Space-time variability of soil salinity in irrigated vineyards of South Africa

Salts present in the soil and surface waters of the Western Cape Province of South Africa represent a limitation to farming activities. Therefore the management of salinity in the landscape, which includes measuring, mapping and monitoring of its behaviour on a regional scale, is the subject of this investigation. Four sites were actively investigated in this study. These are the Robertson experimental farm, Goedemoed farm near Robertson, Broodkraal farm near Piketberg and the Glenrosa farm near Paarl. Regular point measurements and soil samples were taken to study the behaviour of salinity in irrigated soils over several irrigation seasons. The quality of the irrigation water was adjusted to six different levels of salinity, between 30 mS m-1 and 500 mS m-1. At the Broodkraal and Glenrosa farms, large scale investigations were conducted to estimate salinity over larger areas. Broodkraal farm was a large newly established table grape enterprise offering the opportunity to study the initial change in soil salinity when irrigated with saline water. At the Glenrosa farm, there was an opportunity to characterize the soil even before irrigated vineyards were established. Suction cup measurements were taken with self-designed and patented lysimeters and used to follow the seasonal soil water salinity changes through the root zone under different salinity regimes. These results were used to characterize an average salt depth trend, which was found to be best represented by a linear function, and its evolution over time. A method was proposed to reduce the number of samples necessary to determine this salt depth trend and to estimate the quality of the soil water that drains below the rooting zone. One of the important findings was that an ECe threshold for vines of 100 mS m-1 was more suitable than the conventional 150 mS m-1 and that the sensitivity of the vines to levels beyond this threshold increased with the number of years of exposure. The detailed surveys at the Broodkraal and Glenrosa Farms helped the modelling of the regional salinity behaviour. This study allowed gaining a comprehensive understanding of the soil salinity dynamics in an irrigated landscape using saline water

Leaf area changes and transpiration in vineyards under salt stress

Thesis (M.Sc. Agric.) -- University of Stellenbosch, 1999.ENGLISH SUMMARY: Irrigation of vines with saline water has long been a problem in the Western Cape region. Research in this respect financed by the Water Research Commission was done on vines to test the effect of 6 levels of irrigation water quality on production. The experiment consisted of two sites namely one at the Robertson experimental farm of the ARC outside of Robertson and the other on the Nietvoorbij experimental farm outside Stellenbosch. Each site had 6 treatments replicated 4 times. The treatments consisted of water with electrical conductivities of -40, 75, 150, 250, 350, 500 mS/m. The saline water was produced and controlled by a computerised injection system that injected a high concentration stock solution into the irrigation system. The stock solution consisted ofNaCI and CaCl2 mixed to a Na:Ca ratio. Description of the canopy surface and structure per plant is essential to the formulation and description of plant reaction resulting from plant-environmental interaction. This study looked at measurement techniques to non-destructively describe and quantify the reaction of canopies to different saline treatments. Measurement techniques consisted of physical destructive and non-destructive light interception techniques with special reference to the use of the Sunfleck Ceptometer and Dcor C2000 Plant Canopy Analyser. Destructive measurements were only done to calibrate the non-destructive techniques. The Dynamax Heat Balance Sap Flow Meter was used to measure differences in sap flow rate between plants from different treatments. The measured transpiration was compared with weather station derived evapotranspiration as well as the sodium absorption ratio of the different soils. It was found that leaf area indices do show treatment effects very clearly. It was also found that by the time treatment effects were visible, leaf damage was already irreversible. The method clearly highlights treatment effects but cannot be used in a production environment to help prevent leaf damage as a management tool. Sap flow measurement was done to show that sap flow is more sensitive and that differences do occur before leaf damage is visible. Sap flow measurements can therefore be used with greater success as a management and a research tool. A good calibration exercise to determine leaf area indices non-destructively led to the ability of producing reliable transpiration and evapotranspiration data.AFRIKAANSE OPSOMMING: Besproeiing van wingerd met brakwater is reeds 'n wesenlike probleem in die WesKaap Provinsie. Navorsing was deur die Waternavorsingskomrnissie geloods waar wingerd met ses grade van brakwater besproei was om brakwater se invloed op plantprestasie te meet. Die proef was tweeledig van aard met 'n perseel buite Robertson op die NIWW -proefplaas en 'n tweede op die Nietvoorbij proefplaas buite Stellenbosch. Daar was 6 brakwater behandelings nl., -40, 75, 150, 250, 350, 500 mSm-1 met 4 herhalings van elk. Die waterkwaliteit was beheer vanaf In inspuitstelsel gekoppel aan In hoe konsentrasie voorraad oplossing. Die voorraad oplossing het bestaan uit NaCl en CaCl2 gemeng in 'n Na:Ca verhouding. Beskrywing van die blaredak en blaredakstruktuur van 'n gewas is essensieel t.o.v. formulering en beskrywing van plantreaksie a.g. v. plant-omgewing interaksies. Daar was met hierdie studie gekyk na metingstegnieke om die blaredak deur niedestruktiewe metodes te beskryf en dus plantreaksie op verskillende brakwaterbehandelings te kwantifiseer. Metingstegnieke het bestaan uit fisiese destruktiewe metings en ligonderskeppings tegnieke waaronder die Sunfleck Ceptometer en Dcor C2000 Plant Canopy Analizer tel. Destruktiewe metings was slegs gedoen ter kalibrering van die nie-destruktiewe metodes. Die Dynamax Heat Balance Sapflow Meter was gebruik vir sapvloeimetings, om die verskille in transpirasie tussen behandelings waar te neem. Die gemete transpirasie was vergelyk met weerstasie afgeleide evapotranspirasie en ook met die natrium absorpsie verhouding van die verskillende gronde. Daar was gevind dat blaar oppervlakindekse wel duidelik behandelingsverskille uitwys. Daar is ook gevind dat teen die tyd dat verskille sigbaar is, daar reeds onomkeerbare skade aan die blare is. Blaar oppervlakindekse het dus wei gehelp om die behandelingsverskille uit te wys maar dit kan nie gebruik word in 'n produksie omgewing om blaarskade te help voorkom deur dit as 'n bestuurshulpmiddel aan te wend nie. Daarvoor was sapvloei metings gedoen om aan te toon dat verskille in sapvloei reeds bestaan voor blaarskade sigbaar is. Sapvloei metings sou dus met groter sukses aangewend kan word as 'n bestuurshulpmiddel en ook as navorsingshulpmiddel. 'n Goeie kalibreringsoefening om blaaroppervlak indekse akkuraat te bepaal m.b.v niedestruktiewe metodes, het gehelp om transpirasie en evapotranspirasie baie akkuraat te benader.Maste

Predicting gully erosion susceptibility in South Africa by integrating literature directives with regional spatial data

Gully erosion has been identified as a severe land degradation process with environmental and socio-economic consequences. Identifying areas susceptible to gully erosion will aid in developing strategies to inhibit future degradation. Various approaches have been implemented to predict and map gully erosion susceptibility but are mostly restricted to small geographical extents because of process limitations. Here, we introduce a novel method that predicts gully erosion susceptibility on a regional/national scale (1.22 million km 2) by synthesising literature directives with a statistical approach. Findings from a literature review were used to extract physiographic properties associated with gully erosion that was conditioned to characterise susceptibility by using the Frequency Ratio model. The conditioned physiographic properties were aggregated by a weighted overlay procedure using an aggregation of controlling factors derived from the literature review as a weighting system. The gully susceptibility index (GSI) model was validated against a published gully inventory map (n = 163 019) and randomly generated 1-km 2 tessellation zones from which primary validation data were derived. Although uncertainties within the modelling procedure exist (e.g., gully site distribution, the spatial resolution of input data and determination of gully points), the validation shows that the GSI model is generally robust, identifying areas of contrasting susceptibilities. Furthermore, findings converge with other susceptibility metrics, which have been derived by different methodologies. Because empirical gully erosion research has been conducted worldwide, this model could be applied to regional-scale gully susceptibility modelling assessments (as a solitary method or combined with primary data) in other parts of the world. Additionally, the GSI model can be adopted to model environmental change scenarios.</p

Evaluating the effects of generalisation approaches and DEM resolution on the extraction of terrain indices in KwaZulu Natal, South Africa

Digital elevation model (DEM) data are elemental in deriving primary topographic attributes which serve as input variables to a variety of hydrologic and geomorphologic studies. There is however still varied consensus on the effect of DEM source and resolution on the application of these topographic attributes to landscape characterisation. While elevation data for South Africa are available from several major sources and resolutions: Shuttle Radar Topographic Mission (SRTM), Earth ENV and Stellenbosch University DEM (SUDEM). Limited research has been conducted in a local context comparing the extraction of terrain attributes to high resolution Digital Terrain Data (DTM) such as LiDAR (Light Detection and Ranging) that are becoming increasing available. However, the utility of LiDAR to topographic analyses presents its own challenges in terms of operational-relevant resolution, processing demands and limited spatial coverage. There is a need to quantify the impact that generalisation approaches have on simplifying detailed DEMs and to compare the accuracy and reliability of results between high resolution and coarse resolution data on the extraction of localized topographic variables. In this regional study, we analyse the accuracy on selected local terrain attributes: elevation, slope and topographic wetness index derived from DEMs from varying sources, at different spatial resolutions and using three generalisation algorithms, namely: mean cell aggregation, nearest neighbour and hydrological corrected topo-to-raster. We show that topographic variable extraction is highly dependent on DEM source and generalisation approach and while higher resolution DEMs may represent the “true“ surface more accurately, they do not necessarily offer the best results for all extracted variables. Our results highlight the caveats of selecting DEMs not “fit-for-purpose” for topographic analysis and offer a simple yet effective solution for reconciling the selection of DEMs based on neighbourhood size resolution prior to terrain analyses and topographic feature characterization

Assessing the influence of DEM source on derived streamline and catchment boundary accuracy

Accurate DEM-derived streamlines and catchment boundaries are essential for hydrological modelling. Due to the popularity of hydrological parameters derived mainly from free DEMs, it is essential to investigate the accuracy of these parameters. This study compared the spatial accuracy of streamlines and catchment boundaries derived from available digital elevation models in South Africa. Two versions of Stellenbosch University DEMs (SUDEM5 and DEMSA2), the second version of the 30 m advanced spaceborne thermal emission and reflection radiometer global digital elevation model (ASTER GDEM2), the 30 and 90 m shuttle radar topography mission (SRTM30 and SRTM90 DEM), and the 90 m Water Research Commission DEM (WRC DEM) were considered. As a reference, a 1 m GEOEYE DEM was generated from GeoEye stereo images. Catchment boundaries and streamlines were extracted from the DEMs using the Arc Hydro module. A reference catchment boundary was generated from the GEOEYE DEM and verified during field visits. Reference streamlines were digitised at a scale of 1:10 000 from the 1 m orthorectified GeoEye images. Visual inspection, as well as quantitative measures such as correctness index, mean absolute error, root mean squares error and figure of merit index were used to validate the results. The study affirmed that high resolution (&lt;30 m) DEMs produce more accurate parameters and that DEM source and resampling techniques also play a role. However, if high resolution DEMs are not available, the 30 m SRTM DEM is recommended as its vertical accuracy was relatively high and the quality of the streamlines and catchment boundary was good. In addition, it was found that the novel Euclidean distancebased MAE and RMSE proposed in this study to compare reference and DEM-extracted raster datasets of different resolutions is a more reliable indicator of geometrical accuracy than the correctness and figure of merit indices.Keywords: hydrology, catchment delineation, digital elevation model, correctness index, figure of merit index, Euclidean distance inde

Giving gully detection a HAND:Testing the scalability and transferability of a semi-automated object-orientated approach to map permanent gullies

Gully erosion can incur on- and off-site impacts with severe environmental and socio-economic consequences. Semi-automated mapping provides a means to map gullies systematically and without bias, providing information on their location and extent. If used temporally, semi-automated mapping can be used to quantify soil loss and identify soil loss source areas. The information can be used to identify mitigation strategies and test the efficacy thereof. We develop, describe, and test a novel semi-automated mapping workflow, gHAND, based on the distinct topographic landform features of a gully to enhance transferability to different climatic regions. Firstly, topographic heights of a Digital Elevation Model are normalised with reference to the gully channel thalweg to extract gully floor elements, and secondly, slope are calculated along the direction of flow to determine gully wall elements. As the gHAND workflow eliminates the need to define kernel thresholds that are sensitive towards gully size, it is more scalable than kernel-based methods. The workflow is rigorously tested at different gully geomorphic scales, in contrasting geo-environments, and compared to benchmark methods explicitly developed for region-specific gullies. Performance is similar to benchmark methods (variance between 1.4 % and 14.8 %). Regarding scalability, gHAND produced under- and over-estimation errors below 30.6 % and 16.1 % for gullies with planimetric areas varying between 1421.6 m2 and 355403.7 m2, without editing the workflow. Although the gHAND workflow has limitations, most markedly the requirement of manually digitising gully headcuts, it shows potential to be further developed to reliably map gullies of small- to large-scales in different geo-environments

Comparative evaluation of lumpy skin disease virus-based live attenuated vaccines

Vaccines form the cornerstone of any control, eradication and preventative strategy and this is no different for lumpy skin disease. However, the usefulness of a vaccine is determined by a multiplicity of factors which include stability, efficiency, safety and ease of use, to name a few. Although the vaccination campaign in the Balkans against lumpy skin disease virus (LSDV) was successful and has been implemented with success in the past in other countries, data of vaccine failure have also been reported. It was therefore the purpose of this study to compare five homologous live attenuated LSDV vaccines (LSDV LAV) in a standardized setting. All five LSDV LAVs studied were able to protect against a challenge with virulent LSDV. Aside from small differences in serological responses, important differences were seen in side effects such as a local reaction and a Neethling response upon vaccination between the analyzed vaccines. These observations can have important implications in the applicability in the field for some of these LSDV LAVs

Zebrafish : a resourceful vertebrate model to investigate skeletal disorders

Animal models are essential tools for addressing fundamental scientific questions about skeletal diseases and for the development of new therapeutic approaches. Traditionally, mice have been the most common model organism in biomedical research, but their use is hampered by several limitations including complex generation, demanding investigation of early developmental stages, regulatory restrictions on breeding, and high maintenance cost. The zebrafish has been used as an efficient alternative vertebrate model for the study of human skeletal diseases, thanks to its easy genetic manipulation, high fecundity, external fertilization, transparency of rapidly developing embryos, and low maintenance cost. Furthermore, zebrafish share similar skeletal cells and ossification types with mammals. In the last decades, the use of both forward and new reverse genetics techniques has resulted in the generation of many mutant lines carrying skeletal phenotypes associated with human diseases. In addition, transgenic lines expressing fluorescent proteins under bone cell- or pathway- specific promoters enable in vivo imaging of differentiation and signaling at the cellular level. Despite the small size of the zebrafish, many traditional techniques for skeletal phenotyping, such as x-ray and microCT imaging and histological approaches, can be applied using the appropriate equipment and custom protocols. The ability of adult zebrafish to remodel skeletal tissues can be exploited as a unique tool to investigate bone formation and repair. Finally, the permeability of embryos to chemicals dissolved in water, together with the availability of large numbers of small-sized animals makes zebrafish a perfect model for high-throughput bone anabolic drug screening. This review aims to discuss the techniques that make zebrafish a powerful model to investigate the molecular and physiological basis of skeletal disorders