Soil erosion in Mediterranean landscapes : Experimental investigation on crusted surfaces by means of the Portable Wind and Rainfall Simulator

The influence of wind on raindrops and subsequent processes of soil detachment and transport on natural soil surfaces is an essential gap of knowledge. The urgently required data about reactions, interactions and actual impact on soil erosion rates are generally produced under laboratory conditions on highly disturbed substrates, which cannot reflect natural system responses. The Portable Wind and Rainfall Simulator was applied on autochthonous soils in semi-arid Spain to investigate and quantify the relative impact of wind-driven rain on total erosion. On highly degraded crusted soils and freshly ploughed orchard soils in semi-arid Spain, total erosion measured during experiments (30 min; 96 mm h-1) were 28.8 - 150.4 g m-2 and 29.5 - 30.7 g m-2, respectively. Concerning the relative impact of wind-driven rain on total erosion, ambiguous results were obtained: the difference to erosion generated by windless rain ranged from +37.4 to -24.2%, to sediment concentration from +46.7 to -20.6% and to runoff coefficients from +18.8 to -7.4%.The study indicates a potentially very strong impact of wind-driven rain and underlines the paramount importance of experimental data derived on autochthonous soil surfaces for process understanding, realistic assessment of soil erosion rates and application in soil erosion models

A small portable rainfall simulator for reproducible experiments on soil erosion

The importance of distinguishing and discretely studying the subprocesses of runoff generation and erosion has led to the development of rainfall simulations on small plots. We methodically upgraded a small portable rainfall simulator with particular respect to (1) rainfall characteristics that include homogeneous spatial rainfall distribution and drop spectrum, (2) handling, and (3) control of test conditions. We measured simulator characteristics with rain gauges, calibration plate and Laser Precipitation Monitor by Thies (LPM). The upgraded small rainfall simulator, and measurements of the improved rainfall characteristics are presented in this paper.The upgraded configuration shows the desired improvements: regarding drop size distribution, a close relationship to natural rainfall (Marshall & Palmer Distribution) can be observed. Due to low fall heights, measured drop fall velocities are slow; maximum velocities range between 3.4 and 5 m s1. Mean kinetic energy expenditure, mean kinetic energy per unit area and unit depth of rainfall and mean momentum are 214 J m2 h1, 5.8 J m2 mm1 and 0.016 kg m s1, respectively. The spatial rainfall distribution of the upgraded simulator is homogenous with a Christiansen-Uniformity Coefficient of 91%. The measured variables show extremely low variation throughout all tests and should therefore be reproducible in field investigations at any time

Erosion processes on different relief units: the relationship of form and process

Geomorphological relief units are related to certain processes and the history of their development. They are well defined by form and material. This study investigates erosion processes on different relief units. Rainfall simulations, rill experiments and monitoring by aerial photography were performed on dunes, glacis, Holocene fillings, talus material and Quaternary loam terraces in order to analyse the varying process intensities. Splash, interrill erosion and runoff generation were quantified by rainfall simulation experiments, rill erosion by rill experiments and gully growth by monitoring over several years. The test sites are situated in NE- and SE-Spain, S-Morocco and N-Burkina Faso. The results clearly show that the measured processes are very different from those supposed to be relevant for the formation of relief units. Dunes and Holocene fillings are highly erodible by splash and interrill erosion. In contrast, Quaternary loam terraces show a low susceptibility to erosion processes. We conclude that the relief units show very different dominances of erosion processes and process intensities. The differentiation is more significant with increasing scale and complexity of the erosion process: The extent of gully growth varies much more between the different units than those of splash and interrill erosion do.Las unidades geomorfológicas están relacionadas con ciertos procesos y con la historia de su formación. Además, se definen por sus formas y materiales. En este estudio se investigan los procesos de erosión en diferentes unidades de relieve. Simulaciones de lluvia, experimentos en rigolas y seguimientos a través de fotografía aérea se llevaron a cabo en dunas, glacis, rellenos holocenos, taludes y terrazas franco-arcillosos de origen cuaternario, con el objeto de conocer la variación en la intensidad de los procesos. Para ello, se evaluaron el impacto de las gotas de lluvia, la generación de escorrentía, la erosión en inter-rigolas y rigolas durante varios años. Las áreas de estudio se situaron en el NE y SE de España, sur de Marruecos y norte de Burkina Faso. Los resultados muestras claramente que los procesos medidos son muy diferentes entre sí según la formación de las propias unidades de relieve. Las dunas y los rellenos holocenos en los valles son altamente susceptibles a la erosión por el impacto de las gotas de lluvia y la formación de regueros. Por el contrario, las terrazas sobre materiales franco-arcillosos muestran baja susceptibilidad a los procesos de erosión. Por lo tanto, se puede concluir que las distintas unidades de relieve muestran diferentes dominancia e intensidad de procesos. Las diferencias son más significativas con el incremento de la escala y la complejidad del proceso erosivo: el crecimiento de las cárcavas varía mucho más entre diferentes unidades que los efectos del impacto de la gota de lluvia y la formación de rigolas

The role of wind-driven rain for soil erosion – an experimental approach

Recent research has shown that wind can have a significant influence on velocity, impact angle and kinetic energy of raindrops, and subsequently increases soil erosion. The aims of this study were to 1) quantify the influence of wind on water erosion, 2) specifically observe the difference in processes between windless rain (WLR) and wind-driven rain (WDR) simulations and 3) test the device's and test sequence's practicability. The Portable Wind and Rainfall Simulator (PWRS), recently developed at Trier University for plot-scale in situ assessment of differences in soil erosion with and without the influence of wind on raindrops, was used. To facilitate extraction of the influences of WDR on soil erosion, to avoid systematic errors, and to reduce variability between test plots, a defined order of four consecutive test runs was established: 0) wind simulation, 1) WLR simulation on dry soil, 2) WLR simulation on moist soil, 3) WDR simulation. The tests were conducted on homogenous sandy substrate deposited on an area of 15.2 m x 60 m with uniform and smooth surface and low inclination (1 degrees) in the Willem Genet Tunnel of Wageningen University. The results show an increase of eroded sediment ranging from 113% up to 1108% for WDR simulations in comparison to WLR simulations. The increase in runoff was considerably lower (15% to 71%), resulting in an increase of sediment concentration between 56% and 894%. The results indicate an immense impact of WDR on soil erosion of sandy cohesionless substrate. The experimental setting and measurement proved reliable and reproducible and enables a clear process observation and quantification in the field

Characterization of complex pebble movement patterns in channel flow – a laboratory study

For a long time, studies concerning erosion caused by concentrated overland flow mainly dealt with the erosion and the transport of fine material. More recent studies have shown that rock fragments reduce the intensity of soil erosion processes on the one hand, but on the other hand rock fragment movements also have been observed both in the rill- and interrill erosion processes. However, there is little knowledge about the movement process of rock fragments in shallow channel flow. Are certain movement patterns typical for different shapes? Are there relationships between movement patterns and slope and flow velocity? Are all these patterns and relationships reproducible? To answer these questions, we performed laboratory channel experiments. With these experiments, we could obtain information about movement patterns of pebbles, by varying the following parameters: shape (flat, ellipsoidal, nearly spherical), size (diameter between 1.97 and 4.0 cm) and channel slope (5°, 10°). During the experiments, a high-speed camera was used to capture the motion of eight specially painted pebbles. The resulting image sequences were processed using both automatic image processing and manual visual inspection. Besides the movement patterns, the pebbles velocity, the water velocity and the water depth were estimated. We could show that there were different movement patterns depending on the shape and the slope. For the 5° experiments, the big, flat pebbles lie at the beginning of the tests. After the following yawing, the pebbles mainly showed the movement form rolling around the longest axis. For the 10° experiments the big, flat pebbles showed the same movement pattern firstly, but later in the sequence, they started to roll around their shortest axis and in the end this movement form was combined with saltation. These patterns are described using a simple symbolic language: sequences of pictograms describe the consecutive movement forms. Furthermore, we detected five different velocity groups of the pebbles for each slope: different cross-section shapes of the pebbles result in different acceleration behavior. The methodology is limited to clear water in laboratory use. Even a larger water depth restricts the image processing. Thus, in the future the experiments will be combined with a small sensor that is implanted in the pebbles and measures forces (acceleration), compass (magnetic flux density) and rotations (gyroscope).Durante mucho tiempo, las investigaciones sobre la erosión por flujos concentrados se centraban en la erosión y el transporte de materiales finos. Estudios más recientes han demostrado que, por un lado, los fragmentos de rocas pueden reducir la erosión, pero por otro, se han podido observar movimientos de éstos en pequeñas cárcavas o incluso en superficies sin concentración de la escorrentía superficial. Sin embargo, poco se conoce hasta ahora sobre los patrones de movimiento de los fragmentos de rocas dentro flujos efímeros concentrados. ¿Existen patrones de movimiento típicos para diferentes formas? ¿Existe una relación entre los diferentes patrones de movimiento y la pendiente y velocidad del flujo? Y, ¿son reproducibles esos patrones y esas relaciones? Para obtener información sobre los patrones de movimiento de gravas se realizaron experimentos en un canal de laboratorio variando los siguientes parámetros: forma (elipsoide, casi esférico, aplanado), tamaño (diámetros entre 1.97 y 4.0 cm) de las gravas y pendiente (5°, 10°) del canal. El movimiento de 8 piedras diferentes, pintadas especialmente para el caso, fue registrado con una cámara de alta velocidad. Las secuencias de imágenes resultantes fueron analizadas de forma automática al igual que de forma visual. Aparte de la identificación de los patrones de movimiento de las gravas se estimaron también la velocidad de éstas, así como la velocidad y la profundidad del agua. Se pudieron identificar diferentes patrones de movimiento dependiendo de la forma de la piedra y de la pendiente del canal. Al principio de los experimentos con pendiente de 5°, las gravas grandes y aplanadas que descansan sobre la superficie del canal, comienzan con un movimiento de guiñada para después pasar a rodar alrededor del eje mayor. Con una pendiente de 10°, estas mismas gravas comienzan el movimiento con el mismo patrón, para después pasar a rodar alrededor del eje más corto y finalmente combinar el rodamiento con saltos. Estos patrones se describen a base de un lenguaje simbólico simple: secuencias de pictogramas describen formas de movimiento consecutivas. Además, se identificaron 5 grupos de velocidad de las gravas para cada pendiente: cada sección transversal de la forma de la grava resulta en un patrón de aceleración diferente. La metodología está limitada a agua clara y el laboratorio. Mayores profundidades limitan las posibilidades de tratamiento de imágenes. Así, futuros experimentos se combinarán con un sensor instalado dentro de los fragmentos de roca que sea capaz de medir fuerzas (aceleración), orientación (densidad+ del campo magnético) y la rotación (giroscopio)