A virtual isotope injection experiment to unravel the temporal offset between irrigation and water uptake of a potted olive tree.

The temporal origin of root water uptake (RWU) is an indicator of ecosystem functioning that sheds light on how natural and anthropogenic disturbances affect plant resilience. Plant reliance on past rainfall/irrigation is important in season-dry climates and even in humid climates punctuated by drought. It is also essential to know how much water is returned to the atmosphere through evapotranspiration to select the most cost-effective crops to save irrigation water during the growing season and simultaneously reduce groundwater pumping in cropland management. An isotope-enabled module of Hydrus-1D was applied to a potted olive tree to trace water parcels originating from 26 irrigation events in a glasshouse experiment. The soil hydraulic parameters were optimized via inverse modeling by minimizing the discrepancies between observed and simulated soil water content and soil water oxygen stable isotope (18O) values at three soil depths. The model’s performance was validated with observed sap flow z-scores and xylem water 18O. We quantified the source and transit time of irrigation water by analyzing the mass breakthrough curves derived from a virtual tracer injection experiment. On average, 26 % of irrigation water was removed by plant transpiration with a mean transit time of 94 hours. Our proof of concept work suggests that transit time may represent a functional indicator for the uptake of irrigation water in agricultural ecosystems. The virtual tracer injection experiment will be implemented in experimental soil profiles distributed in different agro-climate zones in Italy to evaluate the impact of climate on local vegetation, especially under dry conditions.