The age of evapotranspiration: continental-scale lower-bound constraints
from distributed water fluxes
Abstract
Unlike streamflow, which can be sampled in aggregate at the catchment
outlet, evapotranspiration (ET) is spatially dispersed, challenging
large-scale age estimation. Here, we introduce an approach for
constraining the age of ET via mass balance and present the minimum
flux-weighted age of ET across the continental US using distributed,
publicly available water flux datasets. The lower-bound constraint on ET
age can be calculated by assuming that ET is preferentially sourced from
the most recent precipitation through a last-in, first-out algorithm.
From 2012-2017, ET was at least several months old across large areas of
the western continental US, including in Mediterranean and (semi-)arid
climate zones and shrub and evergreen needleleaf plant communities. The
primary limitation of this approach is that it provides only a minimum
flux-weighted average age to satisfy mass balance of outgoing fluxes;
true ET fluxes are composed of distributions of ages and may be composed
of much older water. The primary advantage of the approach is that flux
timeseries of precipitation and ET are sufficient to constrain ET age,
and model parameterization is unnecessary. ET ages can be used to
validate tracer-aided and modeling approaches and inform studies of
biogeochemistry, water-rock interactions, and plant water sourcing under
drought.