Improving Predictions of Soil and Plant Evapotranspiration in Vadose Zone and Land Surface Models

This study focuses on the surface hydrology to contribute to its better parameterization through inter-comparison and direct improvement with two scopes: (1) soil evaporation process, and (2) the forest hydrological/physiological process based on Community Land Model (CLM). In soil evaporation study, the behaviors of the in-soil system and soil-atmosphere interface are explored and improved. The study of saturated front depths, under steady-state bare-soil evaporation, updates the previous method, which also provides a better understanding of the mechanism of the in-soil system. Through this investigation, the analytical and mass-conservative solution of the front depth is developed from Darcy’s equation by applying two-direction flows in the soil system. More importantly, applying soil-pore heterogeneity at the soil-atmosphere interface in the model has a significant influence on the evaporation rate. Through this study, a practical equation to use soil-pore heterogeneity is selected among known diffusion-based models (except empirical models) for the above the soil layer. Also, this study verifies the fully physical-based model about in and out soil system can mimic the behavior of bare-soil evaporation.

The study of the canopy process identifies that CLM4.5&5 fail to capture the environmental complexity on tropical mountain rainforest in Costa Rica. The newer version (CLM5) shows some improvement. However, it still has discrepancies with observations. This study highlights the issue of the parameter for photosynthesis, but also the lack of in-canopy variability caused by overly simple model structure for sub-canopy layers and site-specific features (e.g., large/frequent precipitation, steep slope). Conversely, multi-layered CLM (CLM-ml) alleviates temperature-related variables and leaf wetness, and it is useful in self-diagnosing through the profiled observation. For this test, the CLM-ml is updated for an in-canopy turbulence transfer, canopy shape, and carbon dioxide (CO2) concentration profile. This study indicates updating sub-canopy structure (e.g., canopy shapes), and the parameter for the turbulent transfer model can have a significant influence on model performance. However, more extensive monitoring of sub-canopies is necessary to increase model reliability for this and other sites with complex terrain and vegetation roughness.