Impacts of root hydraulic redistribution on global evapotranspiration in a climate-scale land model
The redistribution of near-surface soil water by roots is an acknowledged phenomenon that has been shown to have impacts on water availability to plants, carbon and nutrient biogeochemistry, and energy coupling with the atmosphere. However, implementation of the relevant mechanisms in climate-scale models is either missing or numerically ambiguous. To investigate the impacts of root hydraulic redistribution, we implemented the Amenu-Kumar (2008) approach in an updated version of CLM4.5. We tested simultaneous and sequential numerical implementations to solve the coupled root water transport and Richards’ equations.
We found that the sequential implementation has a very strong time stepping dependence, while the tightly coupled implementation is numerically very robust. We compared the two implementations to measured evapotranspiration (ET) at several Ameriflux sites and to the FLUXNET_MTE dataset . The numerical implementation approach has large impacts on predicted global land ET. In particular, the sequential implementation overestimates ET as much as 5 mm H2O day-1 in some grid cells compared to the tightly coupled implementation. Our sensitivity analyses revealed that hydraulic redistribution affects the global soil water cycle significantly by enhancing evapotranspiration, particularly over dry regions. We also found that uncertainty in the soil water retention pedotransfer function resulted in smaller impacts to global ET compared to that by hydraulic redistribution. Finally, we found that, even with hydraulic redistribution, CLM4.5 still predicts enhanced ET associated with vegetation removal, suggesting that additional investigation is warranted to further improve the hydrological modeling components of CLM.