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Rill erosion, mostly affecting steep and long hillslopes, is one of the most severe effects of deforestation and wildfires in natural ecosystems. Specific monitoring and accurate but simple models are needed to assess the impacts of these forest disturbances on the rill detachment process. To address this need, this study has simulated the rill detachment capacity (Dc) through flume experiments on samples of soils collected in hillslopes after deforestation and severe burning. The associations between Dc and organic matter (OM) and the aggregate stability of soil (WSA), two key parameters influencing the rill detachment process, have also been explored under the two soil conditions (deforested and burned soils) using multivariate statistical techniques. Finally, linear regression models to predict Dc from these soil parameters or the hydraulic and morphological variables (water flow rate, WFR, and soil slope, S), set in the flume experiments, have been proposed for both soil conditions. Higher Dc in samples from deforested sites compared to the burned soils (+35%) was measured. This Dc increase was associated with parallel decreases in OM (−15%) and WSA (−34%) after deforestation compared to the wildfire-affected sites. However, the discrimination in those soil properties between the two soil conditions was not sharp. Accurate linear equations (r2 > 0.76) interpolating Dc and the shear stress (τ) have been set to estimate the rill erodibility (Kr) to evaluate soil resistance in erosion models to be applied in deforested or burned sites. [ABSTRACT FROM AUTHOR]

Land use has significant effects on the erosion process, since it influences the soil detachment capacity by causing an overland flow (Dc). The effects of different land uses on the rill detachment capacity have not been explained in depth, and the hydraulic parameters providing accurate estimates of this soil property have not been completely identified. This study quantifies Dc at low flow rates in woodland and forestland, compared to two other land uses (cropland and grassland), in the Saravan watershed (Northern Iran), and develops prediction models of Dc and rill erodibility (Kr). Dc was measured on undisturbed soil samples, collected in the four land uses, and characterized in terms of the main physico-chemical properties in a flume experiment, simulating five slopes and five shallow water flows. The results showed that Dc was significantly lower in woodland and forestland compared to cropland and grassland, as the consequence of the changes in the main soil properties and the more developed vegetation cover and structure. Dc was positively correlated to clay and silt contents of soils, and negatively correlated to sand content, aggregate stability, root density, and organic matter. The stream power and unit stream power were found to be very accurate predictors of Dc in woodland and forestland, respectively. Kr values, which assumed the lowest values in woodland and forestland, were provided by interpolating Dc and the shear stress of water flow. Overall, this study has confirmed that vegetation cover and improved soil properties in forestland and woodland may help to reduce erosion in delicate environment ecosystems, such as the forests of Northern Iran. [ABSTRACT FROM AUTHOR]

Rill erosion is one of the main sediment sources. Rill erodibility and critical shear stress are essential parameters for soil erosion prediction for Water Erosion Prediction Project (WEEP) model. Soil subsurface hydraulic gradient on soil loss is important to an accurate prediction of soil erosion and channel initiation. The objectives of this study were 1) to quantitatively explore the response of parameters of WEPP to different subsurface hydraulic gradients; and 2) to investigate the relationship between measured and calculate values in WEPP of critical shear stress. Taking typical yellow soil as example, runoff scouring experiment and a method of measuring critical shear stress were conducted by using a V-shaped soil pan under a slope of 5%. The experiments were carried out in Jinyun Mountain of the upper and middle reaches of the Yangtze River, China (106°22′E, 29°45′N). A total of 5 hydraulic gradients were -1.43, -0.71, 0, 0.71 and 1.43 m/m and 3 discharges were 0.55, 1.58 and 2.51 L/min, respectively. Average runoff, average sediment, soil detachment rate and shear stress were determined by collecting runoff samples every 30 s intervals in 8 minutes for each experimental treatment. Rill erodibility and critical shear stress were calculated in WEPP model. The measured value of critical shear stress was determined by varying the flow rate until erosion began when soil particles were continuously detached. Results showed that the average runoff for all the experiments increased with the increase of hydraulic gradient. The average sediment increased with the increase of hydraulic gradient when discharges were 0.55 and 1.58 L/min, showing a change range of 2.55-5.77 and 14.58-28.74 g, respectively. However, when the hydraulic gradient was increased from -1.43 to 0 m/m and then to 1.43 m/m for 2.51 L/min discharge, the average sediment exhibited a trend of first sharp decrease from 46.10 to 24.48 g and then slight increase from 24.48 to 29.21 g. When the discharge was increased from 0.55 to 1.58 L/min and then to 2.51 L/min under the drainage conditions (hydraulic gradient from -1.43 to -0.71 m/m), the average sediment increased by 217.40%-217.39% and 64.38%-66.20%, respectively. However, when the discharge was increased from 1.58 to 2.51 L/min under the saturation/seepage conditions (hydraulic gradient from 0 to 1.43 m/m), the average sediment only increased by 1.64%-17.41%. The average sediment under the drainage conditions was 42.24%-88.32% higher than that under saturation/seepage conditions for 2.51 L/min discharge. The soil detachment rate decreased firstly and then changed stably with the increase of scouring time under the saturation/ seepage conditions, and the similar trend of soil detachment rate was found under the drainage conditions for 0.55 L/min discharge. However, the change tread of soil detachment rate were fluctuated under the drainage conditions with the increase of discharge, and the fluctuation of soil detachment rate under 2.51 L/min discharge was stronger than that under 1.58 L/min discharge. The average value of rill erodibility for the 5 hydraulic gradients was 2.51×10-2 s/m. The value of rill erodibility under the saturation/seepage condition was 3.07×10-2 s/m, and was then 1.78 times higher than that under drainage conditions. When the hydraulic gradient was -1.43 m/m, the critical shear stress was nearly equal between the calculated value in WEPP and the measured value. However, when hydraulic gradient ranged from -0.71 to 1.43 m/m, the calculated value overestimated from 14.24% to 55.02% with an average overestimate of 36.85% compared to the measured value. Moreover, an exponential relationship was fitted between the calculated value in WEPP model and the measured value of critical shear stress (R2 was 0.77, Nash coefficient was 0.66, P value was smaller than 0.01). This study not only provides a guidance for controlling soil erosion on yellow soil region, but also offers an important database for correcting critical shear stress of rill erosion in WEPP. [ABSTRACT FROM AUTHOR]

Biochar addition to soil has been found to be a useful practice to control erosion. However, few studies have explored the effects of biochar produced from husk of rice on rill erosion, due to overland flow in deforested areas. This study has evaluated the rill detachment capacity (D c) and erodibility (K r) in soil samples treated with rice husk biochar in comparison to untreated sites (control), collected in deforested hillslopes of Northern Iran. D c was measured in a lab-scale hydraulic flume at four-bed slopes (9.7, 14.3, 19.5, and 24.2%) and five water discharges (0.24, 0.36, 0.47, 0.59, and 0.69 L m−1 s−1). Moreover, organic carbon, aggregate stability and bulk density of soils were measured. D c was lower (on average − 32%) in the treated soil compared to the control. D c was not correlated to the other physico-chemical properties of soil. K r , estimated by linear regressions between D c and shear stress, was noticeably lower (−79%) in the treated soil compared to the control. These results evidence that rice husk biochar is effective at controlling and mitigating soil detachment, and at improving organic matter content and aggregate stability in deforested hillslopes. The proposed values of rill erodibility and shear stress of deforested hillslopes (treated or untreated) are helpful to model soil resistance to rill erosion in process-based erosion models. • Scarce knowledge exists about biochar effects on rill detachment capacity (D c). • D c was lower by 32% in the soil treated with biochar compared to the untreated soil. • This reduction was due to organic matter addition to soil treated with biochar. • D c was not correlated to the other physico-chemical properties of soil. • Rill erodibility was lower (−79%) in the soil treated with biochar. [ABSTRACT FROM AUTHOR]

As forest management scenarios become more complex, the ability to more accurately predict erosion from those scenarios becomes more important. In this second part of a two-part study we report model parameters based on 66 simulated runoff experiments in two disturbed forests in the northwestern U.S. The 5 disturbance classes were natural, 10-month old and 2-week old low soil burn severity, high soil burn severity, and logging skid trails. In these environments the erosion rates were clearly detachment limited, and the rill erodibility parameters calculated from four hydraulic variables increased by orders of magnitude. The soil shear stress based erodibility parameter, K r, was 1.5 × 10−6 s m−1in the natural plots, 2.0 × 10−4 s m−1 in the high soil burn severity plots, and 1.7 × 10−3 s m−1 in the skid trail plots; K r values for the low soil burn severity plots had negative sign. The erodibility value for the skid trail plots fell within ranges reported for tilled agricultural fields and also for forest roads. The K r values decreased as erosion occurred in the plots and therefore should not be a constant parameter. The stream power produced the largest R2 value (0.41) when hydraulic predictors and the sediment flux were log-transformed, but none of the four hydraulic variables (soil shear stress, stream power, unit stream power, and unit length shear force) explained much of the variability in sediment flux rates across the five levels of disturbance when evaluated in the linear form of the erosion models under consideration. [ABSTRACT FROM AUTHOR]

Abstract: Fire in sagebrush rangelands significantly alters canopy cover, ground cover, and soil properties which influence runoff and erosion processes. Runoff can be generated more quickly and in larger volume following fire resulting in increased risk of severe erosion and downstream flooding. The Water Erosion Prediction Project (WEPP) model was developed to predict erosion on cropland, forest, and rangeland. WEPP is a tool that has potential to model the effect of fire on hillslope hydrological processes and help managers address erosion and runoff risks following fire. Experimental results on a steep (35 to 50% slope) sagebrush site suggest that rill erosion is the dominant erosion process following fire and the WEPP parameterization equations related to the rill erosion process need improvements. Rill detachment estimates could be improved by modifying regression-estimated values of rill erodibility. Also, the interactions of rill width and surface roughness on soil shear stress estimates may also need to be modified. In this paper we report the effects of prescribed fire on runoff, soil erosion, and rill hydraulics and compare WEPP estimated erosion for several modeling options with measured erosion. [Copyright &y& Elsevier]

• Rill detachment capacity (D c) was simulated in a flume varying sediment size, bed slope and flow rate. • Dc was significantly higher for sediments over 1 mm compared to other soil fractions. • Unit stream power is the best predictor of Dc using power equations. • Linear regression models between Dc and shear stress were very accurate. • Rill erodibility and critical shear stress are higher for sediments over 1 mm. Rill detachment is the most important erosive process in steep slopes and its comprehension and prediction accuracy is important to properly develop soil conservation practices in forest areas. This process is largely influenced by sediment size, soil slope and water flow characteristics, but the results of the studies that have explored these influences are contrasting. This study has simulated in an experimental flume the rill detachment capacity (D c) of soil with five particle sizes (0–0.25, 0.25–0.5, 0.5–1, 1–2, and 2–3 mm) at five water flow rates (0.26, 0.35, 0.45, 0.56, and 0.67 L m−1 s−1) and five bed slopes (3.5%, 9.1%, 19.2%, 29.1%, and 38.3%) on samples collected in a forestland of Northern Iran. D c was significantly higher (by 70%) for sediments size over 1 mm compared to the other soil fractions and increased primarily with soil slope and secondarily with water depth. A modelling approach has shown that the unit stream power is the best predictor of D c using power equations (NSE over 0.87). Linear regression models between D c and shear stress (τ) were very accurate (r2 over 0.80 with few exceptions) in predicting both rill erodibility and critical shear stress, when developed separately for each particle size class. Sediments with size higher than 1 mm had lower resistance to rill erosion compared to the finer fractions. Overall, the study helps to better understand particle detachment of the erosion process, on which particle size is a key parameter. The modelling activity proposes to land planners values of the rill erodibility and critical shear stress of forested areas for applications in process-based erosion models. [ABSTRACT FROM AUTHOR]

• Rill detachment capacity (D c) was measured in hydromulched and bare plots after deforestation. • D c was 40% to 54% lower in the hydromulched soil compared to the untreated plot. • This reduction was due to the higher weight density and lower diameter of hydromulch roots. • Rill erodibility was much lower (−81%) in the hydromulched soil. • Critical shear stress and rill erodibility for deforested hillslopes (also after hydromulching) are given. Deforestation, which removes soil protection by plant leafs and roots, causes severe soil erosion, since the beneficial effects of plant cover and root actions on soil erodibility are lost. Hydromulching has been found to be a suitable technique for erosion control, but little research has been carried out to evaluate the effectiveness of hydromulch roots in controlling rill erosion in deforested areas due to overland flow. This study has evaluated rill detachment capacity (D c) and erodibility (K r , which is the slope of the equation regressing D c and critical shear stress) in hydromulched and bare plots (1.3-m long and 0.5-m wide) on soils sampled in deforested hillslopes of Northern Iran; the most important root parameters of the grass used for hydromulching were also measured, such as root density and diameter. D c has been measured in a laboratory flume under four bed slopes (10, 15, 20, and 25%) and five water discharges (0.26, 0.35, 0.45, 0.56, and 0.67 L m−1 s−1) with five replications per experiment. D c was lower (on average − 44%, with a minimum reduction of −40% at a slope of 25% and a maximum −50% at a slope of 15%) in the hydromulched soils compared to the untreated plot. D c was positively and negatively correlated, with diameter and density of hydromulch roots, respectively. Rill erodibility was noticeably lower (−81%) in the hydromulched soil compared to the bare plot. By regressing D c on shear stress, rill erodibility and critical shear stress for deforested hillslopes (treated with hydromulching or left bare) were given. These parameters are useful to hydrologists in applications of physically-based erosion models. [ABSTRACT FROM AUTHOR]

• An understanding the soil erosion under natural and planted forestlands is needed. • This study shows the effect of different vegetal species on rill detachment capacity. • Power regression equations to predict the detachment rate were developed. • The natural forest species control better the soil losses compared to the planted species. • Parrotia persica showed the lowest values of rill detachment capacity and Rill erodibility. Planted forests and management operation method can potentially alter the soil properties as a consequence of changes in the characteristics of tree and plant species. However, the effects of planted forest species with different root systems on the soil detachment capacity (D c) have not well been quantified. This study investigates the effects of plantation practice on the soil detachment capacity of soil with six vegetal species (Quercus castaneifolia, Alnus glutinosa, and Zelkova carpinifolia, as planted forest species) and Carpinus betulus , Parrotia persica , and Pinus taeda (as natural forest species) based on a comparison between natural and planted forests in forestland of the Saqalaksar Park (Northern Iran). To this aim, the soil detachment capacity on samples collected under trees of the studied species was measured in an experimental flume at five slopes (0.26, 0.35, 0.46, 0.57 and 0.69 m m−1) and flow rates (0.037, 0.086, 0.185, 0.285 and 0.376 L m−1 s−1). Moreover, simple prediction equations are suggested to evaluate the soil detachment capacity and rill erodibility (K r) for studied species. The soils of natural forestland showed the lower soil detachment capacity compared to soils of the planted forestland, due to the different influence of plant root systems. Moreover, the inappropriate management practices in planted forestland could have worsened the soil properties and thus increased the soil detachment rate. The unit stream power was better predictor of Dc compared to other hydraulic parameters in both natural and planted forestlands. The rill erodibility of soils was higher in planted forestland than in natural forestland. According to the results, the natural forestland is able to show higher resistance against soil erosion in comparison with the planted forestland. These findings indicate that for sustainable management of forest ecosystems, the maintenance of natural vegetal species can be an effective practice to reduce the soil erosion. [ABSTRACT FROM AUTHOR]