Quantifying soil moisture fluxes and evaporative controls in a semiarid floodplain

Arid and semiarid zones cover roughly 70 % of Australia, including the Pilbara region of Western Australia. The Pilbara experiences low seasonal rainfall of ~350 mm/y, with nearly 80 % of precipitation estimated to be lost to evaporation. Subsurface water fluxes are vital to ecohydrological functioning in these environments, yet remain poorly understood. This study integrates stable water isotopes (δ²H and δ¹⁸O) with direct hydrometric observations to characterise soil moisture dynamics in semiarid floodplain soils. The aim is to constrain isotope-based inferences using physically measured soil water fluxes to clarify how episodic water inputs are partitioned, retained, and lost in semiarid floodplains under increasing water scarcity. Vertically resolved isotope profiles were combined with continuous measurements from smart lysimeters and soil-moisture probes across six sites. To our knowledge, this work represents the first combined application of smart lysimeters and stable isotope analyses in natural environments in Australia. Near-surface moisture is strongly affected by evaporation, with elevated δ²H and δ¹⁸O values extending to the depths of ~50 cm. Below this zone, isotope compositions remain comparatively uniform, indicating limited vertical exchange and minimal deep percolation. Only substantial rainfall events (> 60-90 mm) generated transient pulses characterised by low δ-values consistent with episodic deep infiltration. Spatial variability across sites revealed systematic gradients in moisture retention and isotope composition that correspond to soil texture and structure. Shallow infiltration exhibited highly variable instantaneous rates (3.33 – 300 mm/hr), controlled primarily by rainfall intensity and surface conditions. Together, these multi-method observations provide a detailed understanding of flow paths and hydrological transformations in strongly evaporative semiarid environments, demonstrating how stable isotopes, combined with lysimeters and soil moisture probes, can resolve catchment-scale responses and enhance water balance quantification and tracing.