The use of appropriate models for environmental decision-making is crucial for supporting soil conservation strategies, particularly in erosion-prone systems such as Mediterranean basins. In this study, the suitability of the Sediment Delivery Distributed (SEDD) model at the event scale was evaluated in a 231 ha basin in Central Italy, where rainfall-runoff-sediment yield data from four erosive events were available. The model was applied by subdividing the basin into morphological units, defined as areas with uniform aspect, length, and slope steepness. Model parameterization involved the estimation of a single coefficient, beta(e), embedded in the sediment delivery ratio of each morphological unit. The variability of beta(e) with rainfall-runoff event intensity was analysed. The resulting relationship reflects the physical mechanism whereby sediment transport along hillslopes becomes more efficient as event intensity increases, as represented by the runoff coefficient Q(R). Analysis of the empirical cumulative frequency distribution of the sediment delivery ratio for each morphological unit suggests that the influence of the runoff coefficient on beta(e) may vanish during the most erosive events. Under this hypothesis, for Q(R) > 0.15, the variability of sediment delivery processes is governed solely by the structural component of sediment connectivity, while the functional component becomes invariant and characteristic of the investigated basin.

The soil erosion processes involve the detachment of soil particles from the soil surface, followed by their transport by erosive agents, such as rainfall, overland flows, and channelized flows (rills, ephemeral gullies, and gullies) [...]

Even though rill erosion is strongly influenced by flow hydraulics, slope, and channel tortuosity, the influence of path tortuosity on rill flow resistance and soil erosion has been investigated little. In this study, experiments on tortuous rills were performed on a plot with a mean slope sp of 15

The formulation of methods that are both simplified and sufficiently accurate for estimating flow resistance in vegetated channels constitutes a critical challenge in the context of flood hazard mitigation and management of riparian species that directly influence channel conveyance capacity, channel morphological evolution and sediment deposition. In this study, measurements collected by the US Army Corps of Engineers in a large-scale channel with unmanaged spontaneous vegetation were used to verify the suitability of a theoretical flow resistance equation in open-channel conditions and floodplains. This equation, deduced by applying dimensional analysis and the incomplete self-similarity condition for the velocity profile of open-channel flow, was initially reported. The available meausurements, referring to different vegetation density values, were used for relating the Gamma function of the power velocity profile to the slope energy, the flow Froude number and vegetation density. This investigation showed that the Darcy-Weisbach friction factor f can be accurately estimated using the proposed theoretical flow resistance equation, with errors in the f estimate that are less than or equal to 15% for 92.2% of the investigated cases and less than or equal to 10% for 75.5% of the cases. The findings of this study also demonstrated that the characteristics of the investigated vegetation, such as vegetation density, appreciably affect the estimate of flow resistance.

The Carapelle basin (Apulia, Southern Italy) is affected by significant soil erosion processes, exacerbated by its Mediterranean climate and agricultural practices. In this study, the SEdiment Delivery Distributed (SEDD) model is applied to investigate sediment transport and erosion processes over a five-year period. Key factors, including slope steepness and length, land cover, soil erodibility, and climatic conditions, were integrated into the SEDD model to identify areas vulnerable to gross soil erosion. Data collected from rainfall gauges distributed across the basin, together with streamflow and sediment concentration measurements carried out at the basin outlet, were used to calibrate the SEDD model at both annual and mean annual temporal scales. Results identified that 51

Knowledge of the friction factor of overland and channelized rill flows moving over rough hillslopes is required for modeling soil erosion and transport processes. The available literature flume investigations are characterized by different values of the ratio between the hydraulic radius R and the water depth h, and distinguish the two hydraulic cases corresponding to overland (R quasi equal to h) and rill flows (R/h < 1). In this paper, literature flume measurements performed in smooth and rough channels, which are characterized by a fixed bed and a grain resistance condition, were firstly used to highlight the experimental evidence that a continuity in flow resistance from overland (hydraulic radius R quasi equal to water depth h) to rill flows (R/h < 1) can be stated. This analysis pointed out that the measured highest f values correspond to very wide rectangular cross-sections (VWR) and narrow rectangular cross-sections (NR), while for the intermediate geometric conditions, the lowest friction factor values were measured. This detected transition from VWR to NR cross-sections implies a transition in flow resistance that can be represented in the flow resistance equation by R/h. In particular, the proposed flow resistance law was theoretically deduced considering rectangular cross-sections characterized by different R/h values and calibrated by the available datasets. The agreement between the measured f values and those calculated by the theoretical flow resistance equation was characterized by errors, which were normally distributed, less than or equal to +/- 10% for 96.8% of the cases and less than or equal to +/- 5% for 82.0% of the cases. The proposed flow resistance equation is applicable for channels with a rectangular cross-section varying from VWR (overland flows) to NR (rill flows).

Rainfall simulators are essential tools in experimental water and soil management studies. As they enable the generation of precipitation with known energy characteristics, they are useful for understanding the dynamics of several erosion sub-processes and investigating the relationship between rainfall features and soil erosion. The most critical aspect for the energy characterisation of simulated rainfalls is the assessment of the raindrop size distribution and impact velocity, which depends on the fall height and the raindrop diameter. In this paper, the characterisation of drip-type and pressurised rainfall simulators is presented, based on disdrometric measurements carried out using a Parsivel2 disdrometer and drop velocity estimates. In particular, the reliability of a relationship calibrated on literature fall velocity measurements and relating raindrop fall velocity, fall height and drop diameter, is tested for the drip-type and pressurised rainfall simulators considered in the present investigation. This analysis demonstrated that the Parsivel2 produces systematic underestimations of the fall velocity of the raindrops generated by both drip-type and pressurised simulators, probably due to the characteristics of the Parsivel2 (i.e., laser sensor, internal algorithm, design for the natural precipitation). Therefore, simulated rainfall can be characterised considering the drop size distributions detected by the Parsivel2, which enable accurate measurements of raindrop diameter, rainfall intensity and drop count, coupled with the proposed empirical relationship for calculating raindrop fall velocity. Specifically, for the drip-type rainfall simulator, the use of this latter equation simplifies the calculation of raindrop fall velocity, while for pressurised simulators, it represents a robust alternative to the fall velocity measured by Parsivel2.

Soil conservation strategies based on the mean annual value of the rainfall erosivity factor could not guarantee appropriate erosion control for intense rainfall events. In this paper, a new method to define soil loss tolerance at a regional scale, using the crop and management factor of the USLE, C, was applied for estimating the payment for ecosystem service (PES) of soil erosion control. PES is characterized by a target value PEStg corresponding to a post-intervention condition in which soil loss is equal to the tolerance value. The PES was estimated by three different scenarios: (1) only reducing the current C factor, or combining its reduction with that of the (2) support practice factor P or (3) slope length factor L. For Scenario 1, the analysis demonstrated that a drastic reduction of the C factor determines PES/PEStg ≥1 regardless of the investigated climatic conditions. For Scenario 2, when C and P factors are equally weighted, the payment is higher than the target value for slope-steepness ≤15%. Scenario 2 always yields a higher payment compared to Scenario 1. For Scenario 3, the reduction of the topographic factor can lead the PES over the target value. Although PES is generally considered effective for environmental policy, it should be allocated by targeting erosion-prone areas, where soil conservation is a primary objective. These areas experience soil loss greater than tolerable value, which usually lacks a solid scientific basis. By contrast, the proposed method, developed for the Sicilian region, is grounded in a scientific framework and is useful to support policymakers and public authorities in the informed design of economic incentives for farmers who convert their activities in order to preserve soil resources.

ABSTRACT Despite growing interest in biochar as a soil amendment, its effects on erosion control remain uncertain, particularly for rill erosion. In this investigation, the short‐term (i.e., 1 month after biochar addition) effects of wood‐biochar on rill erosion and flow resistance were evaluated. At first, plot experiments were performed discharging a constant clear inflow into a rill incised in the original soil (rill 0 ) and two rills, named rill 3 and rill 5 , incised in clay loam soil amended with different initial biochar concentrations BC (3% and 5% by weight). For each rill, the outflow discharge of the entire run was collected by a storage tank and the run was stopped when a 55 L volume was reached. The collected volumes were used to measure the corresponding weights of the eroded mixture (sediments and biochar), biochar and sediments. 3D photo reconstruction was used to obtain the Digital Elevation Models (DEM) before and after the runs and by the corresponding DEM of difference, the total volume of the eroded mixture was calculated. Measurements of water depth, hydraulic radius and flow velocity were used to calculate the Darcy–Weisbach friction factor f values. The results demonstrated that in the short term, the addition of BC = 3% is effective in reducing soil erosion by 9% as compared to the control condition, while adding BC = 5% determined an increase of 140% in eroded volume as, for this latter, the short‐term soil particle aggregation effects are too weak and the mixture remains cohesionless. Moreover, the analysis demonstrated that the proposed flow resistance equation gives a reliable f estimate and this equation gives, for fixed hydraulic conditions, flow resistance that decreases with increasing BC . Ultimately, for the short term, BC higher than 3%–4% cannot be considered a good soil conservation practice.

Using both laboratory and field samples, this paper investigates how zeolite concentration affects the hydrological connectivity of sandy-loam soil through Fast Field-Cycling Nuclear Magnetic Resonance relaxometry. Laboratory samples (LP) were prepared using four zeolite concentrations (0, 5, 10, and 15%), while field samples (FS) were collected in plots amended with the same concentrations to investigate the differences occurring during incubation time between LP and FS. For each zeolite concentration ZC, the results demonstrated that the F(T1) distribution of the FS systematically shifts towards the right compared to LP. This “scaling” effect between LP and FS was addressed using a dimensionless variable T1/σ(T1), where σ(T1) is the standard deviation of T1 considering the effects of pore size variability. The developed analysis demonstrated that the highest values of the structural connectivity index SCI correspond to ZC = 10% for LP, while ZC = 15% is necessary for FS, even if similar performance corresponds to ZC = 10%. Differences in the functional connectivity index (FCI) of LP and FS, which can be explained by environmental effects, were recognized. In conclusion, for sandy-loam soil, ZC = 10% is sufficient to improve the physical soil characteristics (highest values of structural connectivity) for both samples, while for FS, a ZC = 10% assures the minimum FCI values (the highest water-holding capacity).

The dye-tracer technique is widely applied for measuring the surface velocity of shallow flows, which is jointly used with a correction factor cv to obtain the mean flow velocity. The primary purpose of this paper is to evaluate the effect of the soil roughness height on the correction factor in rill flows. For this aim, experimental runs were performed with clear water flowing in two flumes covered by glued soil particles resulting in different roughness heights (ks = 0.135 mm and 1.317 mm). Ten slopes s, ranging from 0.1 % to 15 %, and five flow discharges Q varying from 0.20 to 0.87 L s- 1 were utilized. Previous measurements, collected in the same flumes for ks = 0 and 0.119 mm, were also used. Using dimensional analysis, Reynolds number, flume slope, and a dimensionless group embedding the roughness height were identified as influential factors of cv. The proposed predictive equation of cv established that the correction factor (i) decreases as the Reynolds number increases, (ii) generally increases with increasing roughness height, and (iii) depends on slope following a non-monotonic trend. The proposed equation can reliably estimate the correction factor, with a mean absolute error of 5.4 % for smooth bed rills and 5.8 % for rough bed rills. Further experiments need to investigate the effect of rill width on the correction factor and test the proposed equation with different roughness heights.

The effect of rock cover on rainfall impact and hydrodynamic flow behavior is a topic that needs to be further explored. In this paper, the effects of rock fragments embedded (trapped in the topsoil layer) in clay soil on runoff and sediment yield under simulated rainfall are investigated. The experiments were performed on 0.25 m × 0.25 m sloping microplots with bare soil (control) or a surface covered by 35 embedded rock elements (rock). For each configuration, three rainfall intensities (260.2, 444.0, and 605.2 mm h−1), obtained by Kamphorst’s simulator, were tested, and the whole suspension was collected to determine runoff and sediment yield. The microplots were surveyed before and after the runs to obtain a Digital Elevation Model (DEM) and a DEM of Difference (DoD) and determine sediment yield. The obtained results demonstrated that both runoff and sediment yield (both measured from bowls and calculated from the DoD) increase with increasing rainfall intensity and rock percentage cover. For embedded elements, a higher rate of the sediments moving along the microplots reaches the downstream end of the microplot due to flow channelization. Finally, for the control condition, the erosive phenomena are concentrated under the nozzles, while for the rock configuration, they are also spread in the areas among the rock elements.

The flow stage-discharge relationship (SDR) of weirs is a well-studied hydraulic topic that is generally concerned with energy considerations and using a discharge coefficient. In this context, dimensional analysis and selfsimilarity theory are alternative theoretical methods to obtain accurate stage-discharge curves. In this paper, the SDR of streamlined-type weirs (streamlined SLW, hydrofoil HW, and modified semi-cylindrical MSCW) is theoretically deduced using dimensional analysis and the self-similarity condition. These new SDRs are estimated using experimental results from the literature. For the SLW, the analysis demonstrated that the new SDR gives discharge estimates similar to those obtained by the theoretical equation proposed by Carollo and Ferro (2021). For the HW, the SDR, calibrated by the measurements of Soydan Oksal et al. (2021), results in errors always less than the accuracy limit of f5 %. Furthermore, for the same geometric parameters and upstream water level, the analysis pointed out that a HW flows a discharge higher than that corresponding to a SLW. For the MSCW, the SDR, calibrated by the measurements of Afaridegan et al. (2023), results in errors lower than or equal to f3 % in 95.6 % of cases. Thereby, for SLW, HW, and MSCW, the developed analysis showed that the proposed SDR results in errors in discharge estimates always less than those obtained for the literature solutions. Finally, a single SDR, useful for all streamlined-type weirs, was also determined, and it results in errors lower than or equal to f5 % for 96.1 % of cases and lower than or equal to f3 % for 76.6 % of cases.

Although several studies regarding flow resistance due to aquatic vegetation for small- and full-scale are available, the scaling of small-scale results to full-scale conditions still needs to be investigated. In this paper, a theoretical flow resistance equation for open channels was tested using literature measurements performed for full-scale field channels and small-scale experimental flumes with aquatic vegetation. At first, the relationship between the scale factor Gamma of the velocity profile, the Froude number and the channel slope was calibrated by using 27 field measurements by Nikora et al. (2008). This relationship was also tested by 39 field experimental series by Okhravi et al. (2022). Then, the proposed relationship for estimating Gamma was coupled with the theoretical flow resistance law to assess the performance in the estimate of the Darcy-Weisbach friction factor values. The results demonstrated that the proposed approach gives a more accurate estimate of the Darcy-Weisbach friction factor as compared to that obtained applying the literature relationships. The relationship between Gamma, the Froude number and the channel slope obtained for the field condition was recalibrated by flume data changing only the scale coefficient. The analysis demonstrated that, for known hydraulic conditions (slope, Froude number), for scaling the Gamma values estimated by field data (streams, rivers), Gamma(F), to values corresponding to laboratory conditions (flumes) Gamma(L), a scale factor of 0.5356 must be used. In conclusion, comparing the same hydraulic conditions, streams or rivers are characterized by friction factor values approximately twice those of a laboratory flume.

The Natural Resources Conservation Service (NRCS)-curve number (CN) method was originally proposed to predict runoff on small and midsize catchments, but it has also been used at the scale of erosion plots. In this case, uncertainties exist with reference to the factors, for example, scale effects, affecting the experimental CN values. In this study, the reliability of the CN method in reproducing plot runoff is analysed by using data collected at the Sparacia erosion plots (Sicily, Southern Italy), which are characterised by different sizes and steepness. This investigation aimed to test the possibility of using simulated runoff within universal soil loss equation (USLE)-type models, including runoff as a term in the erosivity factor. This analysis pointed out that the experimentally determined value of the initial abstraction ratio of the CN method was very low (0.0001). For each plot type (i.e., fixed length and steepness), the calibration was performed for 18 combinations of three rainfall ranges (all data, rainfall depth less than the median, and exceeding the median), two calibration approaches (least-squares and median value) and three datasets (all data, interrill, and rill). The best CN model fit was systematically produced for data with rainfall depth less than the median. The least-squares calibration approach generally performed slightly better than the median value one. Results showed that the CN method can be considered effective only for events producing rills. The CN values generally increased with plot steepness and decreased as plot length increased. For each plot type, CN tendentially increased for increasing soil moisture before the rainfall event, but moisture and rainfall depth were able to explain a minor part (from 19.5% to 41%) of CN variance. Finally, the USLE-MB that incorporates runoff simulated by the CN method was found to satisfactorily predict (relative standard error = 0.69, Nash and Sutcliffe Efficiency Index = 0.54) event soil loss caused by simultaneous interrill and rill erosion due to the higher rainfall depths recorded at the Sparacia station.

Recent studies across Europe highlight the vulnerability of Mediterranean countries to rainfall-induced soil erosion. Climate change is intensifying the water cycle, leading to shifts in rainfall patterns and more frequent extreme precipitation events. Convection-permitting climate models (CPMs) outperform regional models in capturing intense sub-daily rainfall, providing more accurate projections of future extreme events. This study exploits a simplified rainfall erosivity model which uses 2-yr sub-daily rainfall quantiles as input to: investigate the ability of a CPM to provide reliable rainfall data for assessing rainfall erosivity; analyze the projected changes in rainfall quantiles and rainfall erosivity. The study is carried out in Sicily, which offers an ideal case study given its rainfall variability and complex topography. Data from 171 rain gauges are used to evaluate the CPM in simulating 2-year sub-daily rainfall quantiles and rainfall erosivity. Future changes are evaluated from the CPMs projections under the RCP8.5 scenario up to 2070. The results show that model accuracy varies by rainfall duration and elevation, with greater underestimation of rainfall at shorter durations (up to -38 % on average at 1 h duration) and in coastal lowlands. Projections suggest an increase in intense rainfall, particularly for shorter durations, leading to higher rainfall erosivity (about + 20 % on average in the future). These findings underscore the need for bias adjustments in simulated rainfall data, considering both temporal scales and topographic influences. The study also highlights the potential for increased rainfall erosivity under climate warming, stressing the importance of developing soil conservation strategies and policies to address future challenges. This research set the stage for wider-area applications.

ABSTRACTRill erosion is a significant problem worldwide as it determines relevant amounts of soil loss on hillslopes. Although, in the last few years, many studies have focused on rill erosion and biochar as soil amendment, their influence on soil hydrological properties and relevance on soil conservation strategies is still uncertain. In this paper, the effects of rill formation and biochar addition on the physical and hydraulic properties of a clay‐loam soil were assessed by laboratory measurements (water retention, hydraulic conductivity, minidisk infiltrometer data and 1H Nuclear Magnetic Resonance (NMR) relaxometry with the fast field cycling (FFC) setup) and field tests (rill formation tests at the plot scale). The rilled and non‐rilled soils did not show any difference in the volume of pores with a diameter (d) > 300 μm, but the former showed a smaller volume for the pores in the size range between 300 and 0.2 μm. As compared with an untreated rilled soil, the addition of 5% (w w−1) biochar in the soil in which the rill is incised did not change the volume of pores with d > 300 μm, while there were more pores of both 30 ≤ d ≤ 300 μm and 0.2 ≤ d ≤ 30 μm. Moreover, there were less pores with d < 0.2 μm. Shaping the rill did not influence the hydraulic conductivity of the nearly saturated soil (pressure head, h = −1 cm), while it determined a significant decrease of the soil ability to transmit water in more unsaturated conditions (h ≤ −3 cm). The addition of biochar to the soil improved, in general, the soil aptitude to transmit water, regardless of the pore size. However, this improvement was statistically irrelevant in the case of a transport process governed by larger pores. The hydrological measurements also demonstrated that the addition of a large amount of biochar (5%) impedes soil characteristics alteration as the changes due to rilling are balanced by adding biochar in the soil. NMR was also used to measure the structural and functional connectivity of the original soil, the biochar and a mixture with three biochar concentrations (i.e., BC = 1%, 3% and 5% w w−1) traditionally applied in agronomical activity. These measurements revealed that the mixture of soil and biochar was characterised by longitudinal relaxation time (T1) values, which are related to pore sizes, longer than those measured for the soil. In addition, the soil empirical cumulative frequency distribution of T1 was always skewed towards shorter T1 values, thereby suggesting that the macro‐pore component (i.e., the largest T1 values) was never dominant while biochar addition increased the size of mesopores and micropores. Biochar concentrations larger than 3% (w w−1) did not produce appreciable changes in the pore distribution inside the mixture. The biochar component improved the structural connectivity up to BC = 5%, while decreased the functional connectivity up to BC = 3%. A relationship between the water volume contained in soil pores and the NMR data were established for the micropores (d ≤ 0.2 μm). The biochar‐amended soil was characterised by fewer small pores, but these micropores were greater than those in non‐treated soil.

The knowledge of the rainfall drop size distribution (DSD) at the land surface is fundamental for understanding precipitation mechanisms affecting soil erosion processes. The most used analytical form of raindrop size parameterization is that proposed by Ulbrich [1]. While, rarely, the distribution proposed by Weibull [2] has been used. In this paper, the reliability of these theoretical raindrop size distributions to reproduce about 5000 DSDs detected by an optical disdrometer at El Teularet (Spain) is presented. The parameters of both Weibull and Ulbrich distributions were estimated using the Momentum Method, in which the measured values of the median diameter and the median volume diameter were set equal to the ones calculated by each theoretical distribution. The disdrometric measurements allowed to positively test the applicability of the theoretical distributions and to evaluate their reliability in reproducing the main DSD statistics. Both Weibull and Ulbrich distributions, using a specific terminal velocity equation, allowed to theoretically determine the kinetic power of rainfall. These equations demonstrated that the rainfall kinetic power per unit volume of rainfall depends on two parameters of the distribution and, as a consequence, on the disdrometric characteristics of precipitation. Finally, the accuracy of the relationships for estimating the kinetic power of rainfall, theoretically deduced by both Weibull and Ulbrich distributions, was positively tested.