Data-Guided Exploration of the Energy Partitioning at Land Surface in the Contiguous US

This study explores the climatic controls of the long-term energy partitioning of net radiation into sensible heat and latent heat at watershed scale, by following a data-guided approach. Collected data shows that the relationship between the evaporative fraction (ratio of latent heat to net radiation) and the aridity index (ratio of energy supply to water supply), which represents the climatic conditions at watershed scale, follows a trend similar to the Budyko curve. To explore the physical basis of this relationship, we hypothesize that there are mediating factors connecting energy partitioning with climate. Two potential mediating factors are considered, namely the equilibrium root zone soil moisture and land-air temperature difference. We extract empirical relationships between the mediating factors and the partitioning fractions of water and energy fluxes based on observational data in 219 study watersheds across the contiguous US. With the obtained empirical relationships, we analytically derive two alternate Budyko-type equations for energy partitioning, for each of the two mediating factors separately. Both equations show similar trends as the Budyko curve. We also derive a function to relate energy partitioning to water partitioning Budyko-type equations, suggesting a close connection between the two. The study brings out the mediating roles of soil moisture and temperature gradient that connect water/energy partitioning with climate over long timescales in watersheds. In the process, the analysis results also provide useful insights about complex watershed system behavior. The observed relationships can be used to constrain hydrological and Earth system models and thus improve modeling performance.