Throughflow as a mechanism responsible for excessive soil salinisation in non-irrigated, previously arable lands in the Western Australian wheatbelt: A field study

Abstract

Summary A throughflow model of salt accumulation in the Western Australian wheatbelt is introduced and tested by field observations and data from several locations. Areas of excessive soil salinity (salt scalds) in the York-Mawson area are associated with specific soil/geomorphic conditions. Thirty-eight scalds and their catchments were examined: of these ten were mapped in detail and 100 soil profiles in 15 catenary sequences were examined and described. Valley-side slopes of the gently rolling landscape are low-angled, with the modal slopes of the 15 catenas ranging between 1.2 and 3.5 degrees. Interfluves are characterised by sand-textured soils that overlie lateritic duricrust or gravels at varying depths. Valley-sides above the salt scalds are predominantly erosional/transportational landsurface units with a sharp decrease of soil permeability at about plough depth. Further marked reductions in permeability and hence water infiltration occur at greater depth. The salt scalds vary considerably in size (0.02 to 64.4 ha) and nearly all are located in valley bottoms. The latter are usually occupied by well defined stream channels with ephemeral streamflow regimes, and all salt scalds occur on predominantly colluvial and/or alluvial aggradational soil materials. The layering of these materials produces one or more perched soil-water tables at relatively shallow depths below the scald surfaces, and impermeable hardpans occur below the aggradational soil materials. Four areas were examined in detail in the Dalwallinu area, where salt scalding is virtually continuous along the floors of the wide, flat valley bottoms. The results demonstrate that soluble salts move from the catchments to the valley bottoms by a combination of the following processes: (1) occasional overland flow; (2) throughflow, at varying depths below the surface, generally concentrated in percolines; and (3) streamflow down minor tributaries, this streamflow being fed by a combination of (1) and (2). The water supplied to the valley bottoms by these cascades is prevented from percolating to deeper levels by a thick impermeable hardpan which, in turn, also comprises the upper, confining material of the deeper-seated aquifer system in which groundwater is under pressure. Surface or near-surface waterlogging of the valley bottoms is essentially a shallow, perched water phenomenon. Evaporation during summer concentrates the salts in the valley bottom soil materials, leading to very high salinity levels. The water inputs responsible for the increased extent and duration of saturation of scalded soils since clearing appear to be derived primarily from an increase in throughflow and less from overland flow. A successful attempt at salt scald rehabilitation and recent drilling in other parts of the wheatbelt provide further evidence in support of the throughflow model of salt accumulation. An implication of these findings is that valley-side interception of throughflow should result in substantial rehabilitation of many salt scalds in the Western Australian wheatbelt.