Subsurface flow in the unsaturated zone is an important component of the hydrologic cycle and plays a significant role in the water and energy balance through affecting various hydrological processes. Land surface models (LSMs) have been developed and extended during the past decades with various enhanced processes to understand and quantify the complex interaction between atmosphere and land surface systems. However, there are still critical deficiencies (e.g., simplified processes and parameterization) remaining in simulating land surface hydrology for land surface modeling. Thus, this dissertation focuses on understanding land surface processes from various land surface models and improving land surface processes and parameterization in land surface modeling in the unsaturated zone at various spatial scales.
Two main approaches (Bayesian Model Averaging (BMA) based multi-model simulation and physically based hydrologic connectivity approach) to improve the land surface modeling predictability are presented in this dissertation. The BMA-based multi-model simulation approach was developed to reflect the strengths of the models under various land surface wetness conditions and to quantify the model parameter and structural uncertainties. The physically-based hydrologic connectivity concept was proposed to characterize the subsurface flow variability based on spatially distributed patterns of wetness condition or physical controls (e.g., soil type, vegetation, topography). Hydrologic connectivity is an important concept for understanding local processes in the context of catchment hydrology and defining flow path continuity in surface and subsurface flows. These approaches were applied in land surface modeling and tested in various hydro-climate regions and spatial scales showing significant improvement of modeling predictability. Based on the knowledge and experience gained from this dissertation, the proposed concepts will be useful to improve the hydrological model performance and better understand the subsurface flow variability in the unsaturated zone at various scales
