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•Ethanol was used to simulate the sole effect of mechanical breakdown.•The contribution of the mechanisms for aggregate disintegration was quantified.•The contribution of mechanical ...breakdown had significant correlation with RSI/RMI.•Equations for contribution of slaking and mechanical breakdown were established.
The information of aggregate disintegration mechanisms during splash erosion is scant. This study was conducted to quantify contributions of the mechanisms of aggregate disintegration to splash erosion. Six soils with five soil textures were used. Soil aggregate stability was determined by the Le Bissonnais (LB) method. Deionized water was used to simulate the combined effect of slaking and mechanical disaggregation, while ethanol was used to estimate the sole contribution of the mechanical breakdown. Simulated rainfall with intensity of 60 mm h−1 was applied at five fall heights (0.5 m, 1 m, 1.5 m, 2 m and 2.5 m) to achieve different levels of rainfall kinetic energy. The results indicated that slaking caused the most severe aggregate breakdown, and followed by mechanical breakdown, while chemical dispersion in slow wetting with deionized water was the weakest breakdown mechanism. The splash erosion rates due to the effects of slaking and mechanical breakdown increased with an increase in rainfall kinetic energy. The contributions of the slaking (mechanical breakdown) to splash erosion decreased (increased) as rainfall kinetic energy increased. The contribution of mechanical breakdown had a power function relation with rainfall kinetic energy, and had the most significant correlation with RSI (relative slaking index)/RMI (relative mechanical breakdown index). A power and a linear function could be used to describe the relationships between the contributions of mechanical breakdown with rainfall kinetic energy and RSI/RMI, respectively, which could be used to estimate the contribution of mechanical breakdown. The results of this research would be helpful to improving the soil erosion prediction models.
•Three instruments were compared in the lab under simulated rainfall conditions.•The raindrop size was most accurately described using the geometric mean diameter.•The raindrop visual measurement ...method could provide accurate raindrop shapes.•The laser precipitation monitor was more sensitive to measuring raindrops of < 1 mm.•Based on rainfall tests, the applicability of the three instruments was discussed.
Precipitation microphysics, which describes the basic characteristics of rainfall, is important for agricultural praxis, weather prediction, aviation safety, and soil erosion prediction. In this study, three types of instruments (a laser precipitation monitor, LPM; piezoelectric transducer, PT; and particle imaging transient visual measurement technology, PIV) were employed to measure and compare the raindrop size distribution (DSD) and rainfall kinetic energy rate (KEt) under simulated rainfall conditions. Comparisons of the results indicated that under the same simulated rainfall conditions, the number of raindrops per unit area measured by LPM was larger than that measured by PIV. The DSD measured by PIV was more uniform than that of the PT and LPM under the same rainfall conditions. The raindrop size range measured by the LPM was smaller than that measured by the PT and PIV. In addition, the geometric mean diameter was a more accurate representation of raindrop size because PIV can capture the true irregular shape of raindrops. Compared to the PIV sensor, the LPM underestimates the raindrop diameter. The median raindrop diameter measured and calculated by PIV using the geometric mean diameter was approximately 1.61 times that of LPM. The KEt values measured by PIV and PT were approximately similar, while the KEt calculated by LPM was 0.51 and 0.57 times that of PIV and PT for the same rainfall conditions, respectively. A correction factor of 1.75 provided an approximate reference for the calibration of the kinetic energy calculation of the LPM instrument. The above results can provide basic insights for calibration and application of the three instruments.
Characteristics of orographic raindrop size distribution (DSD) in the Tianshan Mountains, China are studied based on second-generation OTT Particle Size Velocity disdrometer installed at the top ...(Tianchi, 43.88°N, 88.12°E, 1941.8 m above sea level) and the foot stations (Urumqi, 43.79°N, 87.65°E, 935 m above sea level) from June to August 2020 and 2021. For the overall rainfall, the concentration of drops of all sizes in Tianchi is greater than that in Urumqi. Furthermore, in both regions, small drops (diameter < 1 mm) primarily contributed to the total number concentration, and both small drops and mid-size drops (1 ≤ diameter ≤ 3 mm) had important contributions to the rainfall rate. For the DSD of different rain rate classes, the concentration of mid-size drops is higher in the first two rain rate classes in Urumqi than that in Tianchi, while the opposite is true when the rain rate exceeds 5 mm h−1. Meanwhile, the concentration of small drops is higher in the first two rain rate classes in Tianchi than that in Urumqi, while the opposite is true for small drops in partial diameter, as the rain rate class increases. In addition, two kinds of rainfall kinetic energy (rainfall kinetic energy flux: KEtime and the rainfall kinetic energy content: KEmm) and various rainfall kinetic energy-rainfall intensity relations (KEtime/KEmm –R relations) were derived based on DSD data. Further, the possible thermo-dynamical and microphysical processes that cause the dissimilarities in DSD between Urumqi and Tianchi are also discussed in this work. Affected by the difference in altitude between the top and the foot stations, the foot station had relatively hotter and drier conditions in the near-surface layer than the top station during the rainfall period, so the evaporation rate of small drops was higher at the foot station than that at the top station, resulting in fewer small drops at the foot station. Meanwhile, lower black body temperature, and stronger seeder-feeder mechanism at the top station during the rainfall period may be partly responsible for more mid-size drops and large size drops (diameter > 3 mm) at the top station
•This work concentrated on the characteristics of orographic raindrop size distribution of the Tianshan Mountains.•Two kinds of rainfall kinetic energy and relations between rainfall kinetic energy and rainfall intensity were derived.
Due to the severe threat of tropical cyclones to human life, recent years have witnessed an increase in the investigations on raindrop size distributions of tropical cyclones to improve their ...quantitative precipitation estimation algorithms and modeling simulations. So far, the raindrop size distributions of tropical cyclones using disdrometer measurements have been conducted at coastal and inland stations, but such studies are still missing for oceanic locations. To the authors’ knowledge, the current study examines—for the first time—the raindrop size distributions of fourteen tropical cyclones observed (during 2003–2007) at an oceanic station, Aimeliik, located in the Palau islands in the Western Pacific. The raindrop size distributions of Western Pacific tropical cyclones measured in the Palau islands showed unlike characteristics between stratiform and convective clusters, with a larger mass-weighted mean diameter and smaller normalized intercept parameter in the convective type. The contribution of the drop diameters to the total number concentration showed a gradual decrease with the increase in drop diameter size. Raindrop size distributions of Western Pacific tropical cyclones measured in the Palau islands differed slightly from Taiwan and Japan. The helpfulness of empirical relations in raindrop size distribution in rainfall estimation algorithms of ground-based (Z–R, μ–Λ, Dm–R, and Nw–R) and remote-sensing (σm–Dm, μo–Dm, Dm–Zku, and Dm–Zka) radars are evaluated. Furthermore, the present study also related the rainfall kinetic energy of fourteen tropical cyclones with rainfall rate and mass-weighted mean diameter (KEtime–R, KEmm–R, and KEmm–Dm). The raindrop size distribution empirical relations appraised in this study offer a chance to: (1) enhance the rain retrieval algorithms of ground-based, remote sensing radars; and (2) improve rainfall kinetic energy estimations using disdrometers and GPM DPR in rainfall erosivity studies.
•Effects of rainfall KE on sediment sorting were investigated.•More sediments were transported as primary particles under higher rainfall KE.•There were at least two different mechanisms affecting ...particle transport.•Relative importance of the two mechanisms was associated with rainfall KE.
Rainfall kinetic energy (KE) can break down aggregates in the soil surface. A better understanding of sediment sorting associated with various KEs is essential for the development and verification of soil erosion models. A clay loam soil was used in the experiments. Six KEs were obtained (76, 90, 105, 160, 270, and 518Jm−2h−1) by covering wire screens located above the soil surface with different apertures to change the size of raindrops falling on the soil surface, while maintaining the same rainfall intensity (90±3.5mmh−1). For each rainfall simulation, runoff and sediment were collected at 3-min intervals to investigate the temporal variation of the sediment particle size distribution (PSD). Comparison of the sediment effective PSD (undispersed) and ultimate PSD (dispersed) was used to investigate the detachment and transport mechanisms involved in sediment mobilization. The effective–ultimate ratios of clay-sized particles were less than 1, whereas that of sand-sized particles were greater than 1, suggesting that these particles were transported as aggregates. Under higher KE, the effective–ultimate ratios were much closer to 1, indicating that sediments were more likely transported as primary particles at higher KE owing to an increased severity of aggregate disaggregation for the clay loam soil. The percentage of clay-sized particles and the relative importance of suspension–saltation increased with increasing KE when KE was greater than 105Jm−2h−1, while decreased with increasing KE when KE was less than 105Jm−2h−1. A KE of 105Jm−2h−1 appeared to be a threshold level beyond which the disintegration of aggregates was severe and the influence of KE on erosion processes and sediment sorting may change. Results of this study demonstrate the need for considering KE-influenced sediment transport when predicting erosion.
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•The reduction in rainfall kinetic energy and soil depth inhibited runoff and erosion.•A power function could describe the relationships between soil erosion rate and stream power ...combined with soil clay content.•Enrichment of fine particles in sediment produces a coarsening quartz layer on slope surface.•The enriched coarse grains restrain subsequent erosion in situ.
Screen covering is a widely used soil conservation practice to control water erosion in coarse-textured soil areas. A better understanding of the effects of screen covers at reducing rainfall kinetic energy (KE) on the erosion behavior will improve erosion prediction and control measures development. The objective of this study was to investigate and quantify the effects of rainfall KE on the soil loss and sediment sorting for developing a prediction model in a coarse-textured soil. A series of rainfall simulation experiments were conducted in three horizons of a coarse-textured soil under five rainfall KEs (628, 443, 324, 231, and 110 J m−2 h-1) which were obtained by covering the soil surface with wire screens of different apertures at a coincident intensity of 90 mm h-1. The results showed that the increase in rainfall KE increased runoff rate and soil erosion rate. The soil erosion rate also increased with depth of the soil layer exposed to the soil surface. Based on the soil clay content, a power relationship could describe the relationships between soil erosion rate and geometric mean diameter of sediment and stream power. With increasing rainfall KE and runoff duration, although the percentage of clay- and silt-sized particles in the sediment decreased, both were still higher than the content in the original soil. The enrichment of fine particles in runoff produced a coarsening layer on the slope surface, which might restrain the subsequent erosion in situ. The observed large preferential loss of clay- and silt-sized particles by erosion emphasizes the importance of sediment sorting for subsequent erosion in these coarse-textured soils. These results suggest that the relationship of rainfall KE to erosion should be considered for practical and effective estimation and prediction of the potential erosiveness of rainstorms.
•Another method was provided to determine Tc of raindrop-impacted interrill flow.•Tc can be calculated using power function equation of slope gradient and rainfall intensity.•Stream power together ...with rainfall kinetic energy can best describe Tc.
Interrill erosion processes typically involve such scientific issues as detachment-limited and transport-limited erosion behaviour. An accurate estimation of the sediment transport capacity (Tc) by raindrop-impacted overland flow is critical for interrill erosion modelling and for evaluating sediment budgets under erosion-limiting conditions. Simulated rainfall experiments with rainfall intensities from 0.8 to 2.5 mm min−1 over a three-area soil pan with slope gradients from 12.7% to 46.6% were conducted to identify the transport-limited cases and determine Tc by raindrop-impacted overland flow within the transport-limited systems of interrill erosion processes. Results indicated that Tc increased as a power function of rainfall intensity and slope gradient (R2 = 0.84, NSE = 0.75), and Tc was more sensitive to rainfall intensity than to slope gradient. In terms of R2 and NSE, stream power was the key hydraulic parameter that influenced Tc among flow velocity (R2 = 0.64, NSE = 0.39), shear stress (R2 = 0.53, NSE = 0.23), stream power (R2 = 0.76, NSE = 0.52) and unit stream power (R2 = 0.49, NSE = 0.16). The addition of rainfall physical parameters in response equations of Tc in addition to hydraulic parameter, could improve an accuracy of Tc modelling. Stream power combined with rainfall kinetic energy can best describe the Tc of raindrop-impacted overland flow within the transport-limited system of interrill erosion processes by a power-exponent function (R2 = 0.90, NSE = 0.72). Rainfall kinetic energy can reduce the Darcy-Weisbach resistance coefficient of raindrop-impacted overland flow and thus benefit sediment transporting. This study provides another method for directly identifying the Tc of raindrop-impacted overland flow in interrill erosion processes on steep loess slopes, and points out that rainfall impacts should be particularly considered when studying Tc by raindrop-impacted overland flow.
The splash erosion-induced soil aggregate turnover is important to understand the mechanisms of soil degradation and soil organic carbon (SOC) cycling. However, the relationship between aggregate ...turnover and rainfall impact has not been explored directly because of methodological difficulties. The soil samples, developed from Quaternary red clay, were adopted from three land use types: natural forest (NF), garden/shrubs (AS), and abandoned meadow (AM). By using rare earth oxides to trace the transformation in soil aggregates of four size-fractions (>2 mm, large macroaggregate; 0.25–2 mm, small macroaggregate; 0.053–0.25 mm, microaggregate; <0.053 mm, silt and clay fraction), we investigated aggregate turnover based on splash experiments at four different rainfall kinetic energies (501.95, 326.00, 252.50 and 153.39 J·m−2·h−1). After splash erosion, the rare earth oxides concentrations and SOC content in different soil aggregate fractions were measured. Our results indicated that the soil aggregates mainly followed the direction of breakdown, with higher cumulative breakdown rates than formation rates. Specifically, with the rainfall kinetic energy increased to 501.95 J·m−2·h−1, the three soil samples (NF, AS, AM) exhibited the highest breakdown rates of 48.35%, 31.95% and 39.29% in large macroaggregates, respectively. Meanwhile, the formation pathway was mainly in the direction of forming small macroaggregates. Compared to soil aggregate from NF, AS and AM aggregates had higher proportion of silt and clay fraction transformed into larger aggregates. Moreover, the changes of SOC varied in aggregates during splash process. With the increase of rainfall kinetic energy, the organic matter content gradually decreased in large aggregates and microaggregates in AM and AS (P < 0.05), in contrast to a trend of first increasing and then decreasing (P < 0.05) in their small aggregates. In the clay particles, with the increase of rainfall kinetic energy, the SOC content gradually decreased in NF (P < 0.05) and fluctuated in AS, with both their organic matter content lower than before the splash test (P < 0.05). The loss of SOC in total soil was linearly related to the net breakdown rate of soil aggregates (P < 0.05, R2 =0.635). Therefore, we proposed a framework to highlight the quantitative relationship between aggregate turnover and SOC degradation under splash, which may enrich the mechanisms of soil erosion and soil degradation in the positions where erosion takes place.
•Aggregate turnover is significantly accelerated by splash erosion.•Aggregate turnover rate depends on aggregate size and rainfall kinetic energy.•Soil carbon dynamics within aggregates are tightly related to aggregate breakdown.
•Differences in splash erosion between 50 and 100 mm h−1 were attributed to surface seals.•Quantified the key factors affecting splash erosion before and after runoff occurrence.•Rainfall intensity ...and its energy exhibit higher contributions on splash erosion whether runoff occurs.•Identified the critical ratio of flow depth and raindrop diameter (h/D) at 100 mm h−1.
Splash erosion has been acknowledged as a key process of water erosion, depending on multiple factors including rainfall and soil properties, antecedent soil moisture, and surface runoff. Previous studies mostly focused on the effect of simple factors on splash erosion, but the interactive effects of multiple factors on splash erosion are still obscure. Therefore, we investigated the characteristics of splash erosion under simulated rainfall and quantified the interactive effects of multiple factors on splash erosion using a modified splash pan for measuring directional splash erosion. The results showed that total splash erosion (summation of four directional splash erosion) and net splash erosion (downslope minus upslope splash erosion) ranged from 14.4 to 804.8 g m−2 and 2.0 to 186.5 g m−2, respectively, and both increased with rainfall intensity and rainfall kinetic energy. The splash erosion rates in the dry run were greater than those in the wet run at the earlier stage, whereas after runoff occurred, the splash erosion rates in the dry run were lower than those in the wet run. Furthermore, the calculated variance contributions indicated that before runoff occurrence, rainfall kinetic energy (KE) and antecedent soil moisture (θa) were the dominant factors affecting splash erosion, accounting for 25.3 % and 15.0 % of variance contributions, respectively. After runoff occurrence, the main factors affecting splash erosion were rainfall intensity (RI), rainfall kinetic energy (KE) and their interactions, explaining 98.2 % of the total splash and 95.7 % of the net splash for all variance contributions. In addition, the effect of shallow flow on splash erosion was dependent on the ratio of flow depth and raindrop diameter (h/D). The thresholds of h/D seemingly ranged from 0.63 to 0.86 and 0.77 to 1.02 for the dry and wet runs under 100 mm h−1 rainfall intensity. This study provides a basis for understanding splash erosion mechanisms and improving process-based erosion models.
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