Assessing the Impact of Subsurface Clay Layers on Water Use Dynamics in Eucalyptus globulus Plantations

ABSTRACT Plantation forests are important for timber production and regional water balances, yet their groundwater interactions are strongly mediated by soil properties. In South Australia's Lower Limestone Coast, groundwater sustainability concerns have highlighted the need to quantify how subsurface clay layers influence plantation water use and growth. This study investigated the effects of subsurface clay thickness on water use, growth, and groundwater extraction of Eucalyptus globulus plantations in South Australia's Lower Limestone Coast, part of the Green Triangle growing region. Measurements included annual rainfall (open and under canopy), forest floor evaporation, stand transpiration, and tree growth (diameter and height). Across eight monitored sites, annual transpiration ranged from 250 to 950 mm, with annual evapotranspiration between 550 to 1250 mm. Sites underlain by thin clay layers exhibited substantially higher transpiration (892 mm year −1 ), annual water balance (−597 mm year −1 ), and stand productivity (periodic annual volume increment, 62 m 3 ha −1 year −1 ) compared with thick clay sites (385 mm year −1 , −23 mm year −1 and 20 m 3 ha −1 year −1 , respectively). At thick clay sites, net groundwater contributions estimated from the annual water balance were small and not significantly different from zero within associated uncertainty bounds. Thus, despite relatively shallow groundwater at some sites (< 6 m), groundwater contributions to stand water use could not be resolved over the monitoring period, consistent with root‐impeding clay layers and reduced soil water availability. These patterns reflect interactions among clay thickness, depth to groundwater, climate variability, and stand properties. The results highlight the importance of incorporating soil profile information into groundwater allocation frameworks for deep‐rooted, non‐irrigated crops like forests, as current policies may overestimate groundwater use in areas underlain by thick clay layers. However, groundwater use estimates should be regarded as indicative because the annual water balance approach cannot distinguish deep soil water from groundwater uptake in the Green Triangle.