with the hydrological data, the TGR sediments were isotopically determined to be primarily fed by upstream input, the hillslope soil erosion, and the catchment weathering of regional tributaries. The geochemical data were in agreement with the results of the Pb isotope analyses. Using the IsoSource model, proportional contributions from distal (e.g., Jinshajiang), regional (surrounding tributaries), and proximal (hillslope soils) sources were calculated to be 50%, 20%, and 30%, respectively, in the flood period, 40%, 30%, and 30%, respectively, in the dry period, and 40%, 25%, and 35%, respectively, in the even period. Thus, the considerable input of hillslope and tributary erosion in recent years should be of concern. This study provides deep insight into the provenance of the bottom sediments in the TGR and will help prevent excessive sediment deposition in this reservoir.

Characterization of Surface Runoff Pathways and Erosion Using Hydrological Attributes Under Simulated Rainfall

Conceptualisation of geo-hydrological characteristic of erosive runoff are of particular importance and has been required in recent soil erosion control. This study aimed to explore the feasibility of applying hydrological attributes to characterize surface runoff pathways in the process of hillslope soil erosion due to rainfall. Combined with sub-millimeter high-resolution laser scanning and computer digital image processing method, three hydrological indicators (i.e., sinuosity, gradient and orientation) were used to investigate the changes of the surface runoff pathways on the slope of three typical southern red soils (i.e., shale (HS), and Quaternary red clay soils (HQ1 and HQ2) under simulated rainfall conditions). The results indicated no significant changes of sinuosity with a mean value of 1.19. After the rainfall with the intensity of 1 mm/min and 2 mm/min, the orientation and gradient changed dramatically. The greatest changes appeared at the first rainfall, which showed that the biggest increase of gradient was 26.78% and it tended to be close to the original slope of the test plot, while the orientation dropped by 5.60–31.44%. Compared with HS and HQ1, the runoff pathway characteristics of HQ2 changed more consistent. The rainfall intensities had a significant impact on the correlation between indicators. The determination coefficients sorting with surface roughness were orientation > graient > sinuosity. And they were significantly linearly related to runoff under 1 mm/min rainfall intensity, while had positive correlation with sediment under 2 mm/min rainfall intensity (p < 0.05). In conclusion, there were more remarkable relationships between orientation, gradient and slope erosion under 1 mm/min rainfall intensity. This provided an innovative idea, that is applying the orientation and gradient to the simulation and prediction model of the rainfall erosion process in the sloping farmland in the southern red soil area.

Variable scale effects on hillslope soil erosion during rainfall-runoff processes

The variation of soil erosion across scales remains a controversial issue. A theoretical framework, coupling the normalized Green-Ampt equation for infiltration, 1D kinematic wave model for overland flow, and WEPP erosion modeling approaches for soil erosion, was used to explain and quantify the direct effect of scale on the soil erosion process. The results show that the relation between interrill erosion and slope length accords with a power-law decreasing trend, while the relation of rill erosion versus slope length shows a power-law increasing trend. Moreover, the power-law scaling of interrill erosion becomes more prominent with an increase of rainfall duration and intensity but is nearly independent of the interrill erodibility factor. The rainfall duration, soil type, soil critical shear stress, sediment transport coefficient, and rill erodibility factor substantially influence the magnitude of scale effect on rill erosion, while rainfall intensity, slope gradient, and Manning's roughness have minor effects. It suggests that the scale effect on soil erosion is variable and dynamic for different erosion types, rainfall and soil characteristics, which can explain why the controversial scaling relationships of soil erosion exist in previous studies. The physically based soil erosion model WEPP is compared to the empirical models USLE and RUSLE at various scales. Only when rill erosion plays a dominant role in soil loss do all these models exhibit consistent scale effect on erosion. These results provide insights into the essence of scaling in the hillslope soil erosion process, which are expected to benefit the development of upscaling techniques for the large-scale hydrological, geomorphological, and ecological processes.

Seasonal changes of soil erosion and its spatial distribution on a long gentle hillslope in the Chinese Mollisol region

Understanding seasonal soil erosion and deposition rates and their spatial distribution along sloping farmlands are necessary for erosion prediction technology and implementing effective soil conservation practices. To date seasonal change of soil erosion and soil redistribution on long gentle hillslopes are not fully quantified due to the variable erosive forces in different seasons. A multi-tracer method using rare earth elements (REE) was employed to discriminate seasonal changes of soil erosion and its spatial distribution on a sloping farmland driven by snowmelt runoff, wind force and rainfall-runoff. A long-slope runoff plot with 5 m wide and 320 m long located in the typical Mollisol region of China was divided into eight segments, each of which was 40 m long and tagged with one of eight REE oxides. The spot method of a partial-area tagging scheme was employed and a grid-based layout was used for REE application. Results showed that annual soil erosion rate was 3251.0 t km−2 for the whole runoff plot, in which snowmelt runoff erosion contributed 537.3 t km−2, wind erosion 363.1 t km−2 and rainfall-runoff erosion 2350.6 t km−2. Surface runoff is the main external erosive force of hillslope soil erosion, accounting for 88.8% of the total annual soil loss. Furthermore, for the eight slope segments of the 320-m long hillslope, the sediment transport ratios of each slope segment caused by snowmelt runoff and rainfall-runoff erosion were more than 23.5% and 34.7%, respectively. The results will enrich the understanding of seasonal soil erosion on long hillslopes.

Land use conversion and lithology impacts soil aggregate stability in subtropical China

Soil aggregate stability is one of the most important indicators of soil structure and soil degradation vulnerability, and it is the dominant factor controlling hillslope soil erosion in the subtropical Ultisols region of China. However, few studies have investigated the synergistic effects of land use conversion and lithology on soil aggregate stability, especially in subtropical Ultisols region of China where soil erosion is a serious concern. Soil samples from 0 to 100 cm depth at increments of 20 cm were collected from four typical land use types (natural forest, artificial forest, orchard, and cropland) and two lithologies (granite and slate) to determine soil aggregate stability using the Le Bissonnais' and Yoder' methods on Ultisols from subtropical China. Results indicated that both land use type and lithology significantly influenced the soil aggregate stability. Specifically, in different soil depths of 0–100 cm, land use changes from natural forest to orchard and cropland significantly decreased the aggregate stability, thus increasing soil erodibility. Following the conversion of natural forest to artificial forest generally enhanced soil stability and resistance to flowing water erosion. For lithology, the slate soil was more resistant to erosion than granite soil. The generalized linear model can explain >60% of the total variation in soil aggregate stability, and the effect of land use on aggregate stability was greater than lithology. Compared with the Yoder' wet-sieving method, the Le Bissonnais method was better at characterizing soil aggregate stability among land uses and lithology, and therefore should be considered as a preferred method in future studies in the subtropical Ultisols region of China.

Flume experiments for assessing the dye-tracing technique in rill flows

Flow velocity controls hillslope soil erosion and is a key hydrodynamic variable involved in sediment transport and deposition processes. The dye-tracer technique is one of the most applied methods for measuring velocity of shallow interrill and rill flow. The technique is based on the injection of a tracer in a specific point and the measurement of its speed to travel the known distance from the injection point to a given channel section. The dye-tracer technique requires that the measured surface flow velocity has to be corrected to obtain the mean flow velocity using a correction factor which is generally empirically deduced. The technique has two sources of uncertainties: i) the method applied for measuring the travel time of the dye-tracer and ii) the estimate of the correction factor, which is the ratio between the mean flow velocity and the surface velocity, in different flow conditions. In this paper the results of a wide experimental investigation, carried out using a fixed bed small flume simulating a rill channel, are presented. At first, the comparison between a chronometer-based (CB) and video-based (VB) technique was carried out for establishing the influence of the travel time measuring technique. For each experimental run, which was characterized by a sample of 20 measurements carried out with the same values of slope and discharge, the developed analysis showed that the empirical frequency distribution of the ratio between the single measurement and the sample mean (i.e., the average of 20 measurements) is more uniform for the VB technique than for the CB one. In any case, this sample mean was representative of surface flow velocity for both the CB and the VB technique. Furthermore, the mean value obtained by the CB measurements lightly underestimated (−1.7%) the corresponding mean obtained by the VB technique. Finally, the effects of slope (0.1–8.7%), flow Reynolds number (3462–10040), Froude number (1.44–5.17) on the correction factor are presented. The analysis demonstrated that the correction factor is independent of flow Reynolds number while a relationship with a Froude number, obtained by surface velocity measurement, or channel slope can be established.

Modeling Soil Erosion With Evolving Rills on Hillslopes

AbstractWhereas current erosion models are successful in quantitative estimates of soil erosion by water flow, modeling the coevolution of geomorphological features, particularly rill network properties and soil erosion on hillslopes, is still a major challenge. In this study, we propose a rill evolution modeling approach and combine it with a rainfall‐runoff and soil erosion model to simulate the feedback loop of hillslope geomorphic development and soil erosion processes. Rill evolution is mainly characterized by three rill network attributes, rill density, orientation angle, and rill width, all modeled with physical equations. The entire rainfall‐runoff‐erosion and rill evolution model is tested against a set of rill network evolution and soil erosion data from an experimental hillslope subjected to successive rainfall events. The simulated spatial and temporal variations of rill network characteristics and soil erosion agree well with the measured data. The results demonstrate that the three rill network characteristics continually alter the partitioning of interrill and rill flows and affect the interrill and rill flow erosivity and soil erosion, which in turn modify the rill geometry and rill network planform. Comparatively, existing approaches such as Water Erosion Prediction Project (WEPP) that ignore the rill evolution processes largely underestimate the hillslope soil erosion when using time independent model parameters. Moreover, a sensitivity analysis indicates that both the rill evolution and soil erosion processes are sensitive to the rill evolution parameters, rainfall intensity, and slope angle. These results can inform the development of general geomorphic evolution and soil erosion models on evolving rilled hillslopes.

Hillslope soil erosion and mobility in pine plantations and native deciduous forest in the coastal range of south‐Central Chile

Significant areas of Latin America, and especially of Chile, have been forested with forest plantations to provide timber and fiber and to decrease soil erosion after deforestation. Despite their potential contribution to reducing erosion processes, the magnitude of this decline has not been adequately assessed. In this study, we estimated soil loss and mobility for one year on steep hillslopes using the traditional erosion-pin approach at five micro-catchments planted with Pinus radiata with different stand ages and compared them to an adjacent secondary broadleaf Nothofagus sp. native forest. Annual net soil loss (ASL) positively correlated with cumulative rainfall (R2 = .49, p < .001). ASL averaged 65.03 ± 9.8 mg ha−1 yr−1, with the lowest ASL in the natural forest with 21.4 ± 3.1 mg ha−1 yr−1 and the highest in one of the 1-year-old plantation micro-catchments (88.9 ± 9.3 mg ha−1 yr−1). ASL under plantation stands of different ages displayed no significant differences. All areas showed similar net soil loss index values, averaging 83.0 ± 1%, suggesting that most of the sediment left the hillslope. The RUSLE model predicted ASL moderately well across all sites and conditions (0.41, p = .018), but it tends to underestimate ASL, particularly for native forests. On average, soils on hilly terrain under plantation forest may suffer losses up to four times greater than natural forests. The latter stresses the need to improve our understanding of erosive processes that may be still active under planted forest, and to better assess the effect of management practices on soil erosion, particularly on previously highly degraded and erodible soils.

Threshold of sub-watersheds for SWAT to simulate hillslope sediment generation and its spatial variations

Abstract Exploring the watershed subdivision threshold in Soil and Water Assessment Tool model is crucial for accurate hillslope soil erosion estimation, which has been rarely explored. This study investigated impacts of watershed partitioning on simulation of hillslope sediment generation and its spatial variations, and determined a partition threshold using three newly proposed evaluation indicators (coefficient of spatial variations for hillslope sediment generation, change rate of hillslope sediment generation, and change rate of coefficient of spatial variations for hillslope sediment generation). The Xixi basin in the south-eastern coastal region of China was taken as an example, and was partitioned into eleven different watershed delineations. Effects of watershed subdivision on model parameter aggregation and areal distribution of hydrological response units were analyzed. The results indicated that (1) the previous method revealing the effects of watershed subdivision on hillslope sediment generation, which was only based on watershed outlet data, and the partition threshold determined by the method, were unreliable. (2) Hillslope sediment generation was seriously affected by watershed subdivision levels, increasing the number of sub-watersheds would decrease the modelled amount of hillslope sediment generation and increase its spatial variations. The partition threshold for the average sub-watershed area was 1.6% of the total watershed area and the number of sub-watersheds was 61 as determined from our new indicators, which resulted in stable amount of hillslope sediment generation and its spatial distribution, thus, the hillslope sediment generation could be adequately simulated by this method. (3) The hydrological response unit area, which was strongly related to the change in hillslope sediment generation, was very sensitive to watershed subdivision until the level of watershed subdivision approached the threshold.

Discrimination of soil losses between ridge and furrow in longitudinal ridge-tillage under simulated upslope inflow and rainfall

Longitudinal ridge-tillage greatly enhances hillslope soil erosion due to increased flow concentration in furrows and sediment delivery from ridge sideslopes. Currently, contributions of upslope inflow and rainfall to soil loss and identification of sediment sources in a ridge-furrow system are still unclear. A set of experiments on 10-m long, 2-m wide field runoff plots at a 5° slope gradient were conducted in the Chinese Mollisol region to quantify the effects of upslope inflow and rainfall on hillslope erosion and to discriminate sediment contributions between ridges and furrows in a longitudinal ridge-tillage system. The experimental treatments included five upslope inflow rates alone (10, 20, 30, 40 and 50 L min−1), two rainfall intensities alone (50 and 100 mm h−1) and the five inflow rates combined with the two rainfall intensities. A stereoscopic photogrammetry method was used to measure micro-topographic conditions before and after each run. The results showed that, compared with the inflow-only treatments, soil losses increased by 1.4–5.2 times and 2.5–14.0 times under the combined treatments of the five inflow rates and two rainfall intensities, respectively. The contributions of the synergistic effects increased with the increase of rainfall intensity. Moreover, in the five upslope inflow-only treatments, soil erosion mainly occurred in the bottom of furrows and ridge toe slopes; while for the combined treatments, ridge sideslope erosion dominated as soil loss from ridge areas accounted for 51.3 %–60.9 % and 53.0 %–61.8 % for the 50 and 100 mm h−1 rainfall intensities, respectively. Therefore, ridge sideslope erosion may be the main sediment source in the longitudinal ridge-tillage system under the bare conditions.

The Daily Erosion Project – daily estimates of water runoff, soil detachment, and erosion

AbstractWater runoff and sediment transport from agricultural uplands are substantial threats to water quality and sustained crop production. To improve soil and water resources, farmers, conservationists, and policy‐makers must understand how landforms, soil types, farming practices, and rainfall interact with water runoff and soil erosion processes. To that end, the Iowa Daily Erosion Project (IDEP) was designed and implemented in 2003 to inventory these factors across Iowa in the United States. IDEP utilized the Water Erosion Prediction Project (WEPP) soil erosion model along with radar‐derived precipitation data and government‐provided slope, soil, and management information to produce daily estimates of soil erosion and runoff at the township scale (93 km2 [36 mi2]). Improved national databases and evolving remote sensing technology now permit the derivation of slope, soil, and field‐level management inputs for WEPP. These remotely sensed parameters, along with more detailed meteorological data, now drive daily WEPP hillslope soil erosion and water runoff estimates at the small watershed scale, approximately 90 km2 (35 mi2), across sections of multiple Midwest states. The revisions constitute a substantial improvement as more realistic field conditions are reflected, more detailed weather data are utilized, hill slope sampling density is an order of magnitude greater, and results are aggregated based on surface hydrology enabling further watershed research and analysis. Considering these improvements and the expansion of the project beyond Iowa it was renamed the Daily Erosion Project (DEP). Statistical and comparative evaluations of soil erosion simulations indicate that the sampling density is adequate and the results are defendable. The modeling framework developed is readily adaptable to other regions given suitable inputs. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd.

Improving the utility of erosion pins: absolute value of pin height change as an indicator of relative erosion

Erosion pins can be an inexpensive and intuitive method to estimate hillslope soil erosion and deposition. It is common practice to calculate annual erosion/deposition rates (also called ground advance/retreat or ground lowering) from pin measurements as the mean net change in pin height over a given area. However, many studies have found this net ‘real number’ change does not produce strong relationships with erosion rates estimated using other methods, or with variables expected to be highly correlated with erosion, calling into question the efficacy of this approach. Here we evaluate an alternative (or complementary) approach - using the absolute value of pin height change to capture the overall magnitude of soil movement as an indicator of erosion. We used measurements from erosion pins in experimental plots across different maize-bean production systems and forest-fallows in northern El Salvador to compare both the absolute and ‘real number’ change in erosion pin height against modeled erosion, related factors (e.g., slope and soil cover), and soil loss collected in erosion pits. We found that the absolute value of pin height change was strongly correlated (r=0.67, p<0.01) with erosion rates predicted from the Revised Universal Soil Loss Equations (RUSLE) and moderately correlated (r=0.82, p<0.10) with erosion measured in collection pits, while no relationships were found for the real number value. The absolute value was also strongly correlated with RUSLE factors related to slope and cover, while no correlations existed for the real number value. Statistically significant differences in RUSLE-predicted erosion were found between plots classified as having ‘high’, ‘medium’ and ‘low’ vegetative cover, and these differences were also detected using absolute value of pin height change. Conversely, such differences were not detected using the net real number value. We conclude that, when using erosion pins for comparative analysis between land management practices or monitoring changes in erosion over time, the absolute value of pin height change is likely a better indicator than net real number change. We encourage additional research using new and existing datasets to further evaluate the utility of absolute value of pin height change as an indicator of relative erosion.

Impacts of rainfall and inflow on rill formation and erosion processes on steep hillslopes

Limited information has isolated the impacts of rainfall on rill formation and erosion on steep hillslopes where upslope inflow simultaneously exists. Field simulation experiments were conducted on steep hillslopes (26°) under rainfall (60mmh−1), inflow (6, 12, 18, 24, 30, 36Lmin−1m−1), and combination of rainfall and inflow to explore the impacts of rainfall on rill formation, and the interaction between rainfall and inflow on soil erosion. Rainfall decreased soil infiltration rate (10%–26%) mainly due to soil crust by raindrop impact. Rainfall strengthened rill formation, which behaved in the increment in rill width (5%–26%), length (4%–22%), and depth (3%–22%), but this increment decreased as inflow rates increased. Additionally, the contribution of rainfall on rill formation was most significant at the initial stage, followed by the final stage and active period of rill development. Rainfall increased rill erosion (8%–80%) and interrill erosion (36%–64%), but it played a dominant role in increasing interrill erosion under relatively high inflow rates. The most sensitive hydrodynamic parameter to soil erosion was shear stress and stream power under inflow and ‘inflow+rainfall’ conditions, respectively. For the lowest inflow rate, the reduction in soil loss by interaction between rainfall and inflow accounted for 20% of total soil loss, indicating a negative interaction. However, such interaction became positive with increasing inflow rates. The contribution rate to rill erosion by the interaction was greater than that of interrill erosion under relatively low inflow rates. Our results provide a better understanding of hillslope soil erosion mechanism.

降雨和汇流对黑土区坡面土壤侵蚀的影响试验研究

降雨和汇流对黑土区坡面土壤侵蚀的影响试验研究