Influences of topography and subsurface heterogeneity on lateral subsurface flow in unsaturated zone

Abstract:

Lateral subsurface flow plays a significant role in redistributing soil water which has a direct effect on biological, chemical, and geomorphologic processes in root zone. However, most of the land surface models (LSMs) are neglecting the horizontal exchanges of water at the grid or subgrid scales, focusing only on the vertical exchanges of entities including water as one-dimensional process. Topography and hydrologic properties are the main controls on the vertical and lateral subsurface flow in heterogeneous land surface. Thus, in this study, we designed three cases to consider the lateral subsurface flow in unsaturated zone using a land surface model (Community Land Model, CLM): Case 1 is to consider only surface topography for the entire soil column, Case 2 is to consider surface topography in upper soil layers and heterogeneous soil hydraulic properties between the soil columns (using uniform anisotropy for lateral hydraulic conductivity) in lower soil layers, and Case 3 is also to consider surface topography and heterogeneous soil hydraulic properties but using spatially distributed anisotropy derived from hydrologic connectivity which can be created by physical controls (e.g., soil type, vegetation cover) at a watershed scale. The results of this study indicated that all of the cases improved the subsurface flow in unsaturated zone compared to the results of original model, especially representing the best performance in the case 3. Hence, this approach could characterize the spatially distributed patterns of subsurface flow and improve a simulation of the hydrologic cycle.