Modeling Transpiration and Evaporation in Parallel Columns.

Moisture flux across the land-atmosphere boundary (through soil evaporation and plant transpiration) is an important component of many large-scale hydrological and ecological processes. In modeling of large scale hydrological and ecological processes, the stream tube/column approach appears convenient way where it is assumed that the soil moisture movement is along a bundle of vertical tubes/columns. Under certain conditions however, lateral interactions between columns might be large enough to introduce larger errors. This study seeks to develop guidance of determining the dimensions of columns in simulating evaporation and transpiration appropriate in large scale landscape and quantifying the associated error. The horizontal flux in comparison to vertical component and the relative error for the total amount of water when applying the stream tube concept are quantified. At the two ends of the column sizes, the approach would induce large errors in representing large scale evaporation and transpiration. At small column size, the main problem is that the interactions among the columns are not captured due to the one-dimensional simplification. For large column size, the main difficulty is to find the appropriate effective parameters in representing plant and soil hydrological processes, which take into account the subgrid variability.