MODELING INTERACTIONS OF MASS AND ENERGY FLUXES ACROSS LAND SURFACE INTERFACE

The terrestrial hydrologic cycle is a key driving force of biogeochemical cycles and is inherently linked to the energy balance through the process of evapotranspiration (ET). Computational methods for simulating the terrestrial hydrologic cycle are often based on a separation of the different components, such as surface and groundwater flow, and the abstraction of the land surface interface, which is treated as a simplified boundary condition. For example, in groundwater flow models, the upper boundary condition commonly does not account for overland flow and the interdependence of ET and soil moisture. On the other hand, land surface models only incorporate the shallow root zone and disregard deeper groundwater flow acting on wide temporal and spatial scales.

In this study, new approaches are tested to couple the surface-subsurface flow equations and examine possible feedbacks across the land surface interface, when groundwater flow is incorporated into the mass and energy balance calculations of land surface models. ParFlow, a parallel, variably saturated groundwater flow model, was extended to simulate surface-subsurface flow in an integrated fashion and modularly incorporate the land surface model CLM (Common Land Model). Hypothetical test cases are used to study the interactions and uncertainty in mass and energy fluxes at the land surface due to subsurface flow and heterogeneity in the hydraulic properties. The Little Washita watershed, Oklahoma, was selected as a real-world test case for comparing with measured data and studying the two-way feedbacks between the subsurface and the land surface. Results show the uncertainty in hydrograph predictions arising from subsurface heterogeneity. Land surface models do not account for the spatial variability in soil moisture caused by heterogeneity and lateral groundwater flow, which leads to uncertainty in the energy and mass balances.

This work was conducted under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory (LLNL) under contract W-7405-Eng-48.