Improving Evapotranspiration Estimates from CMIP5 Simulations: Future Trends and Sensitivity to Changing Climate over North America

Future projections of evapotranspiration (ET) are of critical importance for agricultural, wildfire, and freshwater management, and predicting land-atmosphere feedbacks on the climate system. ET can be measured in situ, estimated from remote-sensing retrievals, or simulated in numerical models. In situ measurements are retrieved at high temporal resolution, but are limited spatially; remote-sensing retrievals can provide near global information, but are typically at lower temporal frequency and can exhibit large regional biases; and numerical simulations can run at high spatial and temporal resolution, but rely on parameterizations that fail to account for sub-grid scale processes. Hence, a synthesis of these methods is necessary to better understand ET. Herein, we demonstrate how a Penman-Monteith (PM)-based, remote sensing retrieval algorithm can be applied to correct ET simulated by a suite of models from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). The algorithm uses as inputs: temperature (T), water vapor pressure (e), atmospheric pressure (P), and surface net radiation (R) from the CMIP5 model output, and mean annual vegetation properties from satellite-based measurements. The reconstructed ET estimates are validated against in situ flux measurements across N. America. This methodology is used to reconstruct ET projections from 2006 through 2100 over North America using output from selected CMIP5 models, and to attribute projected ET trends to specific atmospheric controls.

CMIP5 ET exhibits substantial bias in annual ET relative to in situ flux measurements across N. America (38-73%; 2006-2015), but ET reconstructed from the CMIP5 meteorology with the PM algorithm greatly reduce this bias (-8-+14%). Present-day N. American ET is more sensitive to changes in atmospheric demand for ET (temperature and water vapor pressure) than energy limitation (net radiation), and to a lesser extent vegetation properties (leaf area index). Accordingly, ET is projected to increase 0.26-0.87 mm yr^-1 yr^-1 over N. America through 2100 driven primarily by trends in temperature.