Coupling TOUGH2 with CLM3: Developing a Coupled Land Surface and Subsurface Model

PROCEEDINGS, TOUGH Symposium 2006 Lawrence Berkeley National Laboratory, Berkeley, California, May 15–17, 2006 COUPLING TOUGH2 WITH CLM3: DEVELOPING A COUPLED LAND SURFACE AND SUBSURFACE MODEL Lehua Pan, Jiming Jin, Norman Miller, Yu­Shu Wu, and Gudmundur Bodvarsson Earth Sciences Division, Lawrence Berkeley National Laboratory One cyclotron Road Berkeley, California, 94583, USA e­mail: lpan@lbl.gov models, surface­water models, and vegetation/ecology models often take the land surface as the lower boundary, parameterizing the subsurface processes in various simplified ways (e.g., runoff coefficient, evaporation coefficient). On the other hand, many physically based subsurface or groundwater models often take the land surface as the upper boundary by lumping the complex processes above the surface as known boundary conditions (e.g., net infiltration or hydraulic head). However, such simplified models cannot properly describe how the real system behaves, in many cases resulting in unacceptable errors. During the last few decades, much progress has been made in development of more realistic models to simulate hydraulic interactions through the land surface. Instead of simply taking the land surface as the boundary of the modeling domain, many models simulate the lower portion of the atmosphere and upper portion of the subsurface as an integrated system, by which the atmosphere­land interactions become internal processes (Abromopoulos et al., 1988; Famiglieti and Wood, 1991; Wood et al., 1992; Liang et al., 1994; Bonan, 1998; Dai and Zeng, 1997; Walko et al., 2000; Liang et al., 2003; Olesen et al., 2004). CLM3 is one such model primarily developed to meet the needs of regional climate modeling. In CLM3, radiation, sensible and latent heat transfer, zonal and meridional surface stresses, and ecological and hydrological processes are simulated as interrelated subprocesses, using hybrid approaches (i.e., combinations of physically based dynamic modeling and empirically based parameterization models). However, the model of subsurface moisture flow in CLM3 is still overly simplified. In this regard, TOUGH2 can offer more realistic physical process­ based modeling capability of subsurface hydrologic processes (including heterogeneity, three­ dimensional flow, seamless combining unsaturated and saturated zones, and water table). Therefore, coupling these two models is an attractive way to build a useful model of surface­subsurface hydraulic interactions. The objectives of this study are (1) to improve CLM3 simulation of important atmosphere­land interaction flux, such as ET, runoff, and latent heat flux by incorporating the sophisticated subsurface modeling ABSTRACT An understanding of the hydrologic interactions among atmosphere, land surface, and subsurface is one of the keys to understanding the water cycling system that supports life on earth. The inherent coupled processes and complex feedback structures among subsystems make such interactions difficult to simulate. In this paper, we present a model that simulates the land­surface and subsurface hydrologic response to meteorological forcing. This model combines a state­of­the­art land­surface model, the NCAR Community Land Model version 3 (CLM3), with a variably saturated groundwater model, TOUGH2, through an internal interface that includes flux and state variables shared by the two submodels. Specifically, TOUGH2 uses infiltration, evaporation, and root­uptake rates, calculated by CLM3, as source/sink terms in its simulation; CLM3 uses saturation and capillary pressure profiles, calculated by TOUGH2, as state variables in its simulation. This new model, CLMT2, preserves the best aspects of both submodels: the state­of­the­art modeling capability of surface energy and hydrologic processes (including snow, runoff, freezing/melting, evapotranspiration, radiation, and biophysiological processes) from CLM3 and the more realistic physical­process­based modeling capability of subsurface hydrologic processes (including heterogeneity, three­dimensional flow, seamless combining of unsaturated and saturated zone, and water table) from TOUGH2. The preliminary simulation results show that the coupled model greatly improved the predictions of the groundwater table, evapotranspiration, and surface temperature at a real watershed, as evaluated using 18 years of observed data. The new model is also ready to be coupled with an atmospheric simulation model, to form one of the first top of the atmosphere to deep­ groundwater atmosphere­land­surface­subsurface models. INTRODUCTION The land surface often becomes the boundary between different disciplines in the scientific and engineering community, because of different modeling objectives. For example, many climate