Splash Cups Research Articles

Short communication: Concentrated impacts by tree canopy drips – hotspots of soil erosion in forests

Abstract. The degradation of ground vegetation cover caused by large grazing herbivores frequently results in enhanced erosion rates in forest ecosystems. Splash erosion can be caused by drop impacts with a high throughfall kinetic energy (TKE) from the tree canopy. Notably larger canopy drips from structurally mediated woody surface points appear to induce even higher TKE and generate concentrated impact locations causing severe focus points of soil erosion. However, TKE at these locations has rarely been reported. This pilot study investigated the intensity of TKE at a concentrated impact location and compared it with general TKE locations under the canopy and freefall kinetic energy (FKE) outside the forest. We measured precipitation, TKE and FKE using splash cups at seven locations under Japanese beech trees and five locations outside the forest during the leafless and leafed seasons in 2021 in a mixed forest with evergreen coniferous trees and deciduous broadleaved trees in Japan. The TKE at the concentrated impact location was 15.2 and 49.7 times higher than that at the general locations under the beech and FKE, respectively. This study confirmed that canopy drip from woody surfaces could be a hotspot of soil erosion in temperate forest ecosystems. Throughfall precipitation at the concentrated impact location was 11.4 and 8.1 times higher than that at general locations and freefall, respectively. TKE per 1 mm precipitation (here, “unit TKE”) at the concentrated impact location (39.2 ± 23.7 J m−2 mm−1) was much higher than that at general locations (22.0 ± 12.7 J m−2 mm−1) and unit FKE (4.5 ± 3.5 J m−2 mm−1). Unit TKE in the leafless season was significantly lower than in the leafed season because of fewer redistribution of canopy drips induced only by woody tissue. Nevertheless, unit TKE at the concentrated impact location in the leafless season (36.4 J m−2 mm−1) was still higher than at general locations in the leafed season. These results show that potentially high rates of sediment detachment can be induced not only by throughfall precipitation but also by larger throughfall drop size distributions at the concentrated impact location, even in the leafless season. Further studies with more replication building on this first report are necessary to investigate how many of these concentrated impact locations may occur on average with different tree species to better assess the extent of the erosion risk under forests.

Changeability of runoff and soil loss from inclined mid-sized plots under simulated upward wind-driven rain

The present study investigated the interaction of wind and rain (WDR) on splash and interril erosions under laboratory conditions. The corresponding experiments were carried out with rainfall intensity of 50 mm h−1 with a duration of 30 min under no-wind control conditions and two wind velocities viz., 3 and 6 m s−1 at a slope of 12%. The study was conducted on sandy-loam soil from summer rangelands of the Northern Alborz Mountain in Iran. The soil collected was transported to the Rainfall and Erosion Simulation Laboratory. The prepared 6 × 1 m-plots were concurrent with the simulation of the WDR. Moreover, three splash cups were used to measure splash erosion, and each cup was placed on the soil surface at distances of 1.5, 3.0, and 4.5 m from the plot outlet. The results displayed the significant effects of wind (P < 0.01) on runoff generation and soil erosion components. Therefore, the mean runoff and runoff coefficient decreased while increasing wind velocity increased sediment concentration and soil loss. The mean runoff volume and coefficient were 103.48, 94.62, and 88.82 L and 94.43, 87.23, and 81.62 % at wind velocities of 0, 3, and 6 m s−1, respectively. In addition, the mean soil loss and sediment concentration were 414.94, 1057.47, and 1375.48 g; and 31.35, 156.31, and 303.75 g L−1; at 0, 3, and 6 m s−1 wind velocities, respectively. Besides, the mean net splash increased significantly with increasing wind velocity, from 9.58, 14.93, and 18.63 g, respectively, as wind velocity changed from 0 to 3 and then 6 m s−1. It has been proven that WDR soil erosion was higher than no-wind rain. This research could increase soil managers' awareness of solution strategies.

How much raindrop energy is used for transportation of the two-phase splashed material?

The ejection of a mixture of solid (soil) and liquid (water) phases is one of the aspects of the energy dissipation of the impacting drop during the soil splash phenomenon. All calculations to date have only been taking into account the ejection of solid phase material and were based on significant simplifications of the measured quantities. Therefore, the aim of the study was an improved calculation of the falling drop energy transferred to the ejected material, while considering that such material was a mixture of solid soil particles and water droplets and therefore, “two-phase”. Experimental variants included combinations of soils with different textures and initial moisture contents as well as various falling water-drop energies. Two complementary methods of i)splash cup measurements and ii) image analysis based on high-speed cameras were used for the calculation of quantities i.e. the number of ejected particles, their mass, and their ejection velocity. Based on the obtained results, it was stated that the kinetic energy of ejected particles, expressed as a percentage of falling drop energy, ranged from 1 % to 14 %. This percentage depended strongly on soil texture as well as initial moisture content. The highest values were obtained for soil with the largest amount of sand fraction and the lowest for the soil with the largest amount of finer fractions. All tested soils showed a trend of a higher drop energy transferred to the splashed particles with increasing initial soil moisture content. Taking into account the varied energy of the falling drop variants, the amount of energy transferred to ejected particles was constant.

Aggregate stability and size distribution regulate rainsplash erosion: Evidence from a humid tropical soil under different land-use regimes

Soil erosion by water is a serious global ecological issue that leads to land degradation and threatens ecosystem sustainability. Splash erosion resulting from raindrop impact is the first stage of the erosion process and is mainly responsible for the detachment and migration of soil surface aggregates. In humid tropical regions, however, there has been little quantitative analysis of the relations between splash erosion and variable aggregate characteristics such as stability, particle size distribution and organic matter content. The objective of the current study was to determine how rain splash erosion is related to soil aggregate characteristics under land-use change, i.e., the conversion of tropical rainforest (TR) into rubber plantation (RP). Splash cups containing dry-sieved aggregate samples with eight size classes were exposed to natural rainfall to measure the splash erosion rate. The results showed that the initial aggregate organic carbon, water-stable aggregate index (WSA) and mean weight diameter (MWD) decreased by 31%, 9% and 48%, respectively, after 32 years of rubber cultivation. These degenerations in aggregate properties increased the susceptibility of soil aggregates to erosion and mutually contributed to a higher average splash erosion rate in RP (1.20 kg m−2) than in TR (1.09 kg m−2), regardless of aggregate size fractions. Splash erosion rates for all aggregate sizes were significantly positively correlated with rainfall kinetic energy, rainfall amount and intensity during the study period. The average splash erosion rate of aggregates first increased and then decreased with a decline in aggregate size in both TR and RP. The minimum and maximum splash rates were observed within 10–8 mm and 0.5–0.25 mm size fractions, respectively. In particular, the proportion of small-sized aggregates (i.e., 1–0.15 mm) with a relatively high splash erosion rate increased remarkably after land-use change. This exerted a negative impact on splash erosion control as well as on the sustainable development of rubber cultivation. Increasing the additional input of organic materials (e.g., intercropping cash crops with rubber trees) may help to enhance large aggregate formation and stabilization, which could minimize the risk of splash erosion at the aggregate scale for eroded rubber plantation ecosystems.

Influence of slope incline on the ejection of two-phase soil splashed material.

Soil splash is the first step in the process of water erosion, where impacting raindrops cause the detachment and transport of soil material. One of the factors that strongly influences the magnitude of soil splash is the incline of the surface (slope). The aim of this study was to investigate the effect of the slope on the course of the splash phenomenon caused by single-drop impact (one drop impact per soil sample), with respect to the mass and proportions of the ejected material, taking into account its division into solid and liquid phases i.e. soil and water. The investigation was carried out using three types of soil with different textures, in moistened (pressure head corresponding to -1.0 kPa) and air-dry (-1500 kPa) conditions. The soil samples were on three angles of slope, being 5°, 15°, and 30°, respectively. After a single-drop impact with a diameter of 4.2 mm, the ejected material was collected using a splash cup. The following quantities of splashed material were measured: the total mass, the mass of the solid phase, and the mass of the liquid phase. Additionally, the distribution and proportions (soil/water) of the splashed material were analysed in both the upslope and downslope directions. It was found that: (i) the change of slope had a variable influence on the measured quantities for different soils; (ii) in the case of moistened samples, the measured values were mainly influenced by the texture, while in the dry samples, by the angle of the slope; (iii) with the increase of slope, the splashed material was mostly ejected in the downslope direction (irrespective of moisture conditions); (iv) in the moistened samples, the ejected material consisted mostly of water, while in the dry samples it was soil—this occurred for material ejected both upslope and downslope. The obtained results are important for improving the physical description of the process of splash erosion. A more thorough understanding and better recognition of the mechanisms governing this phenomenon at all stages could contribute to the development of more effective methods for protecting soil against erosion.

Splash detachment of sand particles under varying contact stress field of wind‐driven raindrop impact

AbstractThe effects of wind‐driven rain (WDR) on sand detachment were studied under various raindrop obliquities. Results suggested a significant reduction in compressive stress on sand surfaces for a two‐dimensional experimental set‐up in a wind tunnel. During experiments, sand particles in splash cups were exposed to both wind‐free rain (WFR) and WDR driven by horizontal winds of 6.4, 8.9 and 12.8 m s−1and rainfall intensities of 50, 60, 75 and 90‐mm h−1to assess the sand detachment rate (D, in g m−2 s−1). The effects of sand moisture state (dry and wet) on the detachment of different‐sized particles (0.20–0.50 and 0.50–2.00 mm, respectively) were also tested. Factorial analysis of variance showed that shear and compressive stress components evaluated by horizontal and vertical kinetic energy flux terms (KExand KEy, respectively, in J m−2 s−1) along with their vector sum (KEr, in J m−2 s−1) explained the variation inD. Neither sand size nor sand moisture was statistically significant alone although binary interactions of KEr, KExand KEywith the sand size and three‐way interaction of KEx, sand size and moisture were statistically significant. These results can be explained by size‐dependent variation in sand compressibility and surface friction related to the total stress field developed by a given partition of shear and compressive stresses of wind‐driven oblique raindrops (KEx/KEy). Further analysis of the variation of the unit sand detachment rate (Du = D/KEr = g J−1) with rain inclination (α, in degrees) better revealed the effect of WDR obliquity onDuthat further changed with sand size class and moisture state. Finally, the difference in the resulting stress field differentiable by the oblique raindrop trajectories of the experiment over sand surface significantly affected the non‐cohesive particle detachment rates, to some extent interacted with size‐dependent compressibility and interface shear strength of sand grains.

Comparative Assessment of Splash Erosion Measuring Devices in Auchi Polytechnic, Auchi, Edo State

Soil erosion is recognized as a global threat against the sustainability of the natural ecosystem and the environment because of its severe effects in agricultural productivity, damage to infrastructure and pollution of water bodies. This study aims to compare the splash erosion on open field using Morgan and splash board method. The fabrication of the splash cups and splash board was carried based on the principles of the already existing devices in literature and in the department of Agricultural and Bio-Environmental Engineering Technology Auchi Polytechnic Auchi. The rainstorm depth ranged from 7 to 81 mm and the computed KE obtained ranged from 294 to 3233.4 J/m 2 . A total of 15 rainfall events were used in the season. For site A the amount of soil splashed per rainfall event ranged from 0.02 g/m 2 to 4.77 g/m 2 and zero (no splash) to 2.39 g/m 2 for Morgan cup and Splash board device respectively. Studies of splash erosion on cultivated land should be made to determine the effect of cultivation on the soil in the area. Keywords: Morgan’s Splash cup, splash board, kinetic energy, soil erosion, splash erosion, rainstorm DOI: 10.7176/JEES/11-4-06 Publication date: April 30 th 2021

A new approach to kinetic energy calculation of two-phase soil splashed material

Splash is the first step of the water erosion process initiated when raindrops hit the earth’s surface and eject particles of soil. The ability to physically describe this phenomenon is important in order to counteract it. One of the research aspects of splash is energy dissipation of the impacting drop where the ejection of particles is significant part. Therefore, the main aim of this work is the presentation of an improved method for calculation of the kinetic energy of two-phase splashed material (water/solid soil particles). Measurements were carried out on Haplic Luvisol soil surface and single drop impact. Different methodologies allowed for obtaining key parameters that influence particles’ energy, including measuring the number of ejected particles, their mass, and their ejection velocity. High-speed cameras registered the phenomenon, identified particles, and reconstructed their flight trajectories. Based on the analysis of particle trajectories, it was possible to determine the distribution of particles’ ejection velocity with the average value equal 2.60 m·s−1. Splash cup measurements provided data on the total mass of splashed material, including its division into the solid phase (soil) and the liquid phase (water), and calculation of the average density of ejected particles (1.31 g·cm−3). An alternative method for particles’ mass calculation, the 2D image analysis method, was calibrated with the results obtained with the splash cup. Based on the input data, three variants of kinetic energy calculation of the splashed material were tested. The variants differed in assumptions and approximations, and it was difficult to determine which was preferable. Taking into account the results from all variants, however, it can be stated that the total kinetic energy of splashed particles in relation to the kinetic energy of a water drop hitting the soil surface ranged between 4.6% and 7.4%. To the best of the authors’ knowledge, this study is the first report of kinetic energy calculation which takes into account the two-phase nature of splashed material.

SfM‐MVS Photogrammetry for Splash Erosion Monitoring under Natural Rainfall

AbstractAn understanding of splash erosion is the basis to describe the impact of rain characteristics on soil disturbance. In typical splash cup experiments, splashed soil is collected, filtered, and weighed. As a way to collect additional data, our experiments have been supplemented by a photogrammetric approach. A total of three soils were tested across three sites, one in the Czech Republic and two in Austria, all equipped with rain gauges and disdrometers to measure rainfall parameters. The structure from motion multiview stereo (SfM‐MVS) photogrammetric method was used to measure the raindrops impact on the soil surface. The images were processed using Agisoft PhotoScan, resulting in orthophotos and digital elevation models (DEMs) with a resolution of 0.1 mm/pix. The surface statistics included the mean surface height (whose standard deviation was used as a measure of surface roughness), slope, and other parameters. These parameters were evaluated depending on soil texture and rainfall parameters. The results show a linear correlation between consolidation and splash erosion with a coefficient of determination (R2) of approximately 0.65 for all three soils. When comparing the change in soil volume with rainfall parameters, the best correlation was found with the maximum 30‐minintensity (I30), resulting in R2 values of 0.48 (soil A, silt loam, 26% clay), 0.59 (soil B, silt loam, 18% clay), and 0.68 (soil C, loamy sand, 12% clay). The initial increase in the sample volume for the lowest splashed mass corresponds with the increase in the clay content of each of the soils. Soil A swells the most. Soil B swells less. Soil C does not swell at all and consolidates the most. We derived the relationship between the photogrammetrically measured change in surface height and the splash erosion (measured by weight) by accounting for the effect of the clay content.

How Do Newly-Amended Biochar Particles Affect Erodibility and Soil Water Movement?—A Small-Scale Experimental Approach

Biochar amendment changes chemical and physical properties of soils and influences soil biota. It is, thus, assumed that it can also affect soil erosion and erosion-related processes. In this study, we investigated how biochar particles instantly change erodibility by rain splash and the initial movement of soil water in a small-scale experiment. Hydrothermal carbonization (HTC)-char and Pyrochar were admixed to two soil substrates. Soil erodibility was determined with Tübingen splash cups under simulated rainfall, soil hydraulic conductivity was calculated from texture and bulk soil density, and soil water retention was measured using the negative and the excess pressure methods. Results showed that the addition of biochar significantly reduced initial soil erosion in coarse sand and silt loam immediately after biochar application. Furthermore, biochar particles were not preferentially removed from the substrate surface, but increasing biochar particle sizes partly showed decreasing erodibility of substrates. Moreover, biochar amendment led to improved hydraulic conductivity and soil water retention, regarding soil erosion control. In conclusion, this study provided evidence that biochar amendments reduce soil degradation by water erosion. Furthermore, this effect is detectable in a very early stage, and without long-term incorporation of biochar into soils.

Splash erosion affected by initial soil moisture and surface conditions under simulated rainfall

Soil erosion by water is one of the most severe soil degradation processes. Splash erosion is the initial stage of soil erosion by water, resulting from the destructive force of rain drops acting on soil surface aggregates. Apart from rainfall properties, constant soil physical properties (texture and soil organic matter) are crucial in understanding the splash erosion. However, there is lack of information about the effect of variable soil properties such as soil initial water content and surface condition (seal formation) on splash erosion. The objective of the present study was to determine how initial water content and surface condition affected soil splash erosion under simulated rainfall. The changes in soil surface condition were characterized by hydraulic variability (saturated hydraulic conductivity) due to surface seal formation. Slit loam and loamy sand soil textures were used in the experiment. The soil samples were collected from the top layer; air dried, and filled into modified Morgan splash cups for splash erosion measurements. Rainfall was created in the laboratory using two types of rainfall simulators covering intensity range from 28 to 54 mm h−1 and from 35 to 81 mm h−1. The soil samples were exposed to three consecutive rainfall simulations with different time intervals between simulations and different initial water content and surface conditions (air-dried, wet-sealed, and dry-crusted). Wet-sealed soil samples had up to 70% lower splash erosion rate compared to air-dried samples, due to surface ponding followed by seal formation. A significant decrease in soil saturated hydraulic conductivity indicated the formation of surface seal for silt loam soils. A non-significant decrease in saturated hydraulic conductivity for loamy sand soil was attributed to earlier formation of stable seals. Two different rainfall simulators produced different amount of splash erosion rates; however, the splash erosion development for increasing rainfall intensity was almost equal considering same initial surface condition. These results provide insight into dynamic changes of individual soil parameters affected by rainfall, and could find wider application for more complex soil erosion prediction models.

Rainfall Parameters Affecting Splash Erosion under Natural Conditions

The interaction between rainfall erosivity parameters and splash erosion is crucial for describing the soil erosion process; however, it is rarely investigated under natural rainfall conditions. In this study, we conducted splash erosion experiments under natural rainfall on three sites in Central Europe. The main goal was to obtain the relationship between splash erosion of the bare soil in seedbed condition and commonly used rainfall erosivity parameters (kinetic energy, intensity, and rainfall erosivity (EI30)). All sites were equipped with a rain gauge and an optical laser disdrometer where the splash erosion was measured, with modified Morgan splash cups. In order to investigate which parameter best describes the splash erosion process for all sites, a regression analysis was performed. In total, 80 splash erosion events were evaluated. Splash erosion can be described as a linear function of total kinetic energy and a non-linear function of EI30. However, the use of the total kinetic energy led to underestimation of the splash erosion rates for highly intensive rainfalls. Therefore, better results were obtained when using average rainfall intensity as the splash erosion predictor or the kinetic energy divided by the rainfall duration. Minor differences between the replicates during splash erosion measurements indicate that the modified Morgan splash cup provides a good tool for soil erosion assessment.

Soil Detachment with Organic Mulching Using Rainfall Simulator in Comparison with a Short Duration Natural Rainfall for Effective Soil Conservation

The impact of raindrop on sandy soil was studied using rainfall simulator and natural rainfall to determine average soil detached. Erosion by rainfall is one of the major hazards threatening the productivity of farmlands. This study determined the rate of soil detachment in comparison between natural rainfall and simulated rainfall for effective soil conservation measure. The height of the simulator was varied considering the diameter of the nozzles which were considered during the design stage of the rain simulator. Two plots of dimensions 2 m × 2 m each were cleared with one considered for bare and treated soils for both the natural rainfall and simulated rainfall. Splash cups were installed on each of the plots at half depth of the cup after the clearing of the area at 0.4 m × 0.4 m apart. Two sets of rain gauge were placed at the experimental site to note the volume of natural rainfall on the farm. The average soil detached was analysed using statistical analysis where t-test was also carried out to know the difference in mean. There was a significant difference in the degree of soil detachment between bare and treated soil under natural rainfall experiment; t(18) = 8.917, p η2) = 0.8154) reveals that the nature of soil accounted for 81.5% variance in the average detachment rate. For simulated experiments with a mean value of 7.3360 have higher tendency of detachment than treated soil with a mean value of 4.2240. Size of effect (Eta-square (η2) = 0.630) reveals that the soil types accounted for 63% variance in the average detachment. It was concluded that 40.33% soil was found to be conserved using the cow dungs mixed with bare soil to compact the soil. The nozzle size, simulator height, rainfall intensity and other rainfall parameters all contributed to the amount of average soil detached.

Experimental Setup for Splash Erosion Monitoring—Study of Silty Loam Splash Characteristics

An experimental laboratory setup was developed and evaluated in order to investigate detachment of soil particles by raindrop splash impact. The soil under investigation was a silty loam Cambisol, which is typical for agricultural fields in Central Europe. The setup consisted of a rainfall simulator and soil samples packed into splash cups (a plastic cylinder with a surface area of 78.5 cm2) positioned in the center of sediment collectors with an outer diameter of 45 cm. A laboratory rainfall simulator was used to simulate rainfall with a prescribed intensity and kinetic energy. Photographs of the soil’s surface before and after the experiments were taken to create digital models of relief and to calculate changes in surface roughness and the rate of soil compaction. The corresponding amount of splashed soil ranged between 10 and 1500 g m−2 h−1. We observed a linear relationship between the rainfall kinetic energy and the amount of the detached soil particles. The threshold kinetic energy necessary to initiate the detachment process was 354 J m−2 h−1. No significant relationship between rainfall kinetic energy and splashed sediment particle-size distribution was observed. The splash erosion process exhibited high variability within each repetition, suggesting a sensitivity of the process to the actual soil surface microtopography.

Comparative Analysis of Splash Erosion Devices for Rainfall Simulation Experiments: A Laboratory Study

For the study of soil erosion it is important to set up the experiments well. In the experimental design one of the key factors is the choice of the measurement device. This is especially important when one part of the erosion process needs to be isolated, such as for splash erosion. Therefore, the main aim of this research is to list the general characteristics of the commonly used splash erosion devices and to discuss the performance, to be able to relate them, and make suggestions regarding their use. The devices we selected for this comparative comparison were: the splash cup, funnel, Morgan tray, Tübingen cup, tower, and the gutter. The devices were tested under the same conditions (rainfall characteristics, slope, and soil type) to assess their hydrological response under different intensities of simulated rainfall. All devices were installed on a sloping plot (10°) with sandy soil, and were exposed to 10 min. of simulated rain with intensities ranging from 60 to 172 mm/h to measure the splashed sediment, and to describe problems and differences among them. The results showed that the Tübingen cup was the best performing device to measure kinetic energy of the rain, but, because of its design, it is not possible to measure the detached splashed sediment under natural (field) conditions. On the other hand, the funnel device showed a significant relation with rain intensity because it loses little sediment to washing. In addition, the device is easy to use and cheap. Therefore, this device is highly recommended to estimated splash erosion. to the good performance measuring the actual splash erosion, because it loses little sediment by washing. The device is also cheap and easy to install and manage.

The Role of Water in Fast Plant Movements.

Plants moved onto land ∼450 million years ago and faced their biggest challenge: living in a dry environment. Over the millennia plants have become masters of regulating water flow and the toolkit they have developed has been co-opted to effect rapid movements. Since plants are rooted, these fast movements are used to disperse reproductive propagules including spores, pollen, and seeds. We compare five plants to demonstrate three ways, used alone or in combination, that water powers rapid movements: the direct capture of the kinetic energy of a falling raindrop propels gemmae from the splash cups of the liverwort, Marchantia; the loss of water powers the explosive dispersal of the spores of Sphagnum moss; the alternate loss and gain of water in the bilayer of the elaters of Equisetum drive the walk, jump, and glide of spores; the gain of water in the inner layer of the arils of Oxalis drive the eversion of the aril that jettisons seeds from the capsule; and the buildup of turgor pressure in the petals and stamens of bunchberry dogwood (Cornus canadensis) explosively propels pollen. Each method is accompanied by morphological features, which facilitate water movement as a power source. The urn shaped splash cups of Marchantia allow dispersal of gemmae by multiple splashes. The air gun design of Sphagnum capsules results in a symmetrical impulse creating a vortex ring of spores. The elaters of Equisetum can unfurl while they are dropping from the plant, so that they capture updrafts and glide to new sites. The arils of Oxalis are designed like miniature toy "poppers." Finally, in bunchberry, the softening of stamen filament tissue where it attaches to the anther allows them to function as miniature hinged catapults or trebuchets.

The mass ratio of splashed particles during raindrop splash phenomenon on soil surface

Soil splash is the first step of water erosion, therefore a good understanding of it is important in order to describe this multifaceted phenomenon. One of the methods describing this process is the use of splash cups, which allow us to determine the mass of soil material ejected by the impact of water drops. The aim of this study was to determine the mass ratio of splashed particles during raindrop splash on the soil surface, including their division into a splashed liquid phase and a solid phase. The investigation was carried out using three types of soil with different particle size distributions. In the first stage of the experiment, three initial moisture contents were applied (pressure head equal to 0.1 kPa, 1.0 kPa and 3.16 kPa) and the second stage measurements were based on the soil with the highest splash efficiency in the case of the different kinetic energy of the impacting drop. It was found that: (i) the texture of soil sample had a significant effect on the mass of splashed material; (ii) the lower the initial moisture content of the sample, the lower the mass of splashed material; (iii) the highest values of the mass of splashed material (both the liquid phase and solid phase) were obtained for soil with the highest sand fraction; (iv) for all types of soils, the mass of the splashed water was significantly larger than the collected mass of splashed soil; (v) the ratio of splashed masses (the proportions) was dependent on initial moisture content for soil with a higher amount of finer fractions. The obtained results are important for the physical description of the splash erosion. For instance, the knowledge of the masses of solid (soil) and water parts of the ejected material and their ejection kinetic energy will allow the estimation of the total energy needed for the displacement of splashed material. Moreover, the investigation of the water phase in the splash phenomenon is significant because of the transport processes of pollutants and pathogenic microorganisms.

Influence of freeze-only and freezing-thawing cycles on splash erosion

Soil erosion is recognized as one of the most important types of land degradation in the world particularly in many developing countries like Iran. Water erosion is initiated by splash erosion triggered by raindrop impact. Understanding the process of splash erosion under freezing and thawing conditions is essential to unravel soil erosion mechanisms under temperate conditions leading to appropriate planning of soil and water conservation projects. The present study aimed to study the individual effects of freeze-only as well as freezing-thawing cycle on splash erosion in a loess soil from an erosion prone area in mountainous northern regions of Iran. The study was conducted under laboratory conditions using erosion plots. The erosion plots were subjected to freeze only and freeze-thawing treatments by simulating cold conditions using a large cooling compartment system specifically manufactured for this purpose. The splash erosion under a designed simulated rainfall (1.2 mm min−1 for 30 min) was then measured as upward, downward and net splash erosion in splash cups. The results showed that freeze only decreased the upward, downward and net splash erosion by 0.81 ± 0.43, 0.82 ± 0.29 and 0.85 ± 0.23% while freezing-thawing cycle decreased splash erosion to 0.93 ± 0.83, 0.61 ± 0.43 and 0.57 ± 0.36%. This may be attributed to temporary increase in soil strength and stability or surface sealing during freezing process leading to reduced splash erosion.

Factors influencing the erosivity indices of raindrops in Japanese cypress plantations

Severe soil erosion has been reported in monoculture forests without understory vegetation and litter on the floor. To clarify the factors that affect raindrop erosivity in monoculture forests, we measured raindrop erosivity in Japanese cypress (Chamaecyparis obtuse Endl.) plantations using splash cups. Splash cups can possibly measure raindrop erosivity from loss of sand (LoS). We first examined the relation between raindrop erosivity and LoS under natural rainfall in an open space. LoS was strongly correlated with raindrop erosivity factors, such as the kinetic energy and momentum multiplied by the drop diameter. We then observed LoS using 12–16 splash cups in seven plots with different canopy structures in Japanese cypress plantations. LoS at each point was compared with the amount of throughfall (Tf) and canopy structure factors, such as canopy openness (Co) and distance to the closest branch (Br). The relation between Tf and LoS was similar among the plots, except in one plot where the canopy and crown-base heights were the smallest. Our results imply that Tf is a dominant factor determining raindrop erosivity when canopy and crown-base heights are more than the threshold values (15.2 m for the canopy height and 10.5 m for the crown-base height in our forests). In plot 1, LoS was correlated with not only Tf but also Br. When analyzing all the data together using generalized linear models, LoS was affected by Br besides Tf. In monoculture forests with a high canopy, Tf measurements enable us to estimate raindrop erosivity. In monoculture forests with a low canopy, Br (or the canopy height) should also be considered when estimating the erosivity of raindrops.

Spatial distribution of LAI and its relationship with throughfall kinetic energy of common tree species in a Chinese subtropical forest plantation

The hilly red soil region in southern China is still facing serious soil erosion, even after long-term afforestation projects. This might result from structural shortcomings of the tree species chosen for afforestation. Within the Biodiversity and Ecosystem Functioning China project (BEF China), we used point cloud data from terrestrial laser scanners (TLS) and splash cups to analyze spatial leaf area index (LAI) and to predict the potential of splash erosion in subtropical forests. High LAI of Lithocarpus glaber and Schima superba was measured mainly at the middle and lower parts of the trees while for Sapindus saponaria it was found at the upper parts. LAI was decreasing from the tree stems to the edges of the canopy. Lognormal and exponential linear models were suitable to describe the vertical and horizontal LAI distribution of selected tree species, respectively. Sapindus saponaria generally had the highest values of throughfall kinetic energy (TKE) among the analyzed tree species and measured rainfall events. In the radial direction, higher LAI tended to produce lower TKE, whereas in the vertical direction, higher skewness of LAI distribution had higher TKE. LAI and its spatial distribution both were important for TKE. These findings can help to understand mechanisms of splash erosion in forest plantations related to unsuitable spatial LAI of tree species planted. It might further improve our knowledge how tree diversity may influence splash erosion by enriching the canopy layers in an early successional stage of subtropical forest plantations.