Impact of Fine-Scale Spatially Heterogeneous Canopy and Soil Moisture, and Three-Dimensional Root Water Uptake on Large-Scale Energy Fluxes in Vegetated Environments.

Abstract

The accuracy of transpiration simulation by current land surface models has been limited by several uncertainties. This work focuses on effects of representing fine-scale spatially varying soil moisture, vegetation characteristics, radiation environment, as well as three-dimensional root water uptake processes on larger-scale energy fluxes at the plot- and watershed-scales. The study location is a northern temperate mixed forest with spatially heterogeneous canopy, where intermediate disturbance experiment removed canopy of aspen and birch trees. An initial analysis of empirical data on temporal dynamics of soil water content under the canopies with varying degrees of disturbance demonstrates that crown-scale canopy heterogeneity leads to detectable differences in soil water status, attributed predominantly to a decreased root water uptake in areas of thinner canopy. In order to infer larger-scale implications of crown-scale variations in canopy biomass and soil moisture conditions, an ecohydrologic model tRIBS + VEGGIE is used as an integrating tool to explicitly resolves spatially varying canopy biomass, radiative forcing, and soil moisture. Numerical experimentation shows that root-canopy controls of water uptake exhibit two opposing effects on soil moisture spatial variability. Further, variations in canopy light environments introduce non-linear effects into large-scale response of transpiration to soil moisture conditions: they result in smaller spatially aggregated transpiration and lower water stress as compared to traditional representations. To alleviate the constraints of commonly used heuristic one-dimensional approach for root water uptake in moisture-limited conditions, a novel formulation based on hydraulic representation of three-dimensional uptake process is designed. When mimicking soil drought at the scale of a single root system, the formulation demonstrates plants’ ability to compensate the suppressed root water uptake in water-stressed regions by increasing uptake density in moister regions. Pilot, exploratory experimentation at the plot-scale demonstrates that transpiration and timing of water stress onset are sensitive to different representations of lateral spread. The simulation scenarios with a higher degree of root overlapping exhibit smaller spatial heterogeneity of soil moisture and a later onset of plot-scale water stress. It is argued that hydraulics-based modeling of small-scale root water uptake can be used to guide improvements in the representation of drought effects in land-surface models.