Factors influencing the soil water dynamics beneath native perennial pastures in the high rainfall zone of South-Eastern Australia

This study found that botanical composition has a major impact on the size of soil water deficits created by the end of the summer-autumn period. Native perennial pastures were found to create soilwater deficits up to 85 mm greater than annual ryegrass pastures. When comparing native perennial grasses, soil water deficits beneath Bothriochloa macra was up to 40 mm greater than Austrodanthonia spp. At a broaderscale, such differences are likely to reduce the amount of salt exported from catchments by thousands of tonnes annually. Such an impact is likely to influence management decisions.Soil water extraction was found associated with the establishment of deep roots and the maintenance of green leaf further into summer. Under monoculture conditions, it was found that abundance influences soil water use up to a maximum level, above which the rate of soil water extraction may increase but the amount extracted does not. At Wagga Wagga, this maximum was expected to be approximately 25 plants m-2 for Austrodanthonia spp. Abundance in Bothriochloa macra was not as easily defined due to the plants ability to increase basal area under low competition environments.This study defined some technical areas of weakness which warrant further research in the future. There is a need to develop a more rapid method for assessing green leaf area index and modifications to the Native Pasture Model need to occur. More broadly, this study has assisted in defining the potential benefit of managing native pasture botanical composition with respect to hydrology. However, other impacts such as production and farm economics must also be considered, and mechanisms for increasing the abundance of key species better defined, for any of the findings of this study to be useful to management.