Seasonal water storage and evapotranspiration partitioning controls on
the relationship between continental moisture recycling and
precipitation deuterium excess
Abstract
Moisture recycling via evapotranspiration (ET) is often invoked as a
mechanism for the high deuterium excess signals observed in continental
precipitation (dP). However, a global-scale analysis of precipitation
monitoring station isotope data shows that metrics of ET contributions
to precipitation (van der Ent et al., 2014) explain little dp
variability on seasonal timescales. This occurs despite the fact that ET
contributions increase by ~50% in continental locations
such as the Eurasian interior from wet to dry seasons. To explain this
apparent paradox, we hypothesize that the effects of ET on dP are
dampened during dry seasons due to contributions from
isotopically-evolved residual water storage that act to lower the
d-excess of ET fluxes (dET), in combination with changes in
transpiration fraction (T/ET). To test this hypothesis, we develop a
parsimonious two-season (wet, dry) model for dET incorporating residual
water storage and ET partitioning effects. We find that in environments
with limited water storage, such as shallow-rooted grasslands, dry
season dET is lower than wet season dET despite lower relative humidity.
As global average ratios of annual water storage to precipitation are
relatively low (Guntner et al., 2007), these dynamics may be widespread
over continents. In environments where water storage is not limiting,
such as groundwater-dependent ecosystems, dry season dET is still likely
lower; however, this effect arises instead due to higher seasonal T/ET
when energy-driven plant water use is enhanced and surface evaporation
is relatively limited by water availability. Together, these analyses
also indicate multiple mechanisms by which dET may be lower than dp
during the same season, challenging the view that moisture recycling
feedback increases the dp in continental interiors. This work
demonstrates the potential complexity of seasonal dp dynamics and
cautions against simple interpretations of dP as a process tracer for
moisture recycling. References: Guntner et al., 2007. Water Resour.
Res., 43, W05416. van der Ent et al., 2014. Earth Syst. Dynam., 5,
471–489.