Water content of a red-brown earth subjected to a range of agronomic vegetation options in south-eastern Australia

Abstract

The management of ground water recharge in south-eastern Australia relies on the formulation of agricultural practices that utilise rainfall before it moves below the root-zone. Annual cycles of soil water content were therefore measured in a red-brown earth subjected to 5 fallow-free crop sequences, to 2 crop sequences that included fallow, and to 3 pastures. Changes in soil water content induced by wheat, barley, lupin, pea, safflower, canola, and fallow were compared with those of annual pasture and 2 monocultures of the deep-rooted perennials phalaris and lucerne in 3 years of study.

Mean minimum soil water content (0–1.6 m) seen in December and May was approximately 355 mm in lucerne and phalaris, 410 mm in annuals (crops and pasture), and 475 mm in fallow. Corresponding soil water deficits appropriate to lucerne, annuals, and fallow were 185, 135, and 65 mm, respectively.

Lucerne and annuals both removed approximately 85 mm water from the upper 0.6 m of the soil profile. Differences arose in the subsoil below 0.6 m, where lucerne, annuals, and fallow produced soil water deficits of approximately 100, 50, and 25 mm, respectively. The difference in soil water deficit of deep-rooted perennials and annuals was therefore caused by the extra 50 mm of water extracted by lucerne and phalaris below 0.6 m in the period September–December. The dry subsoil endured through summer to promote the storage, by soil, of rainfall in winter.

The data suggest that the spatial utility of an agronomic recharge control option in south-eastern Australia depends on the magnitude of the soil water deficit associated with the vegetation. The soil water deficit, relative to winter (May–August) rainfall, discriminates between areas where annuals suffice for recharge control, where lucerne and phalaris are required for recharge control, and where agronomic annuals and perennials are both conducive to high rates of drainage.