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A new description of splash erosion in relation to raindrop sizes and vegetation
... The third parameter, the momentum of the rain (M eq ), which is also suggested as a good rain erosivity index to describe splash erosion (e.g. Rose, 1960; Park et al., 1983; Styczen and Hùgh-Schmidt, 1988) is determined from the sum of the momentum (m) of each individual drop. ...
Knowledge of drop size distributions is important for deriving various rain erosivity parameters. This study investigates the potential of an optical spectro pluviometer (OSP) to measure drop size distributions. Particular attention is paid to the impact of drop sample size and derived erosivity parameters. An experimental setup using a rainfall simulator and an OSP is described. The OSP allows a continuous real-time sampling of the drops. Results on drop size distributions and sampling effects are discussed. A simulation aimed at reproducing the sampling made with the widely used flour-pellet or filter-paper method is described. From this simulation, recommendations on the sample size of the collected drops needed for an accurate determination of median drop size and kinetic energy are given. Past studies reporting drop size characteristics have often used too small a sample for an adequate description of rain erosivity.
... 6 to 2 . 1. Styczen and Hùgh-Schmidt (1988) use the square of the momentum as the rainfall parameter to describe splash erosion. The eect of a rainfall erosivity factor, EI (E is the total kinetic energy of the rain) was evaluated by Free (1960). ...
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.
... The third parameter, the momentum of the rain (M eq ), which is also suggested as a good rain erosivity index to describe splash erosion (e.g. Rose, 1960; Park et al., 1983; Styczen and Hùgh-Schmidt, 1988) is determined from the sum of the momentum (m) of each individual drop. ...
Knowledge of drop size distributions is important for deriving various rain erosivity parameters. This study investigates the potential of an optical spectro pluviometer (OSP) to measure drop size distributions. Particular attention is paid to the impact of drop sample size and derived erosivity parameters. An experimental set-up using a rainfall simulator and an OSP is described. The OSP allows a continuous real-time sampling of the drops. Results on drop size distributions and sampling effects are discussed. A simulation aimed at reproducing the sampling made with the widely used flour-pellet or filter-paper method is described. From this simulation, recommendations on the sample size of the collected drops needed for an accurate determination of median drop size and kinetic energy are given. Past studies reporting drop size characteristics have often used too small a sample for an adequate description of rain erosivity. Copyright © 1999 John Wiley & Sons, Ltd.
... The plant volume explained to a much lesser extent the variability of runoff and soil loss under the three species. The influence of this variable, which also implies the plant height, on erosion processes was opposite to that of the vegetation cover, confirming previous studies that reported an increase in erosion with increasing vegetation height (Styczen and Hφgh-Schmidt, 1988). The results show that the influence of plant morphology on runoff and soil loss determined the higher efficiency of Rosmarinus officinalis shrubs in reducing erosion, due to their relatively dense canopy cover, giving rise to a thick permanent litter cover at the soil surface. ...
The influence of plant morphology and rainfall intensity on soil loss and runoff was determined at the plant scale for three representative species of a semi-arid patchy shrubland vegetation of east Spain, representing contrasting canopy structures and plant phenologies (Rosmarinus officinalis, Anthyllis cytisoides and Stipa tenacissima).
Twenty-seven microplots of less than 1 m2, each containing one single plant, were built to quantify runoff volume and sediment yield under the canopies of the three species. Runoff and rates of soil loss measured in these plots under natural rainfall conditions were compared with control microplots built in the bare inter-plant areas. Precipitation was automatic-ally recorded and rainfall intensity calculated over a two-year period.
Results indicated that individual plants played a relevant role in interrill erosion control at the microscale. Compared with a bare soil surface, rates of soil loss and runoff reduction varied strongly depending on the species. Cumulative soil loss was reduced by 94·3, 88·0 and 30·2 per cent, and cumulative runoff volume was reduced by 66·4, 50·8 and 18·4 per cent under the Rosmarinus, Stipa and Anthyllis canopies, respectively, compared with a bare surface. Anthyllis was significantly less efficient than the two other species in reducing runoff volume under its canopy. Differences between species could only be identified above a rainfall intensity threshold of 20 mm h−1. The different plant morphologies and plant compon-ents explained the different erosive responses of the three species. Canopy cover played a major role in runoff and soil loss reduction. The presence of a second layer of protection at the soil surface (litter cover) was fundamental for erosion control during intense rainfall. Rainfall intensity and soil water status prior to rainfall strongly influenced runoff and soil loss rates. The possible use of these species in restoration programmes of degraded areas is discussed. Copyright
Previous studies have identified rainfall intensity as a contributing factor to the amount of suspended particles washed from urban areas during storms. Moreover, it has been postulated that the square of rainfall intensity (I2) provides a measure of the rainfall kinetic energy (KE) available for washoff processes. This paper provides a theoretical analysis of the potential inter-relationships between I2 and rainfall KE. A hypothetical rain drop size distribution (DSD) was used to derive rainfall energy characteristics. A special form of the Marshall-Palmer DSD was developed that ensured conservation of rainfall mass over the analysed range of rainfall intensities. This 'conserved rainfall mass' DSD was used to calculate the specific KE variants of rainfall (the time-specific KEI and the volume- specific KEP). It was found that KEI has a loge-based relationship with rainfall intensity. Empirical relationships widely used in soil erosion studies have generally adopted a log10 basis. A direct relationship between the two KE variants and I2 appears to be absent. A new KE variant (KEIA) is proposed and it is demonstrated that this variant supports the hypothesis that I2 is a measure of the KE of rainfall. KEIA is the kinetic energy potentially transferred from rain drops to the proportion of the unit area impacted at a specific instant in time. It is a function of KEI and rain drop circumferential area. Relationships based in rainfall momentum M are also investigated. The relationship between MI and KEI is nearly linear as a fitted power function has an exponent close to unity (equal to 0.93). This suggests KE and M could be effectively interchangeable if used in particle washoff estimation.
Our goal is to develop a model capable to discern the response of a watershed to different erosion mechanisms. We propose a framework that integrates a geomorphic component into the physically-based and spatially distributed TIN-based Real-time Integrated Basin Simulator (tRIBS) model. The coupled model simulates main erosive processes of hillslopes (raindrop impact detachment, overland flow entrainment, and diffusive processes) and channel (erosion and deposition due to the action of water flow). In addition to the spatially distributed, dynamic hydrologic variables, the model computes the sediment transport discharge and changes in elevation, which feedback to hydrological dynamics through local changes of terrain slope, aspect, and drainage network configuration. The model was calibrated for the Lucky Hills basin, a semi-arid watershed nested in the Walnut Gulch Experimental Watershed (Arizona, USA). It is demonstrated to be capable of reproducing main runoff and sediment yield events and accumulated volumes over the long term. The model was also used to study the response of two first-order synthetic basins representative of landforms dominated by fluvial and diffusive erosion processes to a 100-year long stationary climate. The analysis of the resultant slope-contributing area relationships for the two synthetic basins illustrates that the model is consistent with assumed principles of behavior and capable of reproducing the main mechanisms of erosion.
Sand-filled splash cups were used to study the erosivity of rainfall and throughfall in the humid subtropics of southeast China. Our results showed that the splash cup measurements yielded precise and reproducible results both under open field conditions and under forest vegetation. The splash cups were exposed to forest stands of different age and to selected species (Schima superba, Castanopsis eyrei) in the Gutianshan National Nature Reserve (GNNR). The measurements in the open field revealed a close relationship between unit sand loss (g m− 2), rainfall amount (mm) (R2 = 0.94) and maximum rainfall intensity (mm h− 1) (R2 = 0.90). The highest correlation was obtained between unit sand loss (g m− 2) and the average of the five highest five minute interval rainfall intensities (mm h− 1) (R2 = 0.96). This underlines the reliability of the splash cups used.
Rain falling on soil causes slaking, mechanical disruption of aggregates and compaction. Too few data exist to predict the changes likely to occur in particular soil, landscape and management conditions. Experiments with simulated rain were set up to study and to model mathematically the changes of the pore system within the surface layer of a soil when rain was applied on a field cropped with maize. Macroporosity, pore-size and pore-shape distributions, and the pore volume were measured by image analysis of thin sections and the fractal dimensions of the pore surface roughness were estimated. The general trends of changes in porosity indicated the presence of two different sets of processes at the surface (0–3 cm) and in the layer immediately underneath (3–6 cm). In both layers most of the variation in macroporosity was due to a loss of elongated porosity. A theoretical approach recently developed to link rain and erosion to sealing properties was extended to describing the effect of rain on the elongated porosity and the pore volume fractal dimension in these two layers. The resulting set of equations describe in detail the evolution of soil porosity near the soil surface. Our approach could be useful when modelling the effects of sealing processes in soil erosion.
The effect of slope angle on splash detachment has been a matter of discussion for long because it has not always been clearly observed in experiments. In this paper a model, based on the physics of the drop impact, is developed.The theoretical model indicates that a wide slope effect exists. Data collected by different authors were compared with the theoretical trends showing a satisfactory agreement. Some experiments showed that small changes in soil surface characteristics may hide the slope effect.
Vegetation is widely used for the control of surface erosion on slopes but different vegetation types vary in their effectiveness
and, in some situations, a vegetation cover can have adverse effects and actually increase the rate of erosion. Where climatic
or soil constraints exist, there are also concerns about how quickly an effective cover can be obtained. Simple screening
models can be used to indicate the likely severity of these issues prior to designing an erosion-control system. As soon as
the canopy cover is higher than 0.3 m above the surface, there is a risk that satisfactory protection against soil particle
detachment by raindrop impact will not be obtained. With canopies higher than 1.0 m, detachment rates may exceed those from
natural rainfall on bare ground. The amount of vegetation needed to prevent soil particle detachment by surface runoff depends
upon the steepness of the slope but, for grasses, a stem density of at least 10,000 stems/m2 is recommended. Uniformity of distribution is important because a clumpy vegetation cover can lead to concentrations of flow
between the plants with consequent increases in velocity. A vegetation cover can be used to induce sediment deposition. Where
grass is used as a buffer strip, a width of 10–12 m is usually sufficient to trap even the fine sediment. For large areas
of the world where water erosion is a problem, it is feasible to establish sufficient grass cover within 1 year.
The concept of “soil erodibility”, denoting soil susceptibility to erosional processes, has been extensively used in both theoretical and practical approaches to soil erosion, in part because it was incorporated in the Universal Soil Loss Equation. Soil erodibility has not been rigorously defined, but has evolved around three implicit assumptions:1.a soil erodibility ranking can be defined which is valid for all erosional processes;2.soil erodibility ranking can be uniquely defined by measurement of a few, usually physical, soil properties;3.3. relative erodibility ranking is not affected by short-term changes, particularly in soil moisture status.The concept of erodibility is examined in relation to recent field and laboratory soil erosion process studies which show that none of these assumptions are valid. Different properties determine soil erodibility for each erosional subprocess and erodibility can only be defined for precisely identified processes and erosive forces. The utility of the concept has been limited by failure to define these forces and to establish standard testing procedures. Application has also been restricted because the concept was developed primarily from research on agricultural soils where sheetwash and rainsplash processes are often more dominant than on non-agricultural soils. The precise processes active and the relevant soil properties change almost constantly, reflecting soil moisture conditions and the physical and chemical dynamism of the soil surface. Soil erodibility cannot, therefore, be uniquely defined by a few properties, and only rankings established for the same process, measured under similar conditions can be compared. It also follows that soil erodibility cannot be defined independently from vegetation characteristics which affect erosive forces, soil moisture conditions and soil physical and chemical properties.
The rain kinetic energy (KE) is a widely used indicator of the potential ability of rain to detach soil. However, rain kinetic energy is not a commonly measured meteorological parameter. Therefore, empirical laws linking the rain kinetic energy to the more easily available rain intensity (I) have been proposed based on drop-size and drop-velocity measurements. The various mathematical expressions used to relate rain kinetic energy and rain intensity available from the literature are reported in this study. We focus our discussion on the two expressions of the kinetic energy used: the rain kinetic energy expended per volume of rain or volume-specific kinetic energy (KEmm, J m−2 mm−1) and the rain kinetic energy rate or time-specific kinetic energy (KEtime, J m−2 h−1). We use statistical and micro-physical considerations to demonstrate that KEtime is the most appropriate expression to establish an empirical law between rain kinetic energy and rain intensity. Finally, considering the existing drop-size distribution models from literature, we show that the most suitable mathematical function to link KE and I is a power law. The constants of the power law are related to rain type, geographical location and measurement technique.
The Optical Spectro Pluviometer (OSP), an infrared optical device, is used to measure the rain drop size, fall velocity and intensity. The OSP, which works on the principle of measuring an optical shadow, enables independent measurement of drop size and drop velocity in real time and thus any parameter linked to rain erosivity. Simulated rain characteristics measured with the OSP are reported. Calibrations made in the laboratory using single raindrops allow an estimation of the accuracy of drop size and velocity measurements. The single drop calibration shows that drop diameters are measured with an accuracy that is at least 6% in the range 0.3–4.7 mm. Larger drops are detected but without quantification of their diameter. The accuracy in the drop fall velocity measurements varies from 1% for the lowest velocity (0.2 m s−1) to 20% for the highest velocity (10 m s−1). Simulated and natural raindrop-size distributions measured by the OSP are compared with distribution obtained by the filter paper method. Taking into account the uncertainties associated with the two methods, the raindrop-size distributions derived from the OSP and the filter paper method are in good agreement. To complete the calibration, rain intensity is derived from the raindrop-size distribution measurements and compared with values measured by a nearby rain gauge. For rain intensities less than 35 mm h−1 the OSP underestimates intensity by 12%. However, for rain intensities in excess of 35 mm h−1, the OSP underestimates intensity by 38%. The errors on the two widely used rain erosivity indices, the kinetic energy and the momentum, are estimated to be less than 4%.
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