Assessing the temporal origin of root water uptake and drainage water using a virtual tracer experiment in HYDRUS-1D

The temporal origin of root water uptake and drainage provides insights into the impact of natural and anthropogenic disturbances on plant resilience and aquifer vulnerability. In-situ and virtual tracer experiments provide information on rainfall partitioning and transit times, which help to enhance the understanding of the hydrological response of a soil-plant-atmosphere continuum (SPAC) to climate variability and contaminant transport. A virtual tracer experiment was carried out in a 150-cm-thick soil lysimeter planted with winter rye in Austria. Water flow and tracer transport dynamics were simulated in the SPAC using HYDRUS-1D, previously calibrated with hydrochemical measurements. The root water uptake (τ_R) and drainage (τ_D) transit times (τ) were assessed by identifying arrival times when a prescribed percentage of the tracer mass breakthrough curves was reached. The estimates of τ_R and τ_D were compared to those derived from a particle tracking algorithm that simulates the particle trajectories subject to convective transport. The tracer-based arrival times were in close agreement with those determined using particle tracking when 50% of the tracer output flux was reached. On average, it took 19 or 234 days for water originating from rainfall in the growing season to be taken up by roots (21%) or exit as drainage (36%), respectively. In contrast, in the dormant season, 10% of rainfall water was taken up by roots after 245 days, while 79% became drainage after 241 days. The results from each daily event can be aggregated at any desired temporal resolution to investigate the effects of climate seasonality on water balance and timing. The temporal origin of water can be explored in other plots using the standard guidelines proposed in this study.