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Abstract

Water-use efficiency, defined as the ratio of carbon assimilation over evapotranspiration (ET), is a key metric to assess ecosystem
functioning in response to environmental conditions. It remains unclear which factors control this ratio during periods of
extended water-limitation, and current semi-empirical water-use efficiency models fail to reproduce observed ET dynamics
for these periods. Here, we use dry-down events occurring at eddy-covariance flux tower sites in the FLUXNET database as
natural experiments to assess if and how decreasing soil-water availability modifies water-use efficiency on ecosystem scale.
We demonstrate that an attenuating soil-water availability factor in junction with a previously discovered additive radiation
term is necessary to accurately predict ET flux magnitudes and dry-down lengths of these water-limited periods. In an analysis
of the attenuation, 20–33% of the observed decline in ET was due to the previously unconsidered soil-water availability effect.
We conclude by noting the rates of ET decline differ significantly between FLUXNET sites with tall and short vegetation types
and discuss the dependency of this rate on the variability of seasonal dryness.