The effect of irrigation practices in the Breë River Valley on the salt content of a small river

Summary

The effect of varying irrigation return flow volumes as a possible explanation for the fluctuations in the mean monthly baseflow TDS content of a tributary in the Breë River Valley (South-Western Cape Province, Rep. of South Africa) has been investigated. A conceptual computer model was used to simulate the movement of water and solutes in 769 ha of soil in an area where vineyards are irrigated intensively. Important input requirements of the model are the physical and chemical properties of the irrigated soils, consumptive use data, and root distribution patterns.

It was found that the present system of vineyard irrigation which involves a large amount of water being applied as a pre-bud-burst irrigation in late August or early September, followed by more or less fixed amounts of water being applied at set frequencies during the rest of the irrigation season, results in deep percolation losses and accompanying salt loads which are much bigger during the last six months than during the first six months of the year. The results further indicate that during the period January to June, capillary rise will exceed deep percolation. On a half yearly time basis, these computer predictions were in fair accordance with the observed TDS content of the receiving river, which for three consecutive years (1978–1980) have been substantially higher during the period July to December than from January to June. However, on a monthly time basis, it was found that the fair comparison between predicted (in the irrigation return flow) and observed (in the river) mean monthly TDS contents for the months July to December does not apply to the period January to June.