Modelling water erosion in the Sahel: application of a physically based soil erosion model in a gentle sloping environment     

S. M. Visser  G. Sterk  D. Karssenberg 《地球表面变化过程与地形》2005,30(12):1547-1566

Water is a major limiting factor in arid and semi‐arid agriculture. In the Sahelian zone of Africa, it is not always the limited amount of annual rainfall that constrains crop production, but rather the proportion of rainfall that enters the root zone and becomes plant‐available soil moisture. Maximizing the rain‐use efficiency and therefore limiting overland flow is an important issue for farmers. The objectives of this research were to model the processes of infiltration, runoff and subsequent erosion in a Sahelian environment and to study the spatial distribution of overland flow and soil erosion. The wide variety of existing water erosion models are not developed for the Sahel and so do not include the unique Sahelian processes. The topography of the Sahelian agricultural lands in northern Burkina Faso is such that field slopes are generally low (0–5°) and overland flow mostly occurs in the form of sheet flow, which may transport large amounts of fine, nutrient‐rich particles despite its low sediment transport capacity. Furthermore, pool formation in a field limits overland flow and causes resettlement of sediment resulting in the development of a surface crust. The EUROSEM model was rewritten in the dynamic modelling code of PCRaster and extended to account for the pool formation and crust development. The modelling results were calibrated with field data from the 2001 rainy season in the Katacheri catchment in northern Burkina Faso. It is concluded that the modified version of EUROSEM for the Sahel is a fully dynamic erosion model, able to simulate infiltration, runoff routing, pool formation, sediment transport, and erosion and deposition by inter‐rill processes over the land surface in individual storms at the scale of both runoff plots and fields. A good agreement is obtained between simulated and measured amounts of runoff and sediment discharge. Incorporating crust development during the event may enhance model performance, since the process has a large influence on infiltration capacity and sediment detachment in the Sahel. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Optical remote sensing of submerged aquatic vegetation: Opportunities for shallow clearwater streams     

Fleur Visser  Caroline Wallis  Anne M. Sinnott 《Limnologica》2013,43(5):388-398

Remote sensing has rarely been used as a tool to map and monitor submerged aquatic vegetation (SAV) in rivers, due to a combination of insufficient spatial resolution of available image data and strong attenuation of light in water through absorption and scattering. The latter process reduces the possibility to use spectral reflectance information to accurately classify submerged species. However, increasing availability of very high resolution (VHR) image data may enable the use of shape and texture features to help discriminate between species by taking an object based image analysis (OBIA) approach, and overcome some of the present limitations.This study aimed to investigate the possibility of using optical remote sensing for the detection and mapping of SAV. It firstly looked at the possibilities to discriminate submerged macrophyte species based on spectral information only. Reflectance spectra of three macrophyte species were measured in situ across a range of submergence depths. The results showed that water depth will be a limiting factor for the classification of species from remote sensing images. Only Spiked Water Milfoil (Myriophyllum spicatum) was indicated as spectrally distinct through ANOVA analysis, but subsequent Jeffries–Matusita distance analysis did not confirm this. In particular Water Crowfoot (Ranunculus fluitans) and Pondweed (Potamogeton pectinatus) could not be discriminated at 95% significance level. Spectral separability of these two species was also not possible without the effect of an overlying water column.Secondly, the possibility to improve species discrimination, using spatial and textural information was investigated for the same SAV species. VHR image data was acquired with a Near Infrared (NIR) sensitive DSLR camera from four different heights including a telescopic pole and a Helikite UAS. The results show that shape and texture information can improve the detection of the spectrally similar Pondweed and Water Crowfoot from VHR image data. The best performing feature ‘length/width ratio of sub-objects’ was obtained through expert knowledge. All of the shape and texture based features performed better at species differentiation than the spectrally based features.In conclusion this study has shown that there is considerable potential for the combination of VHR data and OBIA to map SAV in shallow stream environments, which can benefit species monitoring and management.  相似文献   

The Dwarskersbos,South Africa local tsunami of August 27, 1969: field survey and simulation as a meteorological event   总被引：1，自引：1，他引：0  

Emile A. Okal  Johan N. J. Visser  Coenraad H. de Beer 《Natural Hazards》2014,74(1):251-268

We investigate the hitherto unexplained wave which inundated the village of Dwarskersbos, South Africa, in the early hours of August 27, 1969, in the absence of any seismic disturbance or major meteorological storm. A field survey, based on the interview of nine elderly witnesses still residing in the community, documented maximum run-up of 2.9 m, concentrated on an extremely short segment of coastline, less than 2 km in length. These characteristics are incompatible with generation by a seismic source (which, at any rate, should have been felt by the population). A landslide source, located at the only canyon featuring a steep enough ocean floor, is also ruled out since a numerical simulation fails to reproduce the concentration of the wave at Dwarskersbos. By contrast, the wave can be explained as a “meteo-tsunami” resulting from resonance between a meteorological squall propagating at 18 m/s in the azimuth \(\hbox {N101}^{\circ }\hbox {E}\) and a gravity wave propagating in the shallow waters off the eastern shore of St. Helena Bay. This is confirmed by numerical simulation under the formalism of Proudman (Dynamical oceanography. Methuen, London, 1953), which provides a satisfactory model of the distribution of run-up along the beach.  相似文献   

Propeller scars on and known home range of two orca (Orcinus orca) in New Zealand waters     

Ingrid N. Visser 《新西兰海洋与淡水研究杂志》2013,47(4):635-642

Two orca (Orcinus orca, Linnaeus, 1758), photo‐identified as part of an ongoing study in New Zealand waters, have a series of deep scars in their dorsal lumbar‐caudal regions which suggests they were caused by propellers. One of the animals, first noted to have the scars in 1982, is considerably smaller than an adult of similar age. The known home range of these orca are presented. A third orca hit by a boat is presumed dead.  相似文献   

GPS-based precise orbit determination of the very low Earth-orbiting gravity mission GOCE   总被引：5，自引：0，他引：5  

P. N. A. M. Visser  J. van den IJssel 《Journal of Geodesy》2000,74(7-8):590-602

 A prerequisite for the success of future gravity missions like the European Gravity field and steady-state Ocean Circulation Explorer (GOCE) is a precise orbit determination (POD). A detailed simulation study has been carried out to assess the achievable orbit accuracy based on satellite-to-satellite tracking (SST) by the US global positioning system (GPS) and in conjunction the implications for gravity field determination. An orbit accuracy at the few centimeter level seems possible, sufficient to support the GOCE gravity mission and in particular its gravity gradiometer. Received: 21 January 2000 / Accepted: 4 July 2000  相似文献   

In other worlds: on the politics of research in a transforming South Africa   总被引：1，自引：1，他引：0  

Gustav Visser 《Area》2000,32(2):231-235

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Competitive interactions among beam trawlers exploiting local patches of flatfish in the North Sea   总被引：5，自引：4，他引：1  

Rijnsdorp  A. D.; van Mourik Broekman  P. L.; Visser  E. G. 《ICES Journal of Marine Science》2000,57(4):894-902

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Micro-scale distribution of beam trawl effort in the southern North Sea between 1993 and 1996 in relation to the trawling frequency of the sea bed and the impact on benthic organisms   总被引：10，自引：5，他引：5  

Rijnsdorp  A. D.; Buys  A. M.; Storbeck  F.; Visser  E. G. 《ICES Journal of Marine Science》1998,55(3):403-419

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Open‐foldable domes with high‐tension textile membranes: The GREGOR dome     

R.H. Hammerschlag  J.N. Kommers  S. Visser  F.C.M. Bettonvil  A.G.M. van Schie  S.J. van Leverink  G. Sliepen  A.P.L. Jgers  W. Schmidt  R. Volkmer 《Astronomische Nachrichten》2012,333(9):830-839

Double layers of high‐tensioned textile membranes were applied to the completely open‐foldable dome for the GREGOR telescope for the first time. Simultaneous climate measurements inside and outside the dome have proven the thermalinsulating capability of this double‐layer construction. The GREGOR dome is the result of the continuation of the ESO research on open‐foldable domes with textile structures, followed by the research for the DOT dome with high‐tensioned textile membranes. It cleared the way to extreme stability required for astronomical practice on high mountain sites with heavy storms and ice formation. The storm Delta with 245 km/h 1‐minute mean maximum at the location of the GREGOR caused no problems, nor did other storms afterwards. Opening and closing experiences up to wind speeds of 90 km/h were without problems. New technical developments were implemented and tested at the GREGOR dome, opening the way for application to much larger domes up to the 30 m diameter‐class range (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献