Monitoring deep unsaturated zones in Western Australia to reveal crucial insights for water resources management

Soil moisture observations have been collected since the late 1950s and are relatively abundant in the northern hemisphere. These readings are generally taken at shallow depths with sensors rarely installed more than 2 metres below the surface. However, deep soil moisture dynamics can play a crucial role in determining ecosystem services, land-atmosphere water fluxes, plant water use, nutrient cycle and, eventually, groundwater recharge. In thick unsaturated zones, shallow soil moisture observations are likely to fail to capture important hydrological processes, and their feedback with the atmosphere, generating significant uncertainties. 

Here we present the results from a soil moisture monitoring network established as part of the Recharge in a Changing Climate (RiCC) project. The network aims to capture soil moisture dynamics in deep sandy profiles of a Mediterranean-like zone in Western Australia, where traditional shallow and surface soil moisture observations fall short of detecting significant hydrological processes. The monitoring network, deployed since 2022, comprises over 75 sensors strategically distributed across 7 locations over the Swan Coastal Plain at depths of up to 9 m to provide continuous high-frequency soil moisture data. These soil moisture sensors are complemented by 14 access tubes where neutron moisture probe readings are taken to characterize the spatial heterogeneity.

Findings reveal complex patterns of moisture movement through the profile, with significant temporal variations in wetting front depths and propagation patterns, improving the representation of soil water/vegetation interaction, and providing unique insights into groundwater recharge processes in sandy aquifer systems. These observations challenge existing assumptions about soil water movement in sandy soils and provide crucial validation data for improving ecohydrological models and recharge quantification. Information from the RiCC monitoring campaign can significantly reduce uncertainties in water resources management and, by including transpiration from deeper soil moisture pools, enhance the accuracy of modelled land-atmosphere feedback. These insights are also beneficial for understanding the resilience of ecosystems and agroecosystems under transient climate conditions.