Investigating the Coupling of Root Zone and Hillslope Thermo-Hydro-Bio-Geo-Chemical Dynamics in a Mountainous Watershed using Multi-scale Approaches

Abstract:

While geophysics are useful in providing spatially continuous and time lapsed data for hydrogeological studies from meter to kilometer scales, coupled critical zone hydrobiogeochemical processes, such as water and nutrient dynamics in the root zone, often occur at finer, sub-meter scales, and are controlled by local heterogeneities in soil, hydrology and vegetation. Quantifying the coupling between larger scale hydrogeophysical data (often having meter scale or coarser resolution) with fine scale biogeochemical processes is critical for understanding and predicting multi-scale watershed dynamics. This study evaluates the interactions between root zone and larger scale processes along a hillslope of the East River watershed in the upper Colorado river basin, as part of a study to predict how mountainous watersheds respond to hydrological perturbations (such as droughts and early snowmelt), and the ramifications for down-gradient water quality and quantity. This hillslope site is used to explore the dynamic coupling between hydrology and biogeochemistry, primarily driven by the magnitude and timing of snowmelt. Time lapse Electrical Resistivity Tomography (ERT) is acquired along the hillslope, in addition to seismic and other measurements. Root zone hydrobiogeochemical study is performed using soil sensors and samplers installed to cover the entire root zone. Baseline seismic and ERT are used to understand the lithological structure along the hillslope, which includes a surface soil horizon, followed by a saprolite layer underlaid by fractured shale. Time lapse data are used to monitor thermal and hydrological fluxes. Data from soil sensors provide information about water dynamics and its impact on root zone nutrient/carbon movement, driven by the coupling between temperature, snowmelt and vegetation growth. Hydro-, thermal, and geophysical dynamics and couplings are considered in a joint data-model hydrogeophysical assimilation framework involving the Community Land Model (CLM). The numerical approach integrates the multiple datasets across a range of spatial and temporal scales and provides a framework for exploring interactions between various processes and for understanding and predicting hillslope hydrobiogeochemical behavior under future climatic conditions.