01 March 2013

A New Top Boundary Condition for Modeling Surface Diffusive Exchange of Generic Tracers

Science

Different empirical models of soil resistance and soil evaporation efficiency for the soil-atmosphere water exchange have been proposed by many investigators and used in several land surface models. However, ad hoc modifications have to be used when these empirical models are extended to simulate bi-directional soil-atmosphere exchanges of volatile organic compounds. We developed a physically-based top boundary condition that consistently accounts for differences in solubility and diffusivity of volatile chemical species during transport and reaction within the soil and exchange with the atmosphere. We applied this top boundary condition to the bare-soil evaporation problem and found that several widely used empirical soil resistance and soil evaporation efficiency models are flawed in some aspects. We also demonstrated that current field measurements are far from sufficiently accurate to derive accurate empirical formulations of soil resistance. The discrepancy between measurement-derived soil resistance and the theoretical exact soil resistance is severely impacted even when the measurement uncertainty is ~5%.

Impact

Based on our new theoretical developments, we predicted that, in semi-arid regions, soil evaporation is dominated by water vapor transport from below the topsoil, which is in agreement with field measurements. Finally, we found that comprehensive datasets for robust evaluation of this new approach are lacking. We call for more carefully designed lab experiments to help evaluate our physically-based soil resistance model. We conclude that this new capability, to be integrated in CESM/CLM, will improve CLM's estimates of surface to atmosphere exchanges.

Contact
Xiaohong Liu
Acknowledgments

This research was supported by the Director, Office of Science, Office of Biological and Environmental Research of the US Department of Energy under Contract No. DE-AC02-05CH11231 as part of their Regional and Global Climate Modeling (RGCM) Program. The authors thank Guido Salvucci for his comments on an early version of this manuscript. Discussion with John Selker greatly improved this work.