Monitoring and modelling spatio-temporal soil change in a semi-arid irrigated cotton-growing region of south-west NSW, Australia – The impacts of land use and climatic fluctuations

Abstract

Soil is an invaluable finite resource, and it is essential that any changes in soil condition are adequately monitored. In the semi-arid regions of eastern Australia, there has been an expansion of intensive irrigated cotton production, and these regions have also experienced highly variable rainfall patterns in the last decade or so. This combination of climatic and land use changes has the potential to significantly alter soil attributes. This thesis focuses on monitoring the change in several important soil properties – pH, salinity, sodicity, and organic and inorganic carbon – in the semi-arid irrigated cotton-growing district of Hillston, NSW, between 2002 and 2015. Rather than using traditional digital soil mapping techniques, this study focuses on using bi- and multi-variate linear mixed models, and two-step mixture models to model and map soil properties in space and time. The linear mixed models were particularly advantageous for monitoring changes in soil properties as they can account for correlation in space and time, and improve the sensitivity of detecting statistically significant changes. As traditional laboratory methods of measuring certain soil properties can be expensive and laborious, this study also focused on using visible near infrared (VisNIR) spectroscopic techniques to rapidly predict exchangeable sodium percentage (ESP), organic carbon (SOC) content, and inorganic carbon (SIC) content. Various degrees and extents of soil change were observed during the study period in both the top and sub soil. This included an acidification trend in some areas, a contrasting shift in electrical conductivity (EC) under differing land uses, an increase in soil ESP under irrigated land uses, an increase in SOC content at some locations, and no detectable change in SIC content. Overall, it was clear that fluctuating rainfall patterns and agricultural management practices had a notable impact on the degree and direction of changes in soil properties.