Can agricultural management emulate natural ecosystems in recharge control in south eastern Australia

Abstract

Water balance information at locations across southern Australia is analysed to identify mechanistic causes for higher water tables under agricultural systems compared to natural vegetation. Contrasting patterns of water use indicate pronounced physiological activity in summer by natural ecosystems ensuring the persistence of the dominant perennial species. A strategy of tempered water use during periods of rainfall excess in winter/spring enables seasonal carryover of soil water to be withdrawn by deep roots for summer functioning. Characteristic patterns of water use by agricultural systems, comprised mainly of determinant annual species, are truncated in time but feature elevated peak rates coinciding with periods of maximum soil water availability during the cooler months. Low seasonal vapour pressure deficit explains the observed benefits of enhanced biomass production due to high water use efficiency. The associated limited scope for soil water uptake carries the penalty of increased frequency of drainage and the subsequent resource degradation currently associated with farmland. Inclusion of a perennial lucerne phase in rotation with crops confers environmental benefit through summer uptake from a rooting depth double that of crops. An effective buffer is thus provided to restrict drainage under an ensuing cropping phase in a manner that more closely emulates natural systems. Simulations of phase farming point to a halving in long term drainage. They suggest adoption over a range of arable land units could cause local retreat of water tables by 0.3 m y−1 initially, leading to a new equilibrium level which would still remain elevated with respect to that under natural vegetation.