Soil erosion under different vegetation covers in the Venezuelan Andes

This comparative study of soil erosion considered different environments in an ecological unit of the Venezuelan Andes. The soils belong to an association of typic palehumults and humic dystrudepts. Soil losses were quantified by using erosion plots in areas covered by four types of vegetation, including both natural and cultivated environments. The highest soil erosion rate evaluated corresponded to horticultural crops in rotation: reaching a value of 22 Mg ha^–1 per year. For apple tree (Malus sylvestris Miller) plots, soil losses reached values of 1.96 Mg ha^–1 per year. Losses from pasture (Pennisetum clandestinum Hochst. ex Chiov.) plots, without livestock grazing, were as high as 1.11 Mg ha^–1 during the second year of the experiment. The highest soil losses generated from plots under natural forest were equal to 0.54 Mg ha^–1 per year. Environmental factors such as total and effective rainfall, runoff, and some soil characteristics as those related to soil losses by water erosion were evaluated. The type of management applied to each site under different land use type and the absence of conservation practices explain, to a large extent, the erosive processes and mechanisms.     

SPATIAL AND TEMPORAL INFLUENCE OF SOIL FROST ON INFILTRATION AND EROSION OF SAGEBRUSH RANGELANDS1

ABSTRACT: Soil infiltration capacity and interrill erosion are significantly influenced by soil frost on western rangelands which are characterized by cold winters and numerous freeze-thaw cycles. However, little is known about the variable influence of this phenomenon. Infiltration and interrill erosion were measured within a sagebrush-grass plant community during the winter, spring, and summer of 1989. Significant spatial and temporal differences in infiltration capacity and interrill erosion were found for shrub coppice dune and dune interspace soils. Infiltration was generally higher for coppice dune soils compared to interspace soils throughout the year. Infiltration capacity for both soils was lowest early in the year when the soil was frozen or saturated, then increased as the soil dried in the spring and summer. Interrill erosion was consistently lower for coppice dune soils compared to interspace soils. Erosion from interspace soils was greatest during a 19-day period in late winter characterized by diurnal freeze-thaw cycles, saturated surface soil conditions, and soil slaking.     

CHANGES TO INFILTRATION AND INTERRILL EROSION FROM LONG-TERM PRESCRIBED BURNING IN LOUISIANA1

ABSTRACT: Effects of long-term prescribed burning on infiltration and interrill erosion were assessed on two longleaf pine-bluestem sites in Louisiana. Treatments represented biennially-applied winter, spring, or summer burning on an upland sandy loam site for 20 years; and annual winter or spring, and biennial winter or spring burns on a bottomland silt loam site for 10 years, with unburned controls. Immediate effects of burning were a reduction in surface cover, exposing soil to raindrop impact. Burning the sandy loam site increased interrill erosion after winter and spring treatments, but produced no immediate changes in infiltration capacity or time to runoff irrespective of treatment season. Rapid recovery of under-story vegetation mitigated soil exposure. Biennial burning did not increase interrill erosion, or reduce infiltration capacity and time to runoff on the sandy loam site after 20 years. A complete herbaceous understory covered the silt loam site two years after treatment. Interrill erosion was not significantly increased, or infiltration capacity and time to runoff decreased on burning treatments than unburned controls on the silt loam site. Litter biomass was important in predicting interrill erosion. No surface cover condition could be linked to variability in infiltration capacity. This study provides evidence for the resiliency of a longleaf pine-bluestem association to prescribed burning.     

PRECIPITATION RETENTION AND SOIL EROSION UNDER VARYING CLIMATE,LAND USE,AND TILLAGE AND CROPPING SYSTEMS1

ABSTRACT: Nonirrigated crop yields and forage production are limited by low and variable precipitation in the southern Great Plains. Precipitation variation involves production risks, which can be reduced by considering probability of precipitation, precipitation retention, and soil erosion under various production systems. The objective of this study was to probabilistically quantify the impact of precipitation variations, land use, cropping, and tillage systems on precipitation retention and soil erosion. Five 1.6 ha watersheds that had 3 to 4 percent slopes, and similar silt loam soils were selected. One was kept in native grass, and the others were planted into winter wheat (Triticum aestivum L.) under different cropping and tillage systems. Daily runoff and soil erosion were measured at the outlet of each watershed. Precipitation distributions exhibited great seasonal and interannual variations, and precipitation retention distributions resembled those of precipitation. Cropping and tillage systems affected precipitation retention but much less than did precipitation variations. Available soil water storage, which was largely controlled by ET, played an important role in retaining precipitation. This indicates that cropping systems should be adjusted to precipitation patterns, if predictable, for better soil water use. Land use and cropping and tillage systems had a much greater impact on soil erosion than on precipitation retention. Soil erosion risks, which were proportional to the levels of tillage disturbance, were mainly caused by a few large storms in summer, when surface cover was low. This study explored a novel approach for evaluating production risks associated with insufficient precipitation retention and excessive soil erosion for certain crops or cropping systems under assumed precipitation conditions.     

The Use of Carbon and Nitrogen Isotopes to Study Watershed Erosion Processes1

Abstract:  Tracer studies are needed to better understand watershed soil erosion and calibrate watershed erosion models. For the first time, stable nitrogen and carbon isotopes (δ¹⁵N and δ¹³C) and the carbon to nitrogen atomic ratio (C/N) natural tracers are used to investigate temporal and spatial variability of erosion processes within a sub‐watershed. Temporal variability was assessed by comparing δ¹⁵N, δ¹³C, and C/N of eroded‐soils from a non‐equilibrium erosion event immediately following freezing and thawing of surface soils with two erosion events characterized by equilibrium conditions with erosion downcutting. Spatial variability was assessed for the equilibrium events by using the δ¹⁵N and δ¹³C signatures of eroded‐soils to measure the fraction of eroded‐soil derived from rill/interrill erosion on upland hillslopes as compared to headcut erosion on floodplains. In order to perform this study, a number of tasks were carried out including: (1) sampling source‐soils from upland hillslopes and floodplains, (2) sampling eroded‐soils with an in situ trap in the stream of the sub‐watershed, (3) isotopic and elemental analysis of the samples using isotope ratio mass spectrometry, (4) fractioning eroded‐soil to its upland rill/interrill and floodplain headcut end‐members using an unmixing model within a Bayesian Markov Chain Monte Carlo framework, and (5) evaluating tracer unmixing model results by comparison with process‐based erosion prediction models for rill/interrill and headcut erosion processes. Results showed that finer soil particles eroded during the non‐equilibrium event were enriched in δ¹⁵N and δ¹³C tracers and depleted in C/N tracer relative to coarser soil particles eroded during the equilibrium events. Correlation of tracer signature with soil particle size was explainable based on known biogeochemical processes. δ¹⁵N and δ¹³C were also able to distinguish between upland rill/interrill erosion and floodplain headcut erosion, which was due to different plant cover at the erosion sources. Results from the tracer unmixing model highlighted future needs for coupling rill/interrill and headcut erosion prediction models.     

SOME HYDROLOGIC IMPACTS OF PLOWING RANGELAND SOILS1

ABSTRACT: Rainfall simulator studies were conducted during 1982 and 1983 on agricultural and native rangeland soils of the same soil series in northern Utah. Results indicate that the same soil series mapped at different locations on agricultural land will have similar 10, 20, and 30 minute infiltration rates and similar interrill erosion rates. Seasonal differences in infiltration and erosion rates were significant. Comparisons between agricultural and native soils were complicated by three-way statistical interactions. Seasonal variations in both infiltration rates and erosion rates were greatest on agricultural soils. Of four soil series on native rangeland, only one showed significant seasonal variation in infiltration rates, while erosion rates were similar across all seasons for all soil series. Soil and cover factors important in predicting infiltration and erosion were identified.     

STRATIFICATION OF VARIABILITY IN RUNOFF AND SEDIMENT YIELD BASED ON VEGETATION CHARACTERISTICS1

ABSTRACT: Runoff and sediment yield were collected from 100 plots during simulated rainfalls (100 mm/hr for 15 minutes) at antecedent soil moisture conditions. A clustering technique was used to stratify the variability of a single data set within a sagebrush‐grass community into four groups based on vegetation life form and amount of cover. The four cluster groups were grass, grass/shrub, shrub, and forb/grass and were found to be significantly different in plant height, surface roughness, soil bulk density, and soil organic matter. Stepwise multiple regression analyses were performed on the single data set and each cluster group. Results for individual groups resulted in more robust predictive equations for runoff (r²= 0.65–0.73) and sediment yield (r²= 0.37–0.91) than for equations developed from the single data set (r²= 0.56 for runoff and r²= 0.27 for sediment yield). The standard errors of the cluster group regression equations were also improved in three of the four group equations for both runoff and sediment yield compared to the single data set. Runoff was found to be significantly less (p >0.01) in the forb/grass group compared with other vegetation cluster groups, but this was influenced by four plots that produced little or no runoff. Sediment yield was not found to be significantly different among any cluster groups. Discriminant analysis was then used to identify important variables and develop a model to classify plots into one of the four cluster groups. The discriminant model could be incorporated into rangeland hydrology and erosion models. The percentage cover of grasses, shrubs, litter, and bare ground effectively stratified about 12 percent of the variation observed in runoff and 26 percent of the variability for sediment yield as determined by r².     

INFLUENCE OF VEGETATION,ROCK COVER,AND TRAMPLING ON INFILTRATION RATES AND SEDIMENT PRODUCTION1

ABSTRACT: This study evaluated the impact of selected soil surface characteristics on infiltration rates and sediment production from interrill erosion from loam soil. Treatments were two different grass species (crested wheatgrass and intermediate wheatgrass), three levels of grass cover (30, 50, and 80 percent), four levels of rock cover (5, 10, 15, and 20 percent), and six levels of simulated trampling (10 to 60 percent of the respective plot area by 10 percent increments). Results indicated that plots with sod forming grass infiltrated only slightly more water than plots with bunchgrass, though the differences were significant. Trampling reduced infiltration rates significantly. On uncompacted soil, infiltration rates increased as percentage of rock cover increased. Trampling gradually destroyed this relationship however. Rock cover did not significantly affect sediment production. The tradeoff between vegetal cover and rock cover was affected by simulated trampling. Once trampling disturbance reached 20 percent, no relationship between vegetal cover and rock cover existed. Trampling was the most important factor influencing infiltration rates, explaining 35 to 48 percent of the variation in infiltration rates. The most important factor influencing sediment production was grass cover, which explained 40 to 62 percent of the variations associated with sediment yield at various trampling percentages. Results strongly suggest that, for slopes and soils as used here, adequate watershed protection may be obtained by maintaining 50 percent protective ground cover. Additional validation studies are recommended.     

Rainfall Response of Degraded Soil Following Reforestation in the Copper Basin, Tennessee, USA

2 Copper Basin in southeastern Tennessee became the site of increasingly extensive and successful reforestation efforts. To determine the effectiveness of more than 50 years of reforestation efforts, we compared rainfall infiltration, sediment detachment, and soil organic matter of reforested sites to those properties of unvegetated sites and forested reference sites outside the basin. Results of 54 rainfall simulation experiments conducted in zones of the basin known to have been planted during different decades demonstrate that hydrologic recovery of soils in the Copper Basin lags significantly behind the establishment of tree cover and the protection offered by vegetation against soil erosion. Soils in new “forests” have significantly less organic matter and lower infiltration than forests more than 50 years old. The long-term persistence of low infiltration rates serves as a reminder that, at sites where the A and B soil horizons have been lost, restoration of the hydrologic function of a landscape requires decades, at least.     

Erosion control at construction sites on red clay soils

Five single-treatment methods used to stabilize seeded areas at urban and highway construction sites (asphalt-tacked straw, jute netting, mulch blanket, wood chips, excelsior blanket) were tested for their ability to control erosion of red clay soils by comparisons with exposed sites and multiple treatments. Sediment in runoff from experimental plots was characterized during low and high intensity precipitation from natural rainfall episodes during April, May, and June. Reductions in the total sediment concentration of runoff ranged from 28 percent (asphalt-tacked straw, 50 percent slope) to 90 percent (multiple treatments, 40 percent slope), with 85 percent of the eroded material composed of particles <0.04 mm in diameter. Larger size fractions were effectively reduced by all treatments tested regardless of slope (70 percent decrease). Established grass cover exceeded 90 percent on all plots after 60 days, but sediment release remained similar, attributable to high intensity rainfall, poor establishment of root systems, and piping on plots treated with tacked straw or jute netting. Results indicate that current stabilization methods shift sediment compostion toward a smaller particle size, causing single treatments to be minimally effective for controlling erosion of the major component of red clay soils. Because small particles have the greatest direct effect on aquatic biota, certain impacts of sedimentation may not be measurably lessened by single treatments in regions where red clays predominate even though the total sediment load is reduced by as much as 75 percent. Clearly, a multiple-treatment approach offers significantly greater control of erosion on red clay soils, however, current economic and construction policy represents a substantial deterrent to implementation.     

LABORATORY TESTS OF EROSION DEPENDENCE ON PROPERTIES OF SOILS AND RAINFALL1

ABSTRACT: One hundred and sixty-two rainfall-induced soil erosion tests were conducted to assist in predicting soil loss and subsequent increase in total suspended solids leaving a highway construction site during a rainfall event. A rainfall simulator and a water flume were constructed for the tests. Soil shear strength, compressive strength, rainfall intensity, and soil bed slope were treated as variables during the experiments. The soil with a higher shear strength resisted soil erosion better than lower strength soils. Soil loss was nearly independent of shear strength at low rainfall intensity but at high intensities, the shear strength was more important in resisting soil loss. Lower soil loss can be expected for cohesive soils if the compressive strength is high.     

Non-motorized Winter Recreation Impacts to Snowmelt Erosion, Tronsen Basin, Eastern Cascades, Washington

Many recreation impact studies have focused on summer activities, but the environmental impact of winter recreation is poorly characterized. This study characterizes the impact of snowshoe/cross-country ski compaction and snowmelt erosion on trails. Trail cross-sectional profiles were measured before and after the winter season to map changes in erosion due to winter recreation. Compacted snow on the trail was 30 % more dense than snowpack off the trail before spring melt out. Snow stayed on the trail 7 days longer. Soil and organic material was transported after spring snowmelt with ?9.5 ± 2.4 cm² total erosion occurring on the trail transects and ?3.8 ± 2.4 cm² total erosion occurring on the control transect (P = 0.046). More material was transported on the trail than on the control, 12.9± 2.4 versus 6.0 ± 2.4 cm² (P = 0.055), however, deposition levels remained similar on the trail and on the control. Snow compaction from snowshoers and cross-country skiers intensified erosion. Trail gradient was found to be significantly correlated to net changes in material on the trail (R ² = 0.89, ρ = ?0.98, P = 0.005). This study provides a baseline, showing that non-motorized winter recreation does impact soil erosion rates but more studies are needed. Trail managers should consider mitigation such as water bars, culverts and avoiding building trails with steep gradients, in order to reduce loss of soils on trails and subsequent sedimentation of streams.     

Assessment of soil erosion under rainfed sugarcane in KwaZulu‐Natal,South Africa

Soil erosion from agricultural land use runoff is a major threat to the sustainability of soil composition and water resource integrity. Sugarcane is an important cash and food security crop in South Africa, subjected to an intensive soil erosion, and consequently, severe land degradation. This study aimed to investigate soil erosion and associated soil and cover factors under rainfed sugarcane, in a small catchment, KwaZulu‐Natal, South Africa. Three replicated runoff plots were installed at different slope positions (down, mid and upslope) within cultivated sugarcane fields to monitor soil erosion during the 2016–2017 rainy season. On average, annual runoff (RF) was significantly greater from 10 m² plots with 1163.77 ± 2.63 l/m/year compared to 1 m² plots. However, sediment concentration (SC) was significantly lower in 10 m² (0.34 ± 0.04 g/l) compared to 1 m² (6.94 ± 0.24 g/l) plots. The annual soil losses (SL) calculated from 12 rainfall events was 58.36 ± 0.77 and 8.84 ± 0.20 t/ha from 1 m² and 10 m² plots, respectively. The 1 m² plot, SL (2.4 ± 1.41 ton/ha/year) in the upslope experienced 33% more loss than the midslope and 50% more loss than the downslope position. SL was relatively lower from the 10 m² plots than the 1 m² plots, which is explained by high sediment deposition at the greater plot scale. SL was negatively correlated with the soil organic carbon stocks (r = ?0.82) and soil surface cover (r = ?0.55). RF decreased with the increase of slope gradient (r = ?0.88) and soil infiltration rate (r = ?0.87). There were considerable soil losses from cultivated sugarcane fields with low organic matter. These findings suggest that to mitigate soil erosion, soil organic carbon stocks and vegetation cover needs to be increased through appropriate land management practices, particularly in cultivated areas with steep gradients.     

Use of Remote Sensing Techniques to Determine the Effects of Grazing on Vegetation Cover and Dune Elevation at Assateague Island National Seashore: Impact of Horses

The effects of grazing by feral horses on vegetation and dune topography at Assateague Island National Seashore were investigated using color-infrared imagery, lidar surveys, and field measurements. Five pairs of fenced and unfenced plots (300 m²) established in 1993 on sand flats and small dunes with similar elevation, topography, and vegetation cover were used for this study. Color-infrared imagery from 1998 and field measurements from 2001 indicated that there was a significant difference in vegetation cover between the fenced and unfenced plot-pairs over the study period. Fenced plots contained a higher percentage of vegetation cover that was dominated by American beachgrass (Ammophila breviligulata). Lidar surveys from 1997, 1999, and 2000 showed that there were significant differences in elevation and topography between fenced and unfenced plot-pairs. Fenced plots were, on average, 0.63 m higher than unfenced plots, whereas unfenced plots had generally decreased in elevation after establishment in 1993. Results demonstrate that feral horse grazing has had a significant impact on dune formation and has contributed to the erosion of dunes at Assateague Island. The findings suggest that unless the size of the feral horse population is reduced, grazing will continue to foster unnaturally high rates of dune erosion into the future. In order to maintain the natural processes that historically occurred on barrier islands, much larger fenced exclosures would be required to prevent horse grazing.     

Sediment Production From Forest Roads in Western Montana1

Abstract: Unpaved roads are a primary sediment source in forested watersheds. Validation of erosion models and improvements to road management require information on road erosion rates and the factors controlling erosion. This study measured sediment yields from twenty ～0.05 ha unsurfaced (native) road plots in Belt Supergroup and glacial till parent materials of western Montana, and investigated the factors controlling erosion. Annual sediment yields for individual plots ranged from 0 to 96.9 Mg/ha/yr over 3 years (2002‐2004). Annual mean sediment yield ranged from 2.1 Mg/ha in 2003 to 9.9 Mg/ha in 2004 with an overall mean of 5.4 Mg/ha/yr. The mean of log‐transformed sediment yields for sites in glacial till parent materials was higher than Belt Supergroup parent materials (p = 0.063). A regression model with road slope, time since last grading, roadbed gravel content, and precipitation as predictive variables explained 68% of the variability in sediment yield (F = 28.2; p < 0.0001). Road erosion in western Montana is limited by low erodibility of the dominant parent materials and low rainfall. Management procedures such as reducing the frequency of grading can significantly reduce sediment yields from forest roads.     

EVALUATION OF ROSGEN'S STREAMBANK EROSION POTENTIAL ASSESSMENT IN NORTHEAST OKLAHOMA1

ABSTRACT: In this study three components of the Rosgen Level III Stream Reach Condition Assessment were tested for their ability to predict short-term erosion rates. Rosgen's bank erosion potential (BEP) ratings and near bank stress (NBS) estimates and the Pfankuch channel stability ratings were evaluated. Thirty-six banks with a range of BEP ratings and NBS estimates were selected on the 101 km Upper Illinois River in northeast Oklahoma. The Upper Illinois River is a meandering, gravel-dominated, riffle/pool channel. Cumulative erosion data measured with bank pins after four 2.0 to 2.5-year return period flows from September 1996 to July 1997 were used in the analyses. When integrated as indicated in Rosgen (1996), the BEP indices and NBS estimates were poor predictors of bank erosion. Individually, the grouped BEP ratings and Pfankuch ratings performed relatively well compared to grouped NBS estimates in predicting erosion; however, the variability of erosion was large within each rating group. Linear regression between erosion and BEP numerical indices and Pfankuch scores was significant (a = 0.05), but variability was high (illustrated by low r² values). Regression between erosion and NBS estimates was not significant.     

Impact of Vegetative Cover on Runoff and Soil Erosion at Hillslope Scale in Lanjaron, Spain

Soil loss and surface runoff patterns over a four-year period (1997–2000) were studied in erosion plots from three hillslopes under different vegetative covers (Rosmarinus officinalis, Triticum aestivum and natural-spontaneous vegetation) in Lanjaron (Alpujarras) on the south flank of the Sierra Nevada of southeast Spain. The erosion plots were located on the hillslopes at 35.5% incline, at 1,480 m in altitude and with 41.8 m² (21 m×1.9 m) in area. The vegetative covers were tested for effectiveness in controlling the surface runoff and soil loss production. The highest runoff and erosion values, ranging from 114.1 to 1.7 mmyr^–1 and from 14,564.3 to 6.6 kgha^–1yr^–1, respectively, over the entire study period, were measured under the Triticum aestivum. In the Rosmarinus officinalis, runoff ranged from 7.9 to 1.3 mmyr^–1 and erosion from 156.4 to 2.3 kgha^–1yr^–1, while on the hillslope under natural-spontaneous vegetation, runoff ranged from 4.4 to 0.9 mmyr^–1 and erosion from 322.3 to 2.2 kgha^–1yr^–1. According to the results the vegetative covers of Rosmarinus officinalis and natural-spontaneous vegetation reduced the soil losses by 99 and 98%, with respect to the Triticum aestivum, and the runoff losses by 94 and 96%, respectively. Also, the Rosmarinus officinalis and natural-spontaneous plants influenced infiltration by intercepting much of the rainfall water respect to the Triticum aestivum. Monitoring allowed more direct linkages to be made between management practices and their impacts on runoff and soil erosion, thereby enabling to identify problems and take appropriate preventive measures to improve the management practices.     

Determination of Frequency for Remeasuring Ground and Vegetation Cover Factor Needed for Soil Erosion Modeling

Determining a remeasurement frequency of variables over time is required in monitoring environmental systems. This article demonstrates methods based on regression modeling and spatio-temporal variability to determine the time interval to remeasure the ground and vegetation cover factor on permanent plots for monitoring a soil erosion system. The spatio-temporal variability methods include use of historical data to predict semivariograms, modeling average temporal variability, and temporal interpolation by two-step kriging. The results show that for the cover factor, the relative errors of the prediction increase with an increased length of time interval between remeasurements when using the regression and semivariogram models. Given precision or accuracy requirements, appropriate time intervals can be determined. However, the remeasurement frequency also varies depending on the prediction interval time. As an alternative method, the range parameter of a semivariogram model can be used to quantify average temporal variability that approximates the maximum time interval between remeasurements. This method is simpler than regression and semivariogram modeling, but it requires a long-term dataset based on permanent plots. In addition, the temporal interpolation by two-step kriging is also used to determine the time interval. This method is applicable when remeasurements in time are not sufficient. If spatial and temporal remeasurements are sufficient, it can be expanded and applied to design spatial and temporal sampling simultaneously.     

Biosolids applications affect runoff water quality following forest fire

Soil erosion and nutrient losses are great concerns following forest wildfires. Biosolids application might enhance revegetation efforts while reducing soil erodibility. Consequently, we applied Denver Metro Wastewater District composted biosolids at rates of 0, 40, and 80 Mg ha(-1) to a severely burned, previously forested site near Buffalo Creek, CO to increase plant cover and growth. Soils were classified as Ustorthents, Ustochrepts, and Haploborols. Simulated rainfall was applied for 30 min at a rate of 100 mm h(-1) to 3- x 10-m paired plots. Biosolids application rates did not significantly affect mean total runoff (p < 0.05). Sediment concentrations were significantly greater (p < 0.05) from the control plots compared with the plots that had received the 80 Mg biosolids ha(-1) rate. Biosolids application rate had mixed effects on water-quality constituents; however, concentrations of all runoff constituents for all treatment rates were below levels recommended for drinking water standards, except Pb. Biosolids application to this site increased plant cover, which should provide erosion control.