The Role of Topography in Controlling Evapotranspiration Age

Evapotranspiration (ET) age is a key metric of water sustainability but a major unknown partly due to the extreme difficulty in modeling it. Groundwater is found to be important in ET age variations in small-scale studies, yet our understanding is insufficient because groundwater systems are nested across scales. Here, we conducted GPU-accelerated particle tracking with integrated hydrologic modeling to quantify the variations in ET age at a regional scale of & SIM;0.4 M km2. Simulation results reveal topography-driven flow paths shaping the spatial and temporal patterns of ET age variations. On ridges, where root zone decoupling with deep subsurface storage, ET age is generally young, with seasonal variations dominated by meteorological conditions. In the valley bottom, ET age is generally old, with significant subseasonal variations caused by the convergence of subsurface flow paths. On hillslopes with water table depths ranging from 1 to 10 m, ET age shows strong seasonal variations caused by the connections with lateral groundwater regulated by ET demand. Our modeling approach provides insights into the basic linkages between ET age and topography at large scale. Our work highlights the perspective of multiscale studies of ET age, suggesting new field experiments to test these process connections and to determine if such linkages warrant inclusion in Earth System Models. Evapotranspiration (ET) is a major component of the hydrological cycle, and ET age is the time that water takes from precipitation to move through the subsurface and, ultimately, to soil evaporation and/or plant water uptake. A modeling approach for ET age at regional scales is lacking in the hydrologic community, which partly hinders the scientific progress of ET age. Here, we develop a new modeling approach for ET age using particle tracking with an integrated hydrologic model. We conducted a 42-year particle tracking simulation in the Haihe River Basin. Our simulation results showed significant topographic controls on ET age variations due to the topography-driven subsurface flow paths. Simulation results also showed that ET water increased in age from the ridges to valleys throughout our model domain due to the increasing contributions from groundwater. In areas where water table depths vary between 1 and 10 m, the simulated ET age shows the strongest seasonal variability. We hope that our findings can guide the design of new field experiments to test these topographic controls on ET age so that, ultimately, we can include such functions in Earth System Models of land surface processes. Particle tracking based on integrated hydrologic modeling suggests the role of groundwater in Evapotranspiration (ET) age variations at the regional scaleTopography-driven Tothian subsurface flow shapes the basic spatiotemporal patterns of ET ageWhere water table depths are between 1 and 10 m, ET age has a high temporal variability due to the seasonal use of groundwater by ET