Impacts of a new bare‐soil evaporation formulation on site, regional, and global surface energy and water budgets in CLM4

We describe the implementation of a new bare‐soil evaporation formulation in the Community Land Model‐version 4 (CLM4). This new formulation comprises two components: (1) a full‐range (desiccated to fully saturated) soil water retention curve (SWRC) parameterization that extends the classical Clapp‐Hornberger parameterization and (2) a mechanistically based formulation of soil resistance that considers the effects of water vapor diffusion and liquid mass flow. Predictions by the new SWRC parameterization matched measured soil matric pressure data across a wide soil moisture range for six very different soils. We assessed the revised bare‐soil evaporation formulation with two types of global simulations, one with prescribed satellite plant phenology and the other with bare‐soil conditions. Compared with the default CLM4 soil evaporation formulation, the revised model leads to (1) slightly lower (−0.2 ∼ 0 mm d −1 averaged annually) bare‐soil evaporation in moderately wet regions; (2) slightly higher (0 ∼ 0.2 mm d −1 averaged annually) bare‐soil evaporation in semiarid regions; (3) small changes in global surface energy and water balances across all temporal scales for both vegetated and nonvegetated simulations; and (4) a small reduction (−0.2 ∼ 0 mm d −1 averaged annually) in the current overestimation of evapotranspiration in densely vegetated regions such as the Amazon basin. However, contrary to expectation and consistent with the default CLM4, the simulated bare‐soil evapotranspiration remains higher than that of the vegetated soil in the same grid cells. We conclude that further studies are needed to identify the processes that lead to the overestimation of bare‐soil evaporation in CLM4.