The impact of a lucerne phase in a crop rotation on groundwater recharge in south-west Australia Phil Ward, Perry Dolling and Frank Dunin CSIRO Plant Industry, Private Bag No 5, Wembley WA 6913. www.csiro.au Email phil.ward@csiro.au Abstract 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. Key Words LeBuM, phase rotation, simulation, water use Introduction In southern Australia, dryland salinity continues to spread, and without remedial action, is expected to affect 30% of the landscape by 2050 (1). It is caused by an interaction between climate, in which the majority of rainfall arrives at the time of year when potential evaporation is lowest, and the replacement of native vegetation with agricultural crops and pastures. The soil in the shallow root zone of crops and pastures is unable to store the normal winter excess of water, and so leakage to the groundwater is enhanced relative to the native system. The incorporation of deep-rooted perennials such as lucerne into farming systems while the groundwater is still deep enough not to interfere with plant growth is one of the remedial actions suggested to try to restore the groundwater balance (2,3). In theory, lucerne acts by creating a zone of dry soil (referred to as a ‘buffer’) below the normal rooting depth of annual crops and pastures, partially reproducing the effect of the native vegetation. In a phase rotation, the buffer is firstly created during the lucerne phase, and then refilled during the cropping phase with water that would otherwise have contributed to groundwater recharge. The buffer is then re-created in the next lucerne phase, and so on. However, leakage is very variable from year to year (4,5), and can range between 0 and 250 mm. The impact of this variability makes it difficult to assess the likely impact of a lucerne phase on long-term average leakage rates. Although the application of APSIM and similar models to phase rotations is currently under development, there is a need for a simple, user-friendly model to assist with the design of suitable rotations for recharge control. For this reason, we developed the Leakage/Buffer Model (LeBuM), which uses long- term modelled leakage amounts under annual crops or pastures to calculate the average leakage rate for a given phase rotation and buffer size. In this paper, LeBuM is described and then used to assess the impact of varying the length of the annual and perennial phases on long-term average leakage for two contrasting environments and two soil types in south-west Australia. Methods LeBuM Input for the Leakage/Buffer Model (LeBuM) consists of a long-term series of annual leakage amounts (in mm) under traditional crops. In this paper we used 81 years of leakage data under wheat crops generated by the APSIM model (4). Parameters in LeBuM include the length of the crop phase (between 0 and 10 years, © 2003 Australian Society of Agronomy. "Solutions for a better environment". Proceedings of the 11th Australian Agronomy Conference, 2-6 Feb. 2003, Geelong, Victoria. Published on CDROM ISBN 0-9750313-0-9. Web site www.regional.org.au/au/asa 1