Model-Based Interpretation of Hydrological and Biogeochemical Functional Traits of Hillslopes in a Mountainous Watershed
Authors
Lucien Stolze1* (lstolze@lbl.gov), Lijing Wang1* (lijingwang@lbl.gov), Zexuan Xu1, Baptiste Dafflon1, Bhavna Arora1, Dipankar Dwivedi1, Carl Steefel1, Robin Thibaut1, Matthias Sprenger1, Sebastian Uhlemann1, Chen Wang1, Yuxin Wu1, Haruko Wainwright1,2, Kristin Boye3, Nicola Falco1, Craig Ulrich1, Wenming Dong1, Markus Bill1, Curtis Beutler1, Benjamin Gilbert1, Kenneth Hurst Williams1, Michelle E. Newcomer1, Eoin Brodie1 (elbrodie@lbl.gov)
Institutions
1Lawrence Berkeley National Laboratory, Berkeley, CA; 2Massachusetts Institute of Technology, Boston, MA; 3SLAC National Accelerator Laboratory, Menlo Park, CA
URLs
Abstract
Declining snowpack, modification of precipitation patterns, and increasing temperature are rapidly altering the quantity and quality of water exported from headwaters. Hillslopes represent dominant landforms of mountainous watersheds and therefore exert fundamental controls on hydrological cycles and chemical composition of floodplains and rivers. However, subsurface processes controlling water partitioning and solute exports on mountain hillslopes remain poorly understood due to the heterogeneity of watershed traits characterizing the Critical Zone (e.g., bedrock lithology, topography, vegetation cover) and the impacts of local climatic conditions. This knowledge gap limits the ability to predict the evolution of high elevation watersheds experiencing climate change. Physics-based models are powerful tools to mechanistically unravel subsurface hydro-biogeochemical functioning and are key for the quantitative understanding of the traits that modulate water flow and solute exports.
In this study, researchers implement a new generation of HPC-enabled numerical models (e.g., ATS, CrunchFlow, PFLOTRAN) capable of simulating hydro-biogeochemical fluxes from hillslopes at seasonal timescales. Based on a comprehensive dataset characterizing: (1) the subsurface physico-chemical properties (e.g., geophysical measurements of bedrock resistivity); (2) the vegetation cover; and (3) the local weather conditions, a 3D hydrologic model and a 2D reactive transport model (RTM) are developed at two well-instrumented hillslopes of the East River watershed: Snodgrass and the Pumphouse transect, respectively. The models simulate the temporal fluctuations in partially saturated flow and the transient response of biogeochemical processes to climate events. In particular, for the first time, the 2D reactive transport model explicitly accounts for the connection between soil biological processes and rock weathering as well as multiphase exchange with the atmosphere. These results show the capability of the models to reproduce the dynamic change in snowpack, water content, groundwater table, soil respiration, and fluid chemistry monitored at multiple spatial locations. The calibrated models are used to quantify the relative contribution of shallow versus deep groundwater flow paths characterized by different residence times and biogeochemical conditions in shaping the solute fluxes from hillslopes. Furthermore, researchers perform sensitivity analysis to identify and quantify key functional traits controlling hydro-biogeochemical processes.
The identified functional traits will support the selection of new experimental hillslopes in the Taylor River basin using iterative model-experiment (ModEx) approaches and inform the scaling-up efforts. Finally, the implemented physics-based models will be used to explore the evolution of water flow and solute exports under different future climate scenarios for hillslopes with distinctive traits.