A virtual tracer experiment to assess the temporal origin of root water uptake, evaporation, and drainage

Abstract

AbstractThe temporal origin of drainage, evaporation, and root water uptake (RWU) are indicators of ecosystem functioning that shed light on how natural and anthropogenic disturbances affect plant resilience and aquifer vulnerability. A virtual tracer experiment was carried out in HYDRUS‐1D using data from a soil lysimeter planted with winter rye in Austria. The RWU (τR), evaporation (τE), and drainage (τD) transit times (τ) were determined by using the actual dispersivity optimized in a prior study. First, τR and τD were compared to RWU (τPT,R) and drainage (τPT,D) advective transit times estimated using the particle tracking algorithm. The τR values were in agreement with τPT,R while large discrepancies were detected when estimating drainage transit times using the two approaches. A sensitivity analysis revealed that dispersivity had a mild, poor, and strong influence on τR, τE, and τD. The longitudinal dispersivity describes how a tracer spreads along the flow paths. The longer the flow paths, the more dispersion affects the tracer transport. On average, water parcels originating from rainfall in the growing season took 18 and 201 days to reach the roots (with RWU being 21% of rainwater) and the soil profile bottom (with drainage being 36% of rainwater), respectively. In contrast, 10% of rainwater that fell in the dormant season became RWU after 264 days, while 79% became drainage after 294 days. The temporal origin of water can be explored in other plots by using the guidelines proposed in this study.