The impact of a lucerne phase in a crop rotation on groundwater recharge in south-west Australia, Invited presentation, 11th Australian Agronomy Conference: Solutions for a new environment

The south-west of Australia is subject to a Mediterranean-style climate, with cool wet winters and hot dry summers. In the 300-500 mm rainfall zone, broad-scale clearing of native vegetation, and its replacement by annual crops and pastures, has resulted in increased groundwater recharge, and the development of large areas of dryland salinity. The deep-rooted perennial pasture plant, lucerne (Medicago sativa L.), has shown promise in reducing groundwater recharge. However, in a region where recharge in any one year can vary between 0 and 250 mm, lucerne’s impact on average long-term recharge (currently 10-60 mm/year) is not known. In this paper, the impact of changing the length of the lucerne phase in rotation with a phase of annual crops was modelled over 81 years for the 300 and 500 mm rainfall zones, assuming that the buffer created by lucerne was completely filled before recharge commenced. In percentage terms, leakage was more readily controlled in the drier environment, on a duplex soil, where a rotation involving 50 % lucerne (eg. 3 years of lucerne followed by 3 years of crop) reduced leakage by 95%. On an acid loamy sand in the wetter environment, the same rotation reduced leakage by 40%. Once appropriate leakage targets are established, the model described here can be used to design suitable rotations