Toward a Closure of Catchment Mass Balance: Insight on the Missing Link From a Vegetated Lysimeter

Plant transpiration plays a significant role in the terrestrial cycles, but the spatiotemporal origins of water used by plant remains highly uncertain. Therefore, the missing link to fully characterize the water mass balance, for any control volume including significant vegetated surfaces, is identifying and quantifying the key factors that control the age of water used by plants. Here, we bring together an age‐based (tran‐SAS) and a physically based (HYDRUS‐1D) model contrasting information gleaned from soil, drainage, and xylem samples at stand scale. In particular, we focus on the relative role of advection, dispersion, and root distribution on the age of water uptake and drainage. We suggest that the interplay of advective and dispersive forces, subsumed by the local Péclet number, drives the age composition of drainage even in the case of extreme uptake rates. The vegetation influence on the age of drainage is mainly exerted by diversifying the subsurface transport pathways resulting in large dispersivity and spatial heterogeneity of soil hydraulic parameters. We introduce a uniform‐equivalent root length for vegetation and show that its ratio to the effective size of the subsurface water storage controls the age selection of water uptakes. Our results are suggestive of a route forward toward a general toolbox to upscale mass balance closures for catchments embedding large and diverse plant assemblages. Plain Language Summary Mass balance, the fundamental tool of hydrologic analysis, cannot be closed experimentally at the scale of a hydrologic catchment without answering a simple question: what waters do plants uptake? The characterization of vegetation ions, subsumed by water age in the transport volume, requires detailed knowledge of suitable chemical signatures of inflows and outflows and the fate of hydrologic waters by transport. In this study, we combine two conceptually different mathematical models with high‐resolution experimental data (including tracer data from plants' xylem) to identify the main factors determining the origin of water used by vegetation. We suggest that a definition of a uniform‐equivalent root length parameter proves significant in characterizing the age of plant uptake. Key Points We combine data in a vegetated lysimeter with results from two transport models to contrast their convergence on an age‐based mass balance We assess experimentally and computationally the interplay of advective and dispersive processes in drivi