Spatially-explicit knowledge of the origins of water resources for
ecosystems and rivers is challenging when using tracer data alone. We
use simulations from a spatially-distributed model calibrated by
extensive ecohydrological datasets in a small, energy-limited catchment,
where hillslope-riparian dynamics are broadly representative of humid
boreal headwater catchments that are experiencing rapid environmental
transition. We hypothesize that in addition to wetness status, landscape
heterogeneity modulates the water pathways that sustain ecosystem
function and streamflows. Simulations show that catchment storage
inversely controls streamwater ages year-round, but only during the
drier seasons for transpiration and soil evaporation. The ages of these
evaporative outputs depend much less on wetness status in the
oft-saturated riparian soils than on the freely-draining hillslopes that
subsidize them. This work highlights the need to consider local dynamics
and time-changing lateral heterogeneities when interpreting the ages,
and thus the vulnerability, of water resources feeding streams and
ecosystems in landscapes.