Disentangling the relative drivers of seasonal evapotranspiration across a continental-scale aridity gradient

Authors: YOUNG, A.M. - Northern Arizona University; FRIEDL, M.A. - Boston University; NOVICK, K. - Indiana University; Scott, Russell - Russ; MOON, M. - Boston University; FROLKING, S, - University Of New Hampshire; LI, X. - Cornell University; CARRILLO, C.M. - Cornell University; RICHARDSON, A.D. - Northern Arizona University

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2022
Publication Date: 8/10/2022
Citation: Young, A., Friedl, M., Novick, K., Scott, R.L., Moon, M., Frolking, S., Li, X., Carrillo, C., Richardson, A. 2022. Disentangling the relative drivers of seasonal evapotranspiration across a continental-scale aridity gradient. Journal of Geophysical Research-Biogeosciences. 127(8). Article e2022JG006916. https://doi.org/10.1029/2022JG006916.
DOI: https://doi.org/10.1029/2022JG006916

Interpretive Summary: Evapotranspiration (ET), the transfer of liquid water on Earth’s surface into water vapor in the air, is an important hydrological variable. Understanding the seasonal pattern and magnitude of ET is critical for anticipating a range of ecosystem impacts, including drought, heat-wave events, and plant mortality. In this study, we identified the relative controls of seasonal variability in ET, and how these controls vary among ecosystems. We used overlapping data from ET measurement systems and from cameras that monitor an ecosystems growth at 20 sites to explore these linkages, and our study area covered a broad difference in aridity in the U.S. and Canada. Our results revealed that the drivers of ET seasonality varied significantly between wet and dry (i.e., semiarid) ecosystems. Specifically, precipitation had a much higher effect in dry ecosystems, while seasonal patterns in canopy greenness emerged as a stronger control in wet ecosystems. Given that an ecosystem’s growth seasonality is expected to shift under future climate, our findings provide key information for understanding and predicting how that seasonality may impact 21st century hydrology.

Technical Abstract: Evapotranspiration (ET) is a significant ecosystem flux, governing the partitioning of energy at the land surface. Understanding the seasonal pattern and magnitude of ET is critical for anticipating a range of ecosystem impacts, including drought, heat-wave events, and plant mortality. In this study, we identified the relative controls of seasonal variability in ET, and how these controls vary among ecosystems. We used overlapping AmeriFlux and PhenoCam time series at a daily timestep from 20 sites to explore these linkages (# site-years > 100), and our study area covered a broad climatological aridity gradient in the U.S. and Canada. We focused on disentangling the most important controls of bulk surface conductance (Gs) and evaporative fraction (EF = LE/[H + LE]), where LE and H represent latent and sensible heat fluxes, respectively. Specifically, we investigated how vegetation phenology varied in importance relative to meteorological variables (vapor pressure deficit [VPD] and antecedent precipitation) as a driver of Gs and EF using path analysis, a framework for quantifying and comparing the causal linkages among multiple response and explanatory variables. Our results revealed that the drivers of Gs and EF seasonality varied significantly between energy- and water-limited ecosystems. Specifically, precipitation had a much higher effect in water-limited ecosystems, while seasonal patterns in canopy greenness emerged as a stronger control in energy-limited ecosystems. Given that phenology is expected to shift under future climate, our findings provide key information for understanding and predicting how phenology may impact 21st century hydroclimate regimes and the surface-energy balance.