Automated equilibrium tension lysimeter deployment to monitor vadose zone soil water and solute transport.

Accurate quantification of deep percolation is fundamental for understanding vadose zone hydrology, groundwater recharge, and solute transport in agricultural landscapes. Many lysimeter designs impose fixed or passively generated suction that does not continuously respond to changes in the surrounding soil hydraulic environment, limiting the conditions under which representative drainage is collected. Automated equilibrium tension lysimeters (AETLs) aim to address this limitation by continuously adjusting lysimeter suction to match bulk soil matric potential. However, existing AETL systems are constrained by high power demands, limited sensor performance, and complex operation. This paper presents an updated AETL system that integrates contemporary hardware, solar-powered operation, and remote communication to improve system reliability and temporal resolution. Sixteen AETLs installed at 120-cm depth in silt loam soils in south-central Wisconsin were monitored during the 2025 growing season. Performance evaluation indicated that several systems were not fully equilibrated during early spring, suggesting drainage may have been under-collected. Despite this limitation, drainage occurred primarily during late winter and spring when soils approached saturation, whereas summer precipitation rarely produced drainage. Drainage varied substantially among lysimeters (∼1%-40% of cumulative precipitation), reflecting early-season operational variability and microtopographic controls on water redistribution, contextualized using topographic wetness index. An independent soil-water budget showed drainage from better performing lysimeters fell within the expected range, while drainage from the poorer performing lysimeters did not. The updated AETL system enables high-resolution monitoring of deep percolation in non-tile-drained agricultural systems, though performance remains constrained by the power and hydraulic-contact limitations documented here.