Jonathan D.A. Clarke

Modern and palaeogeographic trends in the salinisation of the Western Australian wheatbelt: a review

The Western Australian wheatbelt contains vast areas of agricultural land underlayed by saline and deeply weathered regolith derived from Archaean rocks and recent sediments. The region has been geologically stable since the late Permian, although the Archaean basement sustained some movement during the break-up of Gondwanaland and the northward drift of Australia from Antarctica. During the Early Cretaceous, Eocene and more recently, the wheatbelt region’s weathered mantle has been successively eroded by rivers. The palaeovalleys have been infilled with terrestrial and marine sediments, and subjected to ongoing deep weathering. During the Pliocene and Quaternary the region experienced alternating arid and wetter climates. These cyclic episodes influenced regolith development, affected vegetation species and catchment water balances, and also promoted the accumulation of massive volumes of salt. In more recent times, these salt stores have interacted with vegetation, soils, surface water bodies, and groundwater systems and left a distinctive and pervasive legacy in the landscape. Salinisation was manifest in the wheatbelt from as long ago as 2.8 Ma, concentrating in valley floors as arid and wetter cycles prevailed and while the continent migrated northwards. Today, agricultural development has altered the water balance on 20 Mha of cleared farmland. As a result, salinity is spreading, further degrading 300 000 ha of variably saline landscape that existed before the arrival of Europeans, and affecting an additional 1.1 Mha of formerly arable land. Unchecked by reduced rainfall or human-induced changes to the water balance, salinity may expand even further, potentially affecting 1.7–3.4 Mha of the wheatbelt’s agricultural land and its unique natural resources. This paper reviews the palaeogeography and palaeoclimates of the region and its hydrogeology and examines the nature of its susceptibility to salinisation. It poses questions about the relationship between palaeo-salinity and contemporary salinity, seeking geomorphic evidence to indicate whether salinity is likely to expand beyond extant palaeo-salinity markers. Finally, it considers the likely timeframes involved in salinisation and whether clearing-induced salinity will follow patterns similar to those observed from past saline episodes in the region.

The use of LiDAR in applied interpretive landform mapping for natural resource management, Murray River alluvial plain, Australia

High resolution Light Detection and Ranging (LiDAR) Digital Elevation Models (DEMs) have proved an invaluable tool for landform mapping along the alluvium of the Murray River in southern Australia. They are a key part of an integrated approach to addressing salinity, land management and groundwater resource issues on the Murray River alluvial plain when used in conjunction with satellite imagery, airborne and ground geophysics, and surface and borehole information on soils and sediments. This article describes the processing and application of LiDAR DEMs to the Murray River alluvial plain and how this has informed concepts of landform architecture in the region and confirmed and elaborated on earlier studies. In particular the LiDAR data show how the landform architecture of the Murray River alluvial plain changes dramatically downstream, from the unconfined depositional landscapes of the Riverine Plain to being increasingly confined to an incised valley downstream.

Applied geomorphic mapping for land management in the River Murray corridor, SE Australia

Abstract The River Murray Corridor, in SE Australia, was the subject of a large integrated project aimed at providing answers to a number of important land and environmental management questions and guidance for land use planning. Airborne electromagnetic (AEM) surveys were carried out in conjunction with a light detection and ranging survey in 2007. These data were combined with various satellite remote-sensing data and field observations to produce a number of maps, including maps of landforms and surface materials. These latter maps were used to assess the value of the AEM data, identify potential surface and sub-surface flow pathways and map potential recharge on the Murray River floodplain surface. This allowed the assessment of potential salt storage and the impact of various land use options on salt storage and mobilisation.