Development of a Process-Rich Modeling Framework for Arctic Ecohydrology Using the Open-Source PFLOTRAN and CLM Models

CONTROL ID: 1794491

TITLE: Development of a Process-Rich Modeling Framework for Arctic Ecohydrology Using the Open-Source PFLOTRAN and CLM Models

AUTHORS (FIRST NAME, LAST NAME): Richard T. Mills^{1, 2}, Gautam Bisht³, Glenn E. Hammond⁴, Benjamin J. Andre³, Jitendra Kumar¹, Satish Karra⁵, Scott L. Painter⁵, Peter C. Lichtner⁶, Guoping Tang¹, Fengming Yuan¹, Xiaofeng Xu¹, Forrest M. Hoffman¹, William J. Riley³, Peter E. Thornton¹

INSTITUTIONS (ALL): 1. Oak Ridge National Laboratory, Oak Ridge, TN, United States.
2. University of Tennessee, Knoxville, TN, United States.
3. Lawrence Berkeley National Laboratory, Berkeley, CA, United States.
4. Pacific Northwest National Laboratory, Richland, WA, United States.
5. Los Alamos National Laboratory, Los Alamos, NM, United States.
6. OFM Research, Santa Fe, NM, United States.

ABSTRACT BODY: Current Earth system models greatly simplify or completely omit some key physical processes, such as lateral flow of water and heat, surface-subsurface interactions, realistic groundwater-vadose zone interactions, and freeze-thaw dynamics. Capturing the effects of such processes is critically important for predicting the fate of carbon stored in vulnerable permafrost-affected regions as the climate warms. Towards this end, we have added non-isothermal coupled surface-water groundwater interactions and a multi-phase ice model to PFLOTRAN--an open-source, massively parallel hydrologic flow and reactive transport model--and have developed a framework for coupling it with the Community Land Model (CLM), the state-of-the-art LSM component of the Community Earth System Model (CESM).

In the coupled CLM-PFLOTRAN model, PFLOTRAN replaces the CLM treatment of surface and subsurface thermal hydrology. CLM provides sources and sinks of water (evapotranspiration, snowmelt, and precipitation) and heat flux; while PFLOTRAN evolves the subsurface soil moisture and thermal states (including freeze-thaw dynamics). This allows a unified treatment of the unsaturated and saturated zones (which are decoupled in standalone CLM) and enables lateral redistribution of surface and subsurface water and heat. In addition to hydrologic coupling, we are developing a bigeochemistry linkage to enable interaction between CLM plant functional types and PFLOTRAN subsurface biogeochemistry. As a first step in the eventual use of PFLOTRAN biogeochemistry with CLM, we have reproduced the CLM Carbon-Nitrogen (CLM-CN) soil carbon and nitrogen reaction network in standalone PFLOTRAN, and are working to include explicit nutrient transport processes and methane biogeochemistry using PFLOTRAN.

Treating this rich set of coupled processes operating across a broad range of temporal and spatial scales poses several computational challenges: In freezing conditions, the subsurface thermal hydrology equations are strongly nonlinear, with key physical parameters extremely sensitive to temperatures around the nominal freezing point; time integration is complicated by widely differing rates of flow and transport between the surface and subsurface domains; and choosing an appropriate strategy for coupling the many process models is a challenge. We will discuss some of the approaches we have used to enable our simulations on high-performance computing platforms with PETSc, the Portable, Extensible Toolkit for Scientific Computation, and present some demonstrations of the new model capabilities.

INDEX TERMS: 1805 HYDROLOGY Computational hydrology, 1932 INFORMATICS High-performance computing, 0706 CRYOSPHERE Active layer, 1813 HYDROLOGY Eco-hydrology.
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Contact Details

CONTACT (NAME ONLY): Richard Mills
CONTACT (E-MAIL ONLY): rtm@utk.edu
TITLE OF TEAM: NGEE-Arctic Modeling Team