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now seeking to describe the evolution of the biome (i.e., vegetation cover)
represented in the GCM at a particular place in response to long-term changes in
modeled climate (Foley et al., 1996; 2000; Cox et al. , 2000; 2001; Kucharik et al. ,
2000; Oyama and Nobre, 2003; 2004). Ultimately this capability may also become
relevant in long-term, large-scale hydrological modeling studies.
As yet, this group of sub-models does not consider interactions between the
biogeochemical cycles of carbon, nitrogen and phosphorus. In fact, such
interactions are not yet considered in many terrestrial biogeochemical
models, including the CENTURY model (Wang et al. , 2007). However, including
such interactions is likely necessary for a more comprehensive and realistic
representation of the effect of land surface processes on the carbon cycle. It is also
true that most of the land surface models in this group of sub-models have simpler
treatments of sub-grid spatial variability and hydrological processes than those
described in the last section. Consequently, there is a need to combine the model
improvements in describing hydrological processes with those describing carbon
dioxide exchange and vegetation dynamics in a new generation of land surface
sub-models. Fortunately, ecologists, climatologists, and hydrologists have begun
to work together to improve the realism and functionality of SVATS.
Ongoing developments in land surface sub-models
The impacts of surface and groundwater interactions on the land-atmosphere sys-
tem have hitherto received little attention, but recognizing the role roots (especially
deep roots) may play in the plant-soil-land continuum, researchers have now begun
to investigate the dynamic interactions of surface water and groundwater, and
whether such interactions can affect vegetation and, via vegetation, land-atmos-
phere interactions (e.g., Winter, 2001; Gutowski et al. , 2002; York et al. , 2002; Liang
et al ., 2003; 2006; Maxwell and Miller, 2005; Yeh and Eltahir, 2005; Fan et al ., 2007;
Niu et al. , 2007). Several different approaches are under investigation, including:
(a) A 'TOP model' type based approach (e.g., Walko et al. , 2000).
(b) Solving for soil moisture in unsaturated zones and pressure head profiles in
saturated zones separately by applying (variations of ) the Richards equa-
tion to each zone respectively (e.g., Gutowski et al. , 2002; York et al. , 2002;
Yeh and Eltahir, 2005; Fan et al. , 2007; Niu et al. , 2007). In this approach,
the coupling is essentially one-way rather than two-way.
(c) Solving for the hydraulic pressure profile for the unsaturated and saturated
zones together based on a mixed form of the Richards equation (e.g.,
Maxwell and Miller, 2005). This approach involves two-way coupling.
(d) Solving for soil moisture profile by applying the Richards equation to the
unsaturated zone only, with the groundwater table treated as a moving
boundary (e.g., Liang et al. , 2003). This relatively simple approach does not
introduce any additional parameters other than those already used by a
typical land surface model and also involves two-way coupling.
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