Water- en soutbalanse van geselekteerde besproeide gronde van die benede Bergrivier opvanggebied

Engelbrecht, Hendrik Nicolaas (2004-04)

Thesis (MSc)--University of Stellenbosch, 2004.

Thesis

ENGLISH ABSTRACT: A study was conducted between 2000 and 2002 in the lower Berg River catchment of the semi-arid Western Cape Province to investigate the effect of irrigation developments (wheat fields to table grapes) on the quality of the Berg River as well as the sustainability of these developments. This study was necessitated by the mineralisation of both the Breede and Berg River, probably due to irrigation together with the higher pressure on these water resources from the growing demand for fresh water from the Cape Town metropolitan area. Water and Salt balances of two dominant soil patterns (Glenrosa/Swartland and Oakleaf/Hutton soil forms) from the catchment area were monitored at two localities. Vineyard blocks of different ages in the same soil forms were used to study the effect of irrigation over time. The electrical conductivity of the soil water extracts (ECe) was measured three times over depth at the selected sites and the quality of irrigation and drainage return-flow were monitored. Soil water storing capacities of these stony (high rough fragment' fractions) soils were also characterised for more efficient drainage quantity prediction. Glenrosa/Swartland soils showed the highest ECe's (200-400 mx.m") that decreased significantly after four to five years of irrigation and distribution became more uniform over depth. These high salt concentrations were explained due to natural occurrence of salts, mineral weathering and salts from the irrigation water. Oakleaf/Hutton soils showed lower ECe's « 200 nfS.m") but they are subject to the quality of the irrigation water, which is not that constant over years. Localised salt accumulation was observed due to the irrigation method and became more prominent over years. Drainage quantities were under predicted because of the big/high rough fragment fractions and the occurrence of preferential-flow. This prohibited the accurate calculation of the soil water house holding characteristics. It also led to the under calculation of salt return-flow, but it was very evident that the most salt retumflow occurred from the Glenrosa/Swartland soils. Effective irrigation management to achieve and maintain optimal soil water levels and to reduce the negative impact of high ECe's is essential for the sustainability of these irrigation developments. At present it does not seem that the irrigation return-flow is causing dramatic mineralisation of the lower Berg River, but it is a situation that must be monitored as the developments extent and become older (come into production). It seems that irrigation return-flow only has an influence on the quality of the river at the beginning of such a development, but after about three years the influence directly correlates with the quality of irrigation water and mineral weathering.

AFRIKAANSE OPSOMMING: 'n Studie is gedurende die jare 2000 tot 2002 in die benede Bergrivier opvanggebied van die semi-ariede Wes-Kaap Provinsie gedoen om die effek van besproeiingsontwikkelings (koringlande na tafeldruiwe) op die kwaliteit van die Bergrivier se water, asook die volhoubaarheid van besproeiingslandbou te ondersoek. Hierdie studie was genoodsaak deur toenemende versouting van beide die Breede- en Bergrivier, waarskynlik a.g.v. besproeiing, tesarne met groter druk op hierdie waterbronne van wee die groeiende aanvraag na vars water deur die Kaapstadse metropool. Water- en soutbalanse van twee dominante grondpatrone (Glenrosa/Swartland en Oakleaf/Hutton grondvorms) binne die opvanggebied was by twee lokaliteite gemoniteer. Wingerdblokke van verskillende ouderdomme binne dieselfde grondvorms is gebruik om die effek van besproeiing oor tyd te ondersoek. Die elektriese geleiding van die grondwaterekstrak (EGe) was drie maal op verskillende dieptes by geselekteerde persele gemeet en die kwaliteit van besproeiingswater en dreineringsterugvloei was deurgaans gemoniteer. Grondwaterstoringskapasiteite van hierdie klipryke ( hoe 'growwe fragment fraksie) gronde was ook gekarakteriseer vir meer doeltreffende voorspellings van dreineringshoeveelhede. Glenrosa/Swartland gronde het die hoogste EGe's getoon (200-400 mx.m') wat na vier tot vyf jaar se besproeiing aansienlik afgeneem het en meer uniform met diepte versprei was. Die hoe soutkonsentrasies is veroorsaak deur die natuurlike voorkoms van sout in die grond, minerale verwering en sout vanuit die besproeiingswater. Oakleaf/Hutton gronde het laer EGe's getoon « 200 mS.m-I ), maar hierdie gronde se soutinhoud is ook onderhewig aan die kwaliteit van die besproeiingswater en dreinering wat oor jare nie baie konstant was en nog is nie. Gelokaliseerde soutaansarneling tussen wingerdrye is ook waargeneem a.g.v. die besproeiingsmetode en dit word meer prominent oor jare. Dreineringshoeveelhede was onder-voorspel a.g.v. die groot/hoe growwe fragment fraksies en voorkoms van voorkeurvloei, wat die akkurate modellering van grondwaterhuishouding beperk het. Dit het ook daartoe gelei dat soutterugvloei ondervoorspel was, maar dit was duidelik dat die meeste soutterugvloei vanuit die Glenrosa/Swartland gronde plaasvind. Effektiewe besproeiingsbestuur, om optimale grondwaterpeile te handhaaf en om negatiewe effekte van hoe EGe te beperk, is noodsaaklik vir die volhoubaarheid van hierdie tipe besproeiingsontwikkelings. Tans blyk dit nie of besproeiingsterugvloei drarnatiese versouting van die benede Bergrivier veroorsaak nie, maar dit is egter 'n situasie wat deeglik gemoniteer moet word soos wat ontwikkelings uitbrei en ouer word (in produksie kom). Besproeiingsterugvloei het dus net aan die begin van so 'n ontwikkelling 'n invloed op die EG van die rivier, maar na ongeveer drie jaar hou die invloed direk verband met die kwaliteit van die besproeiingswater en minerale verwenng.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/50227
This item appears in the following collections: