6 results on '"van Blerk, J J"'
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2. Unpacking satellite pixels: UAVs reveal fine-scale drivers of land surface phenology in a winter rainfall shrubland.
- Author
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van Blerk, J J, Slingsby, J A, and West, A G
- Published
- 2024
- Full Text
- View/download PDF
3. Assessing vulnerability to embolism and hydraulic safety margins in reed‐like Restionaceae.
- Author
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West, A. G., Atkins, K., van Blerk, J. J., and Skelton, R. P.
- Subjects
EMBOLISMS ,TURGOR ,TEST methods ,XYLEM ,HYDRAULICS - Abstract
The African Restionaceae (Poales), the dominant graminoid layer in the megadiverse Cape Floristic Region of South Africa, are distributed across a wide range of moisture availability, yet currently there is very little known about the underlying hydraulics of this group.We tested two methods for measuring culm vulnerability to embolism, the optical and pneumatic methods, in three species of Cannomois ranging in habitat from semi‐riparian (Cannomois virgata) to dryland (Cannomois parviflora and C. congesta). Estimates of culm xylem vulnerability were coupled with measures of turgor loss point (ΨTLP) and minimum field water potential (ΨMD) to assess hydraulic safety margins.The optical and pneumatic methods produced similar estimates of P50, but differed for P12 and P88. All three species were quite vulnerable to embolism, with P50 of –1.9 MPa (C. virgata), −2.3 MPa (C. congesta), and −2.4 MPa (C. parviflora). Estimates of P50, ΨTLP and ΨMD aligned with habitat moisture stress, with highest values found in the semi‐riparian C. virgata. Consistent differences in P50, ΨMD and ΨTLP between species resulted in consistent hydraulic safety margins across species of 0.96 ± 0.1 MPa between ΨMD and P50, with onset of embolism occurring 0.43 ± 0.04 MPa after ΨTLP for all three species.Our study demonstrates that restio occupancy of dry environments involves more than the evolution of highly resistant xylem, suggesting that other aspects of water relations are key to understanding trait–environment relationships in this group. [ABSTRACT FROM AUTHOR]
- Published
- 2024
- Full Text
- View/download PDF
4. Analysis of a post-closure safety assessment methodology for radioactive waste disposal systems in South Africa
- Author
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Van Blerk, J. J., Botha, J. F., Van Blerk, J. J., and Botha, J. F.
- Abstract
English: Radioactive waste in South Africa is generated through the nuclear fuel cycle and the application of radioactive materials in industry, science and medicine. The radioactive waste is presently disposed at Vaalputs in Bushmanland and Thabana at Pelindaba in near-surface disposal facilities. No strategy exists at present for the disposal of high level waste. The objective of radioactive waste management and its underlying principles is to ensure that human health and the environment are protected at all times, without imposing an undue burden on future generations. This implies that, before any long-term management strategy of radioactive waste disposal can be implemented, the impact of the disposed waste must be determined as a function of time-a procedure referred to as post-closure safety assessment. In this thesis, a methodology to perform post-closure safety assessments of radioactive waste disposal systems in South Africa and other parts of Africa is described. Not only will it contribute significantly to reassess the suitability of current waste disposal practices, but also lays the foundation for future disposal practices. The proposed methodology-an integrated approach to radioactive waste management-is aimed at: (a) ensuring the safety of the present public and future generations, (b) enhancing the public acceptance of the methodology, (c) keeping the expenditure associated with the implementation of the methodology at a minimum. The methodology recognises the interdependence between operational phase activities and the post-closure behaviour of the disposal system. It is an iterative process that considers site-specific, prospective evaluations of the post-closure phase to ensure that the disposal system will comply with internationally accepted criteria, within reasonable limits. Provision is therefore made to identify the data, design and other needs that will contribute towards the achievement of this objective. The first step in this procedure, Afrikaans: In Suid Afrika word radioaktiewe afval gegenereer deur die kernbrandstofsiklus en die gebruik van radioaktiewe bronne in die industrie, die wetenskap en geneeskunde. Lae en middel energie radioaktiewe afval word tans weggedoen in oppervlak wegdoeningsfasiliteite by Vaalputs in die Boesmanland en geberg in vlak bergingsfasiliteite by Thabana by Pelindaba. Geen strategie bestaan egter tans vir die wegdoening van hoë energie afval nie. Die doel van radioaktiewe afvalbestuur en die onderliggende beginsels daarvan, is om te verseker dat die mens se gesondheid asook die omgewing te alle tye beskerm word, sonder om onnodige druk op toekomstige geslagte te plaas. Dit impliseer dat, alvorens enige langtermyn bestuurstrategie vir radioaktiewe afval geïmplementeer kan word, moet die impak van die gebergde afval as 'n funksie van tyd bepaal word-'n prosedure waarna verwys word as 'n nasluiting veiligheidsvasstelling (post-closure safety assessment). 'n Metodiek om so 'n nasluiting veiligheidsvasstelling vir radioaktiewe afval wegdoeningsisteme in Suid Afrika en ander dele van Afrika uit te voer, word in hierdie proefskrif beskryf. Nie alleen bied dit 'n beduidende bydrae tot die herevaluering van bestaande wegdoeningsisteme nie, maar verskaf dit ook 'n basis vir toekomstige wegdoeningspraktyke wêreldwyd. Die voorgestelde metodiek-'n geïntegreerde benadering tot radioaktiewe afvalbestuurhet ten doelom: (a) die veiligheid van huidige en toekomstige geslagte te verseker, (b) publieke aanvaarding van die metodiek te bevorder, (c) finansiële uitgawes geassosieer met die implementering van die metodiek te minimeer. Die metodiek aanvaar die interafhanklikheid wat daar bestaan tussen aktiwiteite binne die operasionale fase en die nasluitingsgedrag van die sisteem. Dit is 'n iteratiewe proses wat bestaan uit terrein spesifieke, toekomstige evaluasies van die nasluiting fase, met die doel om te verseker dat die wegdoeningsisteem voldoen aan internasionaal aanvaarbare kriteria, Atomic Energy Corporation of South Africa (Ltd.)
- Published
- 2000
5. Numerical solution of partial differential equations on curved domains by collocation
- Author
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Van Blerk, J. J., primary and Botha, J. F., additional
- Published
- 1993
- Full Text
- View/download PDF
6. Post-fire summer rainfall differentially affects reseeder and resprouter population recovery in fire-prone shrublands of South Africa.
- Author
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van Blerk JJ, West AG, Altwegg R, and Hoffman MT
- Subjects
- Climate, Droughts, Soil, South Africa, Fires
- Abstract
Summer rainfall can have strong effects on post-fire mediterranean-type shrubland recovery patterns, with potentially long-lasting implications on communities. Our three-year field rainfall manipulation experiment tested post-fire survival and physiological responses of reseeders and resprouters to contrasting summer rainfall patterns in Fynbos and Renosterveld shrublands in South Africa. Climate projections are uncertain for this region but indicate that increased convective summer rainfall events could occur. We irrigated treatment plots during the hottest summer months (i.e. Jan, Feb, March) to contrast the naturally dry summer conditions. This allowed for assessments of the potential limiting effects of summer drought on post-fire vegetation recovery and the responsiveness of vegetation to moisture inputs during this time. Natural summer droughts led to leaf dehydration, reduced photosynthesis and reduced photosynthetic capacity. This had a particularly severe effect on reseeders during the first summer after fire leading to high mortality rates. Summer irrigations strongly reduced levels of reseeder stress and mortality. Resprouters in both vegetation types were physiologically less sensitive to rainfall patterns and showed little drought-related mortality. Comparisons of final population sizes with emergence and survival patterns showed that summer rainfall during the first summer after fire had the potential to strongly alter reseeder population sizes. The physiological sensitivity of plants to summer rainfall patterns was higher in shrubland communities occurring on fine-textured, moderately fertile soils (e.g. Renosterveld). Shrublands occurring on sandy, nutrient-poor soils (e.g. Fynbos) were remarkably insensitive to summer drought after the first summer with lower irrigation responses. Our study demonstrated the potential for variation in post-fire summer rainfall to strongly affect reseeder and resprouter population recovery patterns., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)
- Published
- 2021
- Full Text
- View/download PDF
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