The Significance of Lateral Flows On Water Flux Across Heterogeneous Fields.

Water flux across the land-atmosphere boundary (through evaporation and transpiration) is an important water budget component of many large-scale hydrological processes, which is often quantified using multiple realizations of local water flow in independent one-dimensional columns.  A main problem of this independent column approach is that it neglects the interactions among different columns. Lateral flows might be significant for long and narrow columns and heterogeneous hydraulic properties and plant covers. This study is to quantify possible errors of using the independent column approach in hydrological simulations. Instead of using the parallel column models of independent hydrologic processes, this study utilizes a multi-dimensional modeling approach to simulate hydrological processes using multiple parallel columns which allow lateral flows/interactions. The results demonstrate that the accuracy of the independent column approach is controlled by the degree of spatial variability of soil hydraulic properties and initial moisture conditions. In particular, the independent column approach may result in up to 50% of relative errors in simulating transpiration at early stage and about 30% of relative errors at late stage of hydrological processes.