Abstract. The Budyko framework posits that a catchment's long-term
mean evapotranspiration (ET) is primarily governed by
the availabilities of water and energy, represented by long-term mean
precipitation (P) and potential evapotranspiration
(PET), respectively. This assertion is supported by
the distinctive clustering pattern that catchments take in Budyko space.
Several semi-empirical, nonparametric curves have been shown to generally
represent this clustering pattern but cannot explain deviations from the
central tendency. Parametric Budyko equations attempt to generalize the
nonparametric framework, through the introduction of a catchment-specific
parameter (n or w). Prevailing interpretations of Budyko curves suggest
that the explicit functional forms represent trajectories through Budyko
space for individual catchments undergoing changes in the aridity index, PETP, while the n and w values represent
catchment biophysical features; however, neither of these interpretations
arise from the derivation of the Budyko equations. In this study, we
reexamine, reinterpret, and test these two key assumptions of the current
Budyko framework both theoretically and empirically. In our theoretical
test, we use a biophysical model for ET to demonstrate that n and
w values can change without invoking changes in landscape biophysical
features and that catchments are not required to follow Budyko curve
trajectories. Our empirical test uses data from 728 reference catchments in
the United Kingdom (UK) and United States (US) to illustrate that catchments rarely
follow Budyko curve trajectories and that n and w are not transferable
between catchments or across time for individual catchments. This
nontransferability implies that n and w are proxy variables for
ETP, rendering the parametric Budyko equations
underdetermined and lacking predictive ability. Finally, we show that the
parametric Budyko equations are nonunique, suggesting their physical
interpretations are unfounded. Overall, we conclude that, while the shape of
Budyko curves generally captures the global behavior of multiple catchments,
their specific functional forms are arbitrary and not reflective of the
dynamic behavior of individual catchments.