Modeling Hydraulic Responses to Meteorological Forcing: From Canopy to Aquifer

325 Land hydrologic responses to meteorological forcing involve complicated exchanges of moisture and energy between soil, vegetation, snowpack, groundwater, and the overlying atmospheric boundary layer. These exchanges occur in the form of many interactive natural hydrologic processes, including precipitation, snow and soil water melting and freezing, infi ltration, storage and movement of soil moisture, surface and subsurface runoff, recharge of groundwater, and evapotranspiration. Through these processes, soil, vegetation, snowpack, groundwater, and the overlying atmospheric boundary layer often become an integrated hydrologic system at various scales. Quantitatively understanding or modeling the behavior of this integrated system is critical not only in modeling regional climate or predicting global energy and water balances but also in assessing the impact of climate change and human modifi cations of the natural hydrologic system on the water resources that sustain our civilizations. However, the integrated system is often modeled separately for each subsystem because the land surface is traditionally the boundary between different disciplines in the scientifi c and engineering community. For example, many climate models, surface-water models, and vegetation and ecology models often take the land surface as the lower boundary, parameterizing the subsurface processes in various simplifi ed ways (e.g., runoff coeffi cient, evaporation coeffi cient). 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 infi ltration or hydraulic head). However, in nature, the hydraulic processes from canopy to aquifer often form an integrated surface–subsurface system through complicated interactions. As a result, such simplifi ed 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 developing 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 recent models simulate with various approaches the lower portion of the atmosphere and upper portion of the subsurface as an integrated system, by which the atmosphere–land 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; Gutowski et al., 2002; York et al., 2002; Liang et al., 2003; Oleson et al., 2004; Niu and Yang, 2006). 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 Modeling Hydraulic Responses to Meteorological Forcing: From Canopy to Aquifer