Global Terrestrial Water–Energy Coupling Across Scales

Abstract

Terrestrial water–energy coupling (WEC), in the form of a 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)-pixel. The field-scale EF and SM were obtained from eddy covariance (EC) and SM sensors at the global FLUXNET and Texas Water Observatory sites. RS-pixel-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 sub-pixel-scale heterogeneity, and, scaling and observational constraints on the evolution of WEC regimes from the field to RS-pixel scale. We argue that the changes in land surface conditions add a temporal variability in the critical thresholds of terrestrial WEC at RS pixel scale. We compare the critical WEC thresholds of the water- and energy-limited regimes with a 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-pixel scale. In contrast, field-scale SM-EF coupling is influenced predominantly by soil conditions and land-use/management practices. 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.