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1.
Simulated raindrops falling in still air have a shape that is mainly determined by surface tension and hydrostatic pressure. Drops released from capillary tips show an initial shape variation ranging from prolate to oblate but eventually this oscillation is damped. At terminal velocity drops have attained equilibrium and have an oblate shape. Measurements of the shape of simulated rain drops produced by capillary tubes were made using a simple, newly-developed photographic set up. The measurements showed that models describing the oscillation frequency and amplitude of drops falling at terminal velocity can also be applied to the simulated drops. A comparison is made between the shape of raindrops in natural storms and simulated drops. Recommendations are given regarding fall heights in simulation in relation to the drop shape in nature.  相似文献   

2.
Three high erosivity conditions (50 mm hr?1, 100 mm hr?1, and 200 mm hr?1) were generated in a laboratory using a rainfall simulator and coherent soil block samples from fourteen different soil erodibility conditions. The data acquired supports the theoretical contention that soil loss should not increase as a simple linear function of storm intensity. Rather, a variable relationship is caused by the rupturing of surface seals and the changing relative significance of splash, wash and rainwash processes. Slope angle appears to influence soil loss at the higher erosivity conditions of 100 mm hr?1 and 200 mm hr?1 on slopes that were either very steep (> 20°) or very shallow (< 3°), but on moderate slopes the relationship is unclear. Examination of the variation of soil loss with erosivity when soil loss for a specific high erosivity condition is known revealed that conversion and power factors are of doubtful value and little generality. A satisfactory predictive equation, a power curve, is seen to be of value only when comparing rainwash soil loss between the higher erosivity conditions. The relationship is most safely considered as soil and site specific. Where the influence of slope and soil erodibility are disregarded, a strong association between soil loss and rainfall intensity is found. That soil loss, and hence, soil erodibility varies non-uniformly with erosivity is clear. The findings indicate caution is required when comparing conclusions drawn from studies based upon different erosivity conditions.  相似文献   

3.
This paper presents a model that simulates the size distribution and erosivity of raindrops and throughfall drops. It utilizes existing models of rainfall drop size distribution and fall velocity and combines them with newly collated evidence of throughfall drop size distributions. A sensitivity analysis reveals that the model is sensitive to parameters that are easily measured or estimated: rainfall intensity, the mean volume drop diameter of the intercepted throughfall, canopy cover, and canopy height. The results of the model may be used at two levels. Firstly, to calculate specifically the size and fall velocity of individual drops, parameters that are needed in studies examining the response of soil surfaces to forces applied by rainfall. Secondly, to produce erosivity indices, based on rainfall intensity but which take account of the effects of a vegetation canopy. The paper shows that while the kinetic energy of rainfall (E(0), J mm?1 m?2) may be calculated from an equation of the familiar form: the kinetic energy of throughfall under any canopy may be calculated by combining this equation with another that relates the energy of drops under a 100 per cent canopy cover (E(100)) and the canopy height: .  相似文献   

4.
Rates of splash detachment from a sandy soil of the Cottenham Series, subjected to a five-minute design storm of 50 mm/h with a kinetic energy of 127 J/m2 and a median volume drop size of 3·2 mm supplied from a rotating-disc rainfall simulator, are determined without a plant cover and with the cover of a single Brussels sprouts plant. Measurements are made at regular intervals throughout the growing season. Plant canopies of 10 to 25 per cent result in reductions of 10 to 25 per cent in rainfall volume and 10 to 81 per cent in rainfall energy. The volume and energy of the rain beneath the plant are significantly (P < 0·05) correlated with its number of leaves (r = ?0·84 and ?0·92 respectively for n = 49). No reduction was observed in the rate of splash detachment which averages 1·2 kg/m2 for the design storm with and without the plant. The detachment rate was found rather surprisingly to be inversely related to the energy of the rainfall under the plant and positively related to the number of leaves. As the number of leaves increases, so does the detachment rate per unit of rainfall energy.  相似文献   

5.
The influence of organic matter on splash detachment was investigated using soils with grass and peat treatments. The relationship between organic matter and aggregate stability to water disruption was positive for soils with grass treatment while it was negative for those with peat treatment. Organic matter from both treatments, however, reduced splash detachment by rainfall. Soils with grass did this by increasing aggregate stability while peat acted as a mulch on the soil surface. This implies that though organic matter always reduces splash detachment, different processes may be involved, depending on the form of the organic material. Because of the different processes involved, both negative and positive relationships between splash detachment and aggregate stability as reported in literature were obtained for the soils with grass and peat respectively.  相似文献   

6.
Interception losses, rain and throughfall drop size spectra and kinetic energy were studied in four relatively undisturbed tropical forest ecosystems along a transect across the Central Andean Cordillera of Colombia at altitudes between 3000 and 1000 m above sea level. Interception amounts ranged from 11 to more than 20 per cent of the total rainfall and fell within the normal range of interception figures observed in natural tropical forests. Drop size spectra were established using the filter paper method; the drop size distributions of the open field rainfall were unimodal while the throughfall had bimodal distributions, with a higher percentage of the volume of rain falling as large drops. Disturbance of the natural forests, for example by logging activities or cattle grazing, will further increase the throughfall kinetic energy and may lead to higher splash erosion rates inside the forests than in the open field. The kinetic energy of the throughfall was higher than that of the open field rainfall (20-70 per cent), even after correcting for interception losses (4-30 per cent). Splash-cup experiments, conducted both in the field and in the laboratory, indicated that the kinetic energy is a good index of rainfall erosivity. Inside the forests the amounts of sand splashed from the splash-cups was, after correction for interception losses, 2-16 per cent higher than outside the forests.  相似文献   

7.
To elucidate splash erosion processes under natural rainfall conditions, temporal variations in splash detachment were observed using a piezoelectric saltation sensor (H11B; Sensit Co., Portland, ND, USA). Preliminary laboratory tests of Sensit suggested that they were suitable for field observations. Field observations were conducted between July and September 2006 in 21‐ and 36‐year‐old Japanese cypress (Chamaecyparis obtusa) plantations with mean stand heights of 9·2 m and 17·4 m, respectively. Splash detachment (in g m?2) was measured seven times using splash cups, and raindrop kinetic energy (in J m?2 mm?1) in both stands was measured using laser drop‐sizing (LD) gauges. Sensit was installed to record saltation counts, which were converted to temporal data of splash detachment (splash rate; in g m?2 10 min?1) using the relationship between splash detachment and saltation counts. Surface runoff was monitored using runoff plots of 0·5 m width and 2·0 m length to obtain temporal data of flow depth (in millimeters). Both total splash detachment and raindrop kinetic energy were larger in the older stand. Increased splash rates per unit throughfall were found in both stands after rainless durations longer than approximately one day in both stands. However, a lower splash rate was found in the 21‐year stand after rainfall events. During extreme rainstorms, the 21‐year stand showed a low runoff rate and a decline in the splash rate, while the 36‐year stand showed a higher splash rate and increased flow depth. The piezoelectric sensor proved to be a useful means to elucidate splash erosion processes in field conditions. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

8.
Splash detachment by raindrops was measured using a factorial experiment involving three soils (sandy loam, clay loam and clay) and two organic materials (cow dung and groundnut haulms) applied at five levels (0, 2, 4, 6 and 8 per cent by mass). A simulated rainfall intensity of 145 mm h?1 for 10 min duration was used for the tests. Detachment was described in terms of the direct effects and the first and second order interactions of the variables mentioned above. Results indicated that mean splash detachment was reduced significantly (P < 0·01) from 1·97 kg m?2 in the sandy loam to 0·67 kgm?2 in the clay soil. For each soil type, detachment was reduced significantly (P < 0.01) with increasing levels of added organic matter. Groundnut haulms produced lower values of detachment than cow dung. There were also significant interaction effects between the study variables. These interactions were used to describe the effect of added organic materials on splash detachment. For each soil type and organic material, negative exponential relationships were established to relate splash detachment to levels of added organic matter.  相似文献   

9.
P. I. A. Kinnell 《水文研究》2005,19(14):2815-2844
Raindrop‐impact‐induced erosion is initiated when detachment of soil particles from the surface of the soil results from an expenditure of raindrop energy. Once detachment by raindrop impact has taken place, particles are transported away from the site of the impact by one or more of the following transport processes: drop splash, raindrop‐induced flow transport, or transport by flow without stimulation by drop impact. These transport processes exhibit varying efficiencies. Particles that fall back to the surface as a result of gravity produce a layer of pre‐detached particles that provides a degree of protection against the detachment of particles from the underlying soil. This, in turn, influences the erodibility of the eroding surface. Good understanding of rainfall erosion processes is necessary if the results of erosion experiments are to be properly interpreted. Current process‐based erosion prediction models do not deal with the issue of temporal variations in erodibility during a rainfall event or variabilities in erodibility associated with spatial changes in dominance of the transport processes that follow detachment by drop impact. Although more complex erosion models may deal with issues like this, their complexity and high data requirement may make them unsuitable for use as general prediction tools. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

10.
This paper describes laboratory testing of 148 samples collected from Southern Alberta for erosion by wash and splash. Rainfall intensity was held constant during these tests. Soil aggregation was the most significant variable explaining soil loss. The significance of other soil properties, such as organic carbon and clay content is variable, depending on the interrelationships among aggregate stability, organic content, and clay content of particular soils. Variations in erodibility of the major soils examined are explained by the resistance of aggregates to compaction and dispersion. Splash detachment and wash transport are the dominant erosion mechanisms in inter-rill areas.  相似文献   

11.
A model has been developed which predicts the dispersion of splash droplets produced by the impact of a water drop on a sloping soil surface. Experimental results of the ejection velocities and ejection angles of the splash droplets are generalized to a planar slope and the resulting splash distances are calculated taking into account the effects of air resistance. The predictions are presented in terms of the numbers of splash droplets from the impact point to surrounding squares arranged in a grid on the slope. The model explains many experimentally observed features of raindrop splash in terms of the mechanics of the processes involved and can make predictions of the effects of slope, wind, raindrop size, and soil properties on droplet dispersion. The component of the raindrop velocity parallel to the surface of the slope is identified as the main factor determining the degree and the direction of the asymmetry in the splash droplet dispersion. By combining the model with a theory of the entrapment of soil in the splash droplets it is possible to extend it to predict the dispersion of soil particles by raindrop impact, which is the basis of a model of soil erosion by rainsplash.  相似文献   

12.
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13.
Abstract

Rainfall simulators have often been used to mimic natural rainfall for studies of various land-surface and water interaction processes. The characteristics of the simulated rainfall are the main indicators used to judge the performance of the rainfall simulators. The aim of this study is to investigate the potential of piezoelectric transducers for measuring and evaluating a dripper-type simulated rainfall drop-size distribution (DSD) and kinetic energy (KE). The directly measured KE was significantly correlated with the estimated KE using the drop-size distribution and empirical rain drop fall velocity relationships. This result emphasizes the potential use of the piezoelectric sensor to directly measure and evaluate rainfall kinetic energy. Also, the relationship between rainfall intensity and KE showed good patterns of agreement between simulated rainfall and natural rainfall.

Citation Abd Elbasit, M. A. M., Yasuda, H. & Salmi, A. (2011) Application of piezoelectric transducers in simulated rainfall erosivity assessment. Hydrol. Sci. J. 56(1), 187–194.  相似文献   

14.
Laboratory experiments have been conducted to study the effects of various rain properties on sand detachment resulting from raindrop impact. Splash cups were exposed to simulated rainfall intensities ranging between 10 and 140 mm h−1. The detached sand was collected and weighed whereas rain intensity, equivalent drop diameter and fall velocity of raindrops were measured with an optical spectro‐pluviometer (OSP). The properties of the simulated rain (i.e. median volume diameter and kinetic energy) were compared with those observed in natural conditions. Statistical analyses have been undertaken in order to evaluate which rain variable best predicts the mass of sand detached. Linear and non‐linear correlations between the mass of detached sediment and the product of drop size (d) by drop velocity (v), i.e. DαVβ, with values of α varying between 1 and 6 and β between 0 and 3, have been computed. The results indicate that the coefficient of determination (R2) for α ranging between 3 and 5 and β lower or equal to 2 are satisfying. Although kinetic energy (D3V2) described splash detachment relatively well, the product of momentum by drop diameter (D4V) was slightly superior in describing splash detachment. Therefore, the momentum multiplied by the drop diameter is recommended as the best rain variable to describe splash detachment. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

15.
Raindrop impact can be a major contributor to particle mobilization for soils and other granular materials. In previous work, water repellent soils, comprised of hydrophobic particles, have been shown to exhibit greater splash erosion losses under multiple drop impact. However, the underlying principle differences in splash behavior between hydrophobic and hydrophilic granular surfaces have not been studied to date. In this study the effects of particle hydrophobicity on splash behaviour by a single water drop impact were examined using high‐speed videography. Water drops (4 mm in diameter) were dropped on beds of hydrophilic and hydrophobic glass beads (sieved range: 350–400 µm), serving as model soil particles. The drop velocity on impact was 2.67 m s‐1, which corresponds to ~30% of the terminal velocity of a raindrop of similar size. The resulting impact behaviour was measured in terms of the trajectories of particles ejected from the beds and their final resting positions. The response to the impacting water drop was significantly different between hydrophilic and hydrophobic particles in terms of the distance distribution, the median distance travelled by the particles and number of ejected particles. The greater ejection distances of hydrophobic particles were mainly the result of the higher initial velocities rather than differences in ejecting angles. The higher and longer ejection trajectories for hydrophobic particles, compared with hydrophilic particles, indicate that particle hydrophobicity affects splash erosion from the initial stage of rainfall erosion before a water layer may be formed by accumulating drops. The ~10% increase in average splash distance for hydrophobic particles compared with hydrophilic particles suggests that particle hydrophobicity can result in greater net erosion rate, which would be amplified on sloping surfaces, for example, by ridges in ploughed agricultural soils or hillslopes following vegetation loss by clearing or wildfire. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

16.
Crop residues in conservation tillage systems are known to cause both a reduction in the erosive runoff power and an increase in the topsoil erosion resistance. In this study, the relative importance of both mechanisms in reducing soil loss by concentrated flow erosion is examined. Therefore, a method to calculate the effective flow shear stress responsible for soil detachment in the presence of a residue cover is applied. The determination of effective flow shear stress is based on the recalculation of the hydraulic radius for residue treatments. The method was tested in a laboratory flume by comparing soil detachment rates of identical pairs of soil samples that only differ in the presence or absence of crop residues. This shear stress partitioning approach and a soil detachment correction were then applied to a dataset of soil detachment measurements on undisturbed topsoil samples from a no‐till field plot on a loess‐derived soil, sampled during one growing season. Results indicate that only a small fraction (10% on average) of the difference in soil detachment rate between conventional and conservation tillage can be attributed to the dissipation of shear forces on the residues. The remaining decrease in soil detachment during concentrated runoff after a two‐year application of conservation tillage can be explained by the increased dry bulk density and root and crop residue content in the topsoil that reduces soil erodibility. After correcting for the presence of residues, the temporal variability in soil detachment rates (Dr) during concentrated flow for a given flow shear stress (τ) for both treatments can be predicted fairly well (R2 = 0·87) from dry soil bulk density (DBD, representing consolidation effects), soil moisture content (SMC, representing antecedent rainfall conditions), the dry mass of organic material (OM, representing root growth and residue decomposition) and saturated soil shear strength σs, sat using an equation of the form: This study is the first to show that the effect of conservation tillage on soil detachment rates is a result of soil property modifications affecting soil erodibility, rather than a result of the surface residue decreasing flow erosivity. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

17.
Changes in rainfall erosivity are an expected consequence of climate change. Long‐term series of the single storm erosion index, EI, may be analysed to detect trends in rainfall erosivity. An indirect approach has to be applied for estimating EI, given that long series of rainfall intensities are seldom available. In this paper, a method for estimating EI from the corresponding rainfall amount, he, was developed for Sicily. This method was then applied at 17 Sicilian locations, representative of different climatic zones of the region, to generate a long series (i.e. from 1916 to 1999 in most cases) of EI values. Linear and step (step located at 1970) trends in annual and seasonal erosivity were detected by both classical approaches (Mann–Kendall test, Wilcoxon‐Mann‐Whitney rank‐sum test) and a new empirical approach (quantile approach, QA), based on the determination of the erosivity values corresponding to selected probability levels. A power relationship between EI and he with a space‐ and time‐variable scale factor and a time‐variable process parameter yielded the most accurate predictions of EI. However, a simpler model, using a time‐variable scale factor and a constant process parameter, yielded reasonably accurate EI estimates. Annual erosivity did not increase in Sicily during the twentieth century. At the most, it decreased at a few locations (three of the 17 considered locations). Significant trends were observed more frequently for winter erosivity (six locations) than for summer erosivity (two locations), suggesting that the erosive storms of winter determined the occasional occurrence of a negative trend in annual erosivity. In general, the QA compared reasonably well with more classical approaches. The QA appears promising since step trends for different return periods may be detected but efforts are needed to statistically formalize the proposed approach. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

18.
During 1974 and 1975, measurements of splash and wash were carried out in the cultivated area, in a station installed on 6·5 per cent slope covered with a loess in which a grey-brown podzolic soil has developed. Splash has been measured using an apparatus prepared for this purpose and wash has been measured on plots of standard length (22·13m). The splash is some tens t/ha.year but the splash loss calculated using the results of the measurements of splash is only a few tens kg/ha.year. Splash is positively correlated with the erosion index of the rains and with the structural stability but negatively with the crop cover. As splash, wash is positively correlated with the erosion index of the rains and negatively with the crop cover, but unlike splash it is negatively correlated with structural stability. The mean value of the wash loss is a few t/ha.year but very important differences are observed from one plot to another with regard to the structural stability. However, on a given plot wash loss and splash are positively correlated because the particles of soil detached by splash are easily carried off by runoff, but the relation between wash loss and splash is very different from one plot to another because splash is positively and wash loss negatively correlated with the structural stability of soils.  相似文献   

19.
Rain and throughfall drops were sampled during rain events in a New Zealand beech forest and the frequency distributions of drop mass and kinetic energy calculated. The kinetic energy of throughfall under the canopy was always greater than that of rainfall in the open, notwithstanding interception losses. During a typical rain event in which 51 mm fell in 36 h, the total kinetic energy of throughfail was 1.5 times greater than that of rainfall, and the mean amount of sand splashed from sample cups was 3.1 times greater under the canopy than in the open. It appears that where mineral soil is exposed at the surface, by animal trampling or burrowing for example, rates of soil detachment by splash under a forest canopy will probably exceed those in the open.  相似文献   

20.
Rainfall erosivity represents the primary driver for particle detachment in splash soil erosion. Several raindrop erosivity indices have been developed in order to quantify the potential of rainfall to cause soil erosion. Different types of rainfall simulators have been used to relate rainfall characteristics to soil detachment. However, rainfall produced by different rainfall simulators has different characteristics, specifically different relationships between rainfall intensity and rainfall erosivity. For this reason, the effect of rainfall characteristics produced by a dripper‐type rainfall simulator on splash soil erosion (Ds) has been investigated. The simulated rainfall kinetic energy (KE) and drop size distribution (DSD) were measured using piezoelectric transducers, modified from the Vaisala RAINCAP® rain sensor. The soil splash was evaluated under various simulated rainfall intensities ranging from 10 to 100 mm h?1 using the splash‐cup method. The simulated rainfall intensity (I) and kinetic energy relationship (I–KE) was found to be different from natural rainfall. The simulated rainfall intensity and splash soil erosion relationship (I–Ds) also followed this same trend. The I–KE relationship was found to follow the natural rainfall trend until the rainfall intensity reached 30 mm h?1 and above this limit the KE started to decrease. This emphasizes the importance of the I–KE relationship in determining the I‐Ds relationship, which can differ from one rainfall simulator to another. Ds was found to be highly correlated with KE (r = 0·85, P < 0·001), when data produced by the rainfall intensity ranged from 10 to 100 mm h?1. However, when the threshold rainfall intensity (30 mm h?1) was considered, the correlation coefficient further improved (r = 0·89, P = 0·001). Accordingly, to improve the soil splash estimation of simulated rainfall under various rainfall intensities the I–KE characterization relationship for rainfall simulators has to be taken into account. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

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