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dc.contributor.advisorBennie, A. T. P.
dc.contributor.authorHoffman, Josias Eduard
dc.date.accessioned2018-05-09T05:59:02Z
dc.date.available2018-05-09T05:59:02Z
dc.date.issued1997-12
dc.identifier.urihttp://hdl.handle.net/11660/8254
dc.description.abstractEnglish: Research results worldwide show that approximately 70% of the annual rainfall is lost due to evaporation from the soil. It is therefore of utmost importance to store as much water in the root zone during the fallow period as possible. The aim of this study was to find methods of reducing evaporation losses through different cultivation practices and to develop procedures to predict soil evaporation. The soil evaporation process was studied in detail by using micro Iysimeters with an inside diameter of 60 mm and a length of 315 mm. Soil water flux and diffusivity of the top 100 mm was studied in 20 mm layers, and from 200 - 300 mm in 50 mm layers. The different stages of evaporation as well as the duration of the constant phase were determined in this study. The potential evaporation was determined with different methods and compared with the class A-pan. Several methods to reduce soil evaporation were investigated. This included different soil manipulation practices and levels of shading. The influence of soil texture on. the evaporation rate and on the cumulative evaporation was also investigated. It was established that the constant evaporation stage (phase I) lasted only a few hours, and was therefore omitted from the development of evaporation procedures. Good correlation was found between the measured evaporation of evaporation trays and the class A-pan. The results of this study showed that soil evaporation continues during the first two nights after wetting (rainfall or irrigation). The atmospheric evaporative demand has an influence on the evaporation of soil up to the third day after wetting. An increase in evaporation due to a temperature increase was higher in undisturbed soil than in disturbed soil. The evaporation rate decreased with an increase in the level of disturbance (looseness) of the soil. During the first week after wetting the evaporation losses were mainly from the top 100 mm. After nine days the soil started drying out from deeper than 100 mm and from all the deeper layers simultaneously. The evaporation due to some of the different soil surface treatments differed significantly. The difference was mainly ascribed to the initial soil water content rather than to the soil manipulation as such. The soil water content available for evaporation (evaporativity) had the greatest influence on the evaporation rate and on the cumulative evaporation of the soil. Shading of the soil surface decreased evaporation losses over the short term for example over the first ten days and 20 days after wetting for sandy and sandy clay loam soils respectively. This decrease increased with an increase in the shading percentage. Over long drying periods, shading had little effect on evaporation losses. The cumulative soil surface evaporation equations of Rose (1968), Ritchie (1972), Kijne (1973) and AI-Khafaf et. al. (1989) fitted the data of the different experiments well. Equations were developed with which the regression coefficients and empirical constants could be calculated from measurable parameters. Such were the silt plus clay content, initial soil water content (0), desorptivity (01-00) and the evaporativity water content (01-00)Z1 of the soil layer. Equations were also developed to calculate the water content at which evaporation stop (00) and the depth of the evaporating soil layer (Z1). The Ritchie equation predicted the actual soil evaporation the most accurately. This equation is therefore recommended for inclusion in computer modeling of evaporation. The following two equation are recommended: See full text for formulae.en_ZA
dc.description.abstractAfrikaans: Navorsingsresultate wereldwyd beraam dat sowat 70 % van die jaarlikse reënval as gevolg van grondoppervlakverdamping onder droë en halfdroë klimaatstoestande verlore gaan. Dit is daarom belangrik dat so veel as moontlik reënval gedurende periodes van wateropgaring in die bewortelingsone van die profielopgegaar moet word. Die studie het gevolglik ten doel gehad om metodes te vind waarmee grondoppervlakverdamping gedurende die wateropgaringsperiode verminder kan word, en om prosedures te ontwikkel met behulp waarvan grondoppervlakverdamping voorspel kan word. 'n Detail studie van die verdampingsproses is met behulp van mikrolisimeters met 'n binnedeursnee van 60 mm en 'n lengte van 315 mm uitgevoer. Die vloed deur en die diffusiwiteitskoëffisiënte van die boonste 100 mm grond, is in 20 mm lagies, en dié van die 100 tot 300 mm diepte interval, in 50 mm lagies ondersoek. Die verskillende stadiums waarin die verdampingsproses verdeel word, en die tydsduur van die konstante tempo fase van verdamping is in die studie bepaal. Die potensiële grondoppervlakverdamping is met behulp van 'n aantal metodes bepaal, en met die klas A-pan vergelyk. J Verskillende metodes om grondoppervlakverdamping te verminder, soos byvoorbeeld verskillende, grondoppervlaktoestande asook verskillende vlakke van beskaduwing, is met behulp van verskillende eksperimente nagevors. Die invloed van grondtekstuur op die verdampingstempo asook op die kumulatiewe grondoppervlakverdamping, is ondersoek. Die konstante tempo fase het in die ondersoek slegs 'n paar uur geduur en is daarom nie in die ontwikkeling van voorspellingsprosedures in ag geneem nie. Verdamping vanuit verdampingsbakke het 'n baie goeie korrelasie met die waardes van die klas A-pan gehad. Die studie het aangetoon dat grondoppervlakverdamping gedurende die nag ook plaasvind. Die styging in verdampingstempo met temperatuur was hoër by die onversteurde grond as by dié van die bewerkte gronde. Die verdampingstempo van die gronde van die verskillende behandelings het met 'n toename in losheid van die bewerkte grond afgeneem. Gedurende die eerste week na benatting deur reënval of besproeiing, het die waterverliese uit die grondprofiel hoofsaaklik vanuit die boonste 100 mm plaasgevind. Na sowat nege dae het die uitdroging oor die hele diepte van 300 mm plaasgevind. Grondoppervlaktoestande het wel betekenisvolle verskille in verdamping tussen sommige van die verskillende grondbewerkingspraktyke veroorsaak. Die verskil word hoofsaaklik aan 'n verskil in die aanvangswaterinhoud eerder as die bewerkingspraktyk per se toegeskryf. Die hoeveelheid verdampbare waterinhoud het die grootste invloed op die verdampingstempo en kumulatiewe verdamping van gronde gehad. Grondoppervlakverdamping kan met behulp van beskaduwing verminder word. Die effek van beskaduwing was groter hoe korter die verdampingsperiode, en die effektiwiteit het verder met 'n toename in kleipersentasie en persentasie beskaduwing verhoog. Die Ritchiemodel het die kumulatiewe grondoppervlakverdamping met ongeveer 4 % oorberaam terwyl die Kijnemodel die kumulatiewe verdamping 30 % te laag voorspel het. 'n Aanqepaste Kijnevergelyking was egter baie akkurater en was met minder as 5 % uit. Die vergelykings waarmee die k-empiriese konstante van die Rosemodel vanaf die desorpsiwiteit (01-00) en verdampbare waterinhoud 01-00)Z1 beraam is, het die kumulatiewe verdamping met 18 tot 20 % onderskat. Deur die k-empiriese konstante vanaf slik- plus kleipersentasie te beraam, het 'n 17 tot 20 % oorberaming tot gevolg gehad. Die reglynige verwantskappe vir die beraming van die AI-Khafafmodel se regressiekoëffisiënte en empiriese konstantes het nie baie goeie passings opgelewer nie. Die beramingsprosedure van die Ritchiemodel (Ritchie, 1972) het grondoppervlakverdamping die akkuraatste voorspel. Die volgende twee vergelykings wat as insette die tydsverloop sedert benatting of tussen benattings (t), die aanvangswaterinhoud (01), volumetriese waterinhoud waar grondoppervlakverdamping staak (00), die dikte van die verdampingsone (Z1) en die slik- plus kleipersentasie gebruik, word vir die doel aanbeveel, naamlik: Sien volteks vir formules.en_ZA
dc.description.sponsorshipWaternavorsingskommissieen_ZA
dc.description.sponsorshipInstituut vir Tropiese en Subtropiese Gewasse, Nelspruiten_ZA
dc.language.isoafen_ZA
dc.publisherUniversity of the Free Stateen_ZA
dc.subjectGroundwater -- Samplingen_ZA
dc.subjectGroundwater -- Managementen_ZA
dc.subjectEvaporation -- Measurementen_ZA
dc.subjectEvapotranspiration -- Measurementen_ZA
dc.subjectThesis (Ph.D. (Soil, Crop and Climate Sciences))--University of the Free State, 1997en_ZA
dc.titleKwantifisering en voorspelling van grondwaterverdamping by droëlandgewasproduksieen_ZA
dc.typeThesisen_ZA
dc.rights.holderUniversity of the Free Stateen_ZA


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