Integrating Tree Hydraulic Trait, Forest Stand Structure, and Topographic Controls on Ecohydrologic Function in a Rocky Mountain Subalpine Watershed

Complex mountainous terrain comprises more than 25% of the terrestrial

terrestrial

The geographical and biological features that are associated with the Earth's solid, rocky surface, residing on or resembling the land or terrain found on our planet.

Source: NASA Source

surface, and water discharge from mountain zones accounts for at least half of the world’s freshwater resources. In the Colorado River Basin alone, mountain watersheds

watershed

An area of land that channels rainfall and snowmelt to creeks, streams, and rivers, and eventually flows to the lowest point in a basin.

Source: USGS Source

provide the primary water source for more than 40 million people. Accurately representing interactions among vegetation, terrain, and the hydrological cycle in mountain zones remains one of the critical unsolved challenges in integrated Earth system modeling. The complex topography

topography

The land's physical attributes within a specific area or region, comprising both natural formations like mountains, valleys, and rivers, alongside human-made additions such as roads and buildings.

Source: Source

and subsurface heterogeneity constrain water flow paths to produce hydrological heterogeneity that is currently not well represented in land-surface or hydrological models.  In forested watersheds, trees mediate ecohydrological processes via physiological controls on water use and drought tolerance, but also via tree size, density and species composition, which alter precipitation interception and water retention. Therefore, plants respond and contribute to hydrological heterogeneity in ways that are only very recently integrated into some land surface models. This project aims to improve understanding and spatially explicit 3D prediction of tree-mediated water and energy fluxes

flux

The amount of some type of particle or energy crossing a unit area per unit time.

Source: NRC Library Source

and subsurface flow that together regulate ecohydrologic function in a forested subalpine watershed. Four hypotheses will be tested:

Rooting depth and stem capacitance, more so than other hydraulic traits, explain the differences in diurnal and seasonal transpiration patterns and in growth sensitivity to climate across the dominant species in subalpine watersheds.

Across forest stands, soil moisture, canopy water content, transpiration, and radial growth covary and depend on non-linear interactions between stand density, species dominance, and topographic setting. The highest canopy water content and transpiration rates are expected in broadleaf tree-dominated forest stands.

Interannual differences in forest stand-scale soil moisture, canopy water content, transpiration, and growth are smallest in more convergent, high-elevation topographic positions with low incident solar radiation. Interannual differences are expected to be largest in convergent zones with high incident radiation, where maximum transpiration and the potential range of fluxes is very high.

Forest structure and composition have little influence on seasonal soil moisture and transpiration dynamics at more convergent landscape positions where subsurface lateral flow contributions are higher, but have stronger influence on these dynamics in more divergent and neutral positions, particularly during periods of drought.

    The research approach integrates field measurements of tree hydraulic traits, transpiration, canopy water content, tree ring-width variation, and soil moisture; airborne observations of transpiration and canopy water content; and coupled 3D hydrologic-vegetation demographic modeling using ParFlow-ELM-FATES across a heterogeneous watershed in the Upper Colorado River basin. The project will partner with DOE’s field-based Watershed Function SFA and the Rocky Mountain Biological Laboratory, hydrologic model developers at PNNL, and DOE’s computational facilities to achieve project goals. Project data will be archived in DOE’s ESS-DIVE archive.