The current project was devised in the knowledge that increased sodicity and salinity of percolating water will alter the saturated conductivity of many soils, especially sodic soils. Additionally, on commencement of the project the Cotton industry had minimal data or experimentation on the scale or driving forces behind deep drainage (DD) with furrow irrigation. The project had two principal objectives: to assess the effect of increasing salinity and sodicity of irrigation water on the DD under various cotton soil types, and to utilise drainage lysimeters to directly measure DD and correlate responses in column experiments to field response.

To obtain data for the first objective, a glasshouse experiment was conducted to assess the effect on DD of changes in salinity and sodicity levels, waters being applied to large intact cores collected from each field site. For the second objective, nine drainage lysineters were installed in 3 cotton fields (3 lysimeters over each field) and data collected over a period of two cotton seasons.

There were four major sets of results. Firstly, physico-chemical analysis of the soils (to 150 cm) at each site, showed the three sites to be quite different, particularly in their clay content being on average 75%, 55%, and 45% for the Dalby, Goondiwindi and St George sites, respectively.

Secondly, the quality of the irrigation water applied at each site was quite different, in terms of its salinity, with electrical conductivity of 3358, 498 and 137 (μS/cm) for Dalby, Goondiwindi and St George, respectively.

Thirdly, the drainage lysimeters (located at 115 to 150 cm below the soil surface collected water that is deemed “lost from the cotton root-zone”and hence is deep drainage. At the Goondiwindi and St George sites in both seasons (2002-3 and 2003-4) the ranking of the amount of DD at the three in-field locations was the same, with the head ditch receiving the most DD, then the mid and the least at the tail ditch end. Long-term inundation at the head ditch rationalises these results, whereas the tail end may remain dry if irrigation siphons are stopped early. In terms of quantities of DD, the Dalby site had the greatest recorded DD; 222 mm (= 2.2 ML of water) at the head ditch end in the first season but the other two sites also recorded several instances of >90 mm of DD in one season (0.9 ML) at the head and mid field locations.

Fourthly, in a glass house experiment, irrigation with high Electrical Conductivity (EC) and low Sodium Adsorption Ratio (SAR, a measure of sodicity) water increased drainage by 4, and 2 fold in Dalby and St George soils, respectively, compared with irrigating with fresh water; however, Goondiwindi did not show any change. Irrigation with low EC and high SAR water resulted in 2, 4, and 3 fold greater drainage in Dalby, St George, and Goondiwindi soils, respectively compared to fresh water. Irrigation with high EC and high SAR water showed 5, 3, and 1.3 fold drainage increase in Dalby, St George, and Goondiwindi soils, respectively compared to fresh water. These results demonstrate an interaction between soil type (probably clay content) and water quality on deep-drainage. Water lost to deep drainage was increased more by salinity than sodicity of the irrigation water in Dalby soil (high clay content), and more by the sodicity rather than salinity of the irrigation water in St George and Goondiwindi soils.