A lagrangian particle tracking approach of evapotranspiration partitioning to explore evaporation and transpiration travel time distributions in the root zone

Evapotranspiration is a key component of the hydrological cycle, influencing water and biogeochemical cycles in the critical zone. Particularly, the travel time of evapotranspired water is critical for describing the origin and young water contribution to evapotranspiration, but is yet poorly understood. In this study, we revisited the Lagrangian particle-tracking model, EcoSLIM, to separate evaporation and transpiration particles using a mass balance approach. This separation allowed us to determine the travel time distribution of particles with different sources (i.e., rainfall, snowmelt, and pre-stored groundwater) captured by evaporation and transpiration. We validated the mass of evapotranspiration, evaporation and transpiration particles against those computed by the physically-based ParFlow-CLM model, which yields the expected mass of target fluxes by simulating the physical processes controlling water and energy balances. Our results demonstrated the high accuracy of the modified EcoSLIM in closing the mass balance, as evidenced by R² > 0.9 between the tracked and expected masses of the target fluxes. Using this model, we further studied the impact of plant growth cycle and vertical root distribution on source water partitioning and travel time of evaporation and transpiration via hillslope-scale virtual experiments. The results of three scenarios of different plant phenology indicated that the root depth and the state of synchronicity between climatic forcing and the timing of plant growth cycle influence the distribution of water travel times available to plants in contrasting ways. Finally, we found that plants show preference to young ages, unless the deep rooting network allows for the extraction of old ages during dry periods.