Abstract
Terrestrial water energy coupling (WEC), in the form of the non-linear
relationship between Soil Moisture (SM) and evaporative fraction (EF,
ratio of actual and potential evapotranspiration), controls critical
ecohydrological processes. We investigate and parameterize the evolution
of global SM–EF coupling from the field to remote-sensing
(RS)-footprint. The field-scale EF and SM were obtained from 163 eddy
covariance (EC) and SM sensors at various network (Texas Water
Observatory and FLUXNET) sites around the globe. Remote-sensing
(RS)-scale EF and SM estimates were obtained from Moderate-resolution
Imaging Spectroradiometer (MODIS) and Soil Moisture Active Passive
(SMAP) sensors, respectively. We estimate the effective
thresholds of the WEC regimes from both EC and satellite datasets to
highlight the influence of subgrid-scale heterogeneity, scaling, and
observational constraints on the evolution of WEC regimes from field to
RS-footprint scale. We compare the critical WEC thresholds of the water-
and energy-limited regimes with an SM drydown-based approach and
highlight the similarities between both methods in partitioning dominant
WEC regimes. EF and SM are strongly coupled in dryland arid and
semi-arid regions compared to humid climates. WEC regimes and thresholds
have strong interseason variability due to dynamic interactions between
soil, vegetation, and atmosphere at the RS-footprint scale. In contrast,
field-scale SM-EF coupling is influenced predominantly by agricultural
/land-use practices and soil conditions. Hence, future development of
Earth-System/Land-Surface models must account for the inter-scale
differences in the coupling between terrestrial water and energy fluxes
representative of the “ effective” processes at large spatial
scales.