The hydrology of shallow and deep aquifers in relation to secondary soil salinisation in Southwestern Australia

Abstract

Results are reported from detailed research which has been carried out since 1984 on several Western Australian wheatbelt catchments, ranging in size from 12 ha to 1000 km2, in order to quantify the nature and roles of groundwater aquifers in relation to secondary salinisation. Secondary soil salinisation in the semi-arid wheatbelt has increased by 600% over the past 34 years to 440 000 ha, and is likely to increase by at least another 500% in the next 30-50 years. Changes in groundwater hydrology following the replacement of the native, woodland vegetation with agricultural crops and pastures during this century, have been responsible for the increasing problem. Current recharge rates to groundwater aquifers are approximately two orders of magnitude greater than pre-clearing discharge rates, resulting in increased groundwater pressures and the accumulation of saline water in low-lying parts of the landscape. In wetter (>400 mm mean annual rainfall), western wheatbelt areas with texture-contrast soils, ephemeral to seasonal throughflow may contribute about 50% of the water discharging in salt-affected areas, but only about 2% of the soluble salts. Deep aquifers contribute the remainder. In drier (<400 mm/yr) and flatter areas in the central and eastern wheatbelt, contributions from ephemeral throughflow in shallow soils appear to decrease. However, perennial aquifers perched above silicified mottled or pallid zones of the deeply-weathered soils, at the base of deep, coarse, yellow «sandplain» materials, are wholly responsible for the formation of small, saline, «sandplain seeps», which account for about 10% of secondary, salt-affected land in the region. In all parts of the wheatbelt, both types of perched aquifers provide significant quantities of recharge to the deep, perennial aquifers which are responsible for most of the large-scale, valley salinisation.