Wolfgang Fister

Comparative measurements with seven rainfall simulators on uniform bare fallow land

To assess the inflfl uence of rainfall simulator type and plot dimensions on runoff and erosion, seven small portable rainfall simulators from Freiberg, Tubingen, Trier (all Germany), Valencia, Zaragoza (both Spain), Basel (Switzerland) and Wageningen (the Netherlands) were compared on a prepared bare fallow fifi eld. The experiments were carried out during an international rainfall simulator workshop, organized at Trier University (Germany) from 30th of June to 1st of July 2011.The tested rainfall simulators differ in design, rainfall intensities, rain spectra, etc. and represent most of the devices which have been used over the last decade in Europe. The plots for the different rainfall simulators were selected as similar as possible concerning soil physical and chemical properties, aspect and inclination and were chosen to be placed side by side in horizontal direction. Test procedure was standardized in order to examine the inflfl uence of the rainfall simulator andplot dimension only. The results show a clear and consistent relationship in runoff, erosion and infifi ltration behaviour of the different used rainfall simulators. With all the devices total soil loss is measurable, but different plot sizes, intensities and kinetic energies of the simulated rainfall caused differences in soil loss and runoff quantities per unit of area. Regarding course characteristics over runs, similarities could be observed especially in runoff behaviour. The rainfall simulators (> 1 m² plot size) are able to reproduce infifi ltration and interrillerosion processes. With an increase of plot size (≥ 1 m²), rill-erosion will be also reflfl ected. Therefore it can be concluded that up to a certain plot size, the results of the different simulators are comparable and depend in their magnitude on the properties of the applied rainfall. The increase in process complexity with increasing plot size shows, that the scale of the simulation is one of the most important parameters to be taken intoaccount when comparing values of erosion and runoff.

Comparison of rainfall characteristics of a small portable rainfall simulator and a combined wind and rainfall simulator

In this investigation two rainfall simulators have been compared, concerning drop size distribution, drop fall velocity, and spatial rainfall distribution. Additionally, the effect of wind onto rainfall characteristics is measured with the combined wind and rainfall simulator.The results indicate that the used nozzles in both simulators have very small water discharge fluctuations (< 2%). regarding drop size distribution a very close relation of both simulators and natural rainfall (marshall & palmer distribution) can be observed. owing to low fall heights, measured drop fall velocities are too slow ranging from 3.4 to 5 m s−1. The spatial rainfall distribution of the small rainfall simulator is very homogenous with a Chistiansen Coefficient (CU) of 91%. In contrast, the combined wind and rainfall simulator has CU values of 60%, which clearly improve with simultaneous simulation of wind (CU = 76%). The collected variables of both simulators show extremely low fluctuations throughout all tests and are therefore reproducible in field investigations at any time. In dieser Untersuchung geht es um den direkten Vergleich der Regeneigenschaften zweier Regensimulatoren, insbesondere hinsichtlich des Tropfengrosenspektrums, der Fallgeschwindigkeit und raumlichen Verteilung der Tropfen sowie der Reproduzierbarkeit der Ergebnisse. Mit der kombinierten Wind-/Beregnungsanlage wird zudem untersucht, welchen Ein fluss der zugeschaltete Wind auf die Regeneigenschaften dieser Anlage ausubt.Die Ergebnisse der Kalibrierungen belegen, dass die eingesetzten Dusen beider Regensimulatoren eine auserst geringe Mengenabweichung pro Zeiteinheit aufweisen (< 2%). bezuglich der tropfengrosenverteilung wurde eine sehr gute Ubereinstimmung beider anlagen mit dem naturlichen niederschlagsspektrum (marshall & palmer-verteilung) festgestellt. im gegensatz dazu sind die tropfenfallgeschwindigkeiten entsprechend der geringen fallhohe zu niedrig (3.4-5 m s−1). Die raumliche Verteilung des Niederschlags der Kleinberegnungsanlage ist sehr homogen (Cu = 91%); im Gegensatz dazu weist die Wind-/Beregnungsanlage in Simulationen ohne Wind Inhomogenitaten (Cu = 60%) auf, wahrend mit Wind diese deutlich ausgeglichen werden (Cu = 76%). Alle genannten Messgrosen weisen uber alle Versuche beider Anlagen auserst geringe Schwankungen auf und sind damit im Gelande jederzeit sicher reproduzierbar.

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

Recent research has shown that wind can have a signififi cant inflfl uence on velocity, impact angle and kinetic energy of raindrops, and subsequently increases soil erosion. The aims of this study were to 1) quantify the inflfl uence of wind on water erosion, 2) specififi cally observe the difference in processes betweenwindless 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 inflfl uence of wind on raindrops, wasused. To facilitate extraction of the inflfl uences of WDR on soil erosion, to avoid systematic errors, and to reduce variability between test plots, a defifi ned 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 60 m with uniform and smooth surface and low inclination (1°) 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 comparisonto 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 quantififi cation in the fifi eld.

Impact of severe rain storms on soil erosion: Experimental evaluation of wind-driven rain and its implications for natural hazard management

Abstract Prediction and risk assessment of hydrological extremes are great challenges. Following climate predictions, frequent and violent rainstorms will become a new hazard to several regions in the medium term. Particularly agricultural soils will be severely threatened due to the combined action of heavy rainfall and accompanying winds on bare soil surfaces. Based on the general underestimation of the effect of wind on water erosion, conventional soil erosion measurements and modeling approaches lack related information to adequately calculate its impact. The presented experimental-empirical approach shows the strong impact of wind on the erosive potential of rain. The tested soils had properties that characterize three environments 1. Silty loam of semi-arid Mediterranean dryfarming and fallow, 2. clayey loam of humid agricultural sites and 3. cohesionless sandy substrates as found at coasts, dune fields and drift-sand areas. Total erosion was found to increase by a factor of 1.3 to 7.1, depending on site characteristics. A complementary laboratory procedure was applied to quantify explicitly the effect of wind on raindrop erosion as well as the influence of substrate, surface structure and slope on particle displacement. These tests confirmed the impact of wind-driven rain on total erosion rates to be of great importance when compared to all other tested factors. To successfully adapt soil erosion models to near-future challenges of climate change induced rain storms, wind-driven rain should be included into the hazard management agenda.

Use of Field Experiments in Soil Erosion Research

Abstract Field experiments are widely used while investigating runoff and soil erosion. This chapter presents some general thoughts on the appropriate design and use of field experiments, as well as two case studies of experimental research to highlight their relative advantages and disadvantages over laboratory experiments and field monitoring. Both examples are structured so that the reader is led from the initial conception of a hypothesis, through the planning and field campaign phases, before some final conclusions are drawn and suggestions made for further work. Despite meticulous planning, however, each example highlights the fact that specific compromises are frequently needed in order to overcome the many constraints that may be encountered when undertaking field experiments. Both case studies also illustrate that field experiments are an important tool for investigating processes through measuring their rates under controlled driving conditions, but variable natural surface properties. This basic principle enables a detailed insight into the functioning of specific process domains, that is, plots, but is also limited, mostly in spatial scale and the number of scenarios of interaction between driving rainfall and runoff and the surfaces they act upon. The interpretation of field experiments therefore requires a careful consideration of the context in which they were collected.