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

[摘要] ENGLISH ABSTRACT:A study was conducted between 2000 and 2002 in the lower Berg River catchment ofthe semi-arid Western Cape Province to investigate the effect of irrigationdevelopments (wheat fields to table grapes) on the quality of the Berg River as well asthe sustainability of these developments. This study was necessitated by themineralisation of both the Breede and Berg River, probably due to irrigation togetherwith the higher pressure on these water resources from the growing demand for freshwater from the Cape Town metropolitan area.Water and Salt balances of two dominant soil patterns (Glenrosa/Swartland andOakleaf/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 effectof irrigation over time. The electrical conductivity of the soil water extracts (ECe) wasmeasured three times over depth at the selected sites and the quality of irrigation anddrainage return-flow were monitored. Soil water storing capacities of these stony(high rough fragment' fractions) soils were also characterised for more efficientdrainage quantity prediction.Glenrosa/Swartland soils showed the highest ECe's (200-400 mx.m) that decreasedsignificantly after four to five years of irrigation and distribution became moreuniform over depth. These high salt concentrations were explained due to naturaloccurrence 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 thequality of the irrigation water, which is not that constant over years. Localised saltaccumulation was observed due to the irrigation method and became more prominentover years. Drainage quantities were under predicted because of the big/high roughfragment fractions and the occurrence of preferential-flow. This prohibited theaccurate calculation of the soil water house holding characteristics. It also led to theunder calculation of salt return-flow, but it was very evident that the most salt retumflowoccurred from the Glenrosa/Swartland soils. Effective irrigation management toachieve and maintain optimal soil water levels and to reduce the negative impact ofhigh ECe's is essential for the sustainability of these irrigation developments. Atpresent it does not seem that the irrigation return-flow is causing dramaticmineralisation of the lower Berg River, but it is a situation that must be monitored asthe developments extent and become older (come into production). It seems thatirrigation return-flow only has an influence on the quality of the river at the beginningof such a development, but after about three years the influence directly correlateswith the quality of irrigation water and mineral weathering.

[发布机构] Stellenbosch University