To understand the factors involved in lucerne reducing drainage below the root-zone and influencing lucerne biomass production and water extraction were analysed in the south of Western Australia. The lucerne was grown for 3 years before removal. The factors investigated as part of the water extraction analysis included the rate of advance of the extraction front or extraction front velocity (EFV, mm/day), the soil plant-available water-holding capacity (PAWC, mm/m soil), and the temporal change in soil water deficit (drainage buffer, mm). The drainage buffer is related to the EFV and PAWC.

A site with deep sand had the highest EFV (mean of 9.2 mm/day) but the lowest PAWC (mean of 32 mm/m soil) to a depth of 4 m. In the duplex soils the EFV was 18–34% of the deep sand EFV and the PAWC was 60–222% higher than the deep sand PAWC to a depth of 1.6–2.1 m. The EFV was reduced by the higher clay content and sodicity in the B horizon of the duplex soils. The highest drainage buffer measurements occurred in the deep sand site and the better structured duplex soils and therefore these soils will have the greater effect on reducing drainage below the root-zone. However, lucerne was able to create a drainage buffer to at least a depth of 1.5 m over 3 years and therefore contribute to a reduced drainage even on the most sodic and saline sites. Low soil pH did not affect the drainage buffer as much as soil texture and structure.

Variation in biomass production of lucerne-based pastures was positively related to rainfall and water use (taking into account soil water storage and drainage losses) across sites, explaining approximately 50% of the biomass variation. Rainfall and water use could therefore be used for predicting lucerne biomass production in Western Australia. Biomass water use efficiency was highest in spring (15 kg/ha.mm) and least during autumn (4.5 kg/ha.mm).