Linking Ground Hydrology to Ecosystems and Carbon Cycle in a Climate Model

The paper discusses land surface processes in a climate model, linking the roles of hydrology, ecosystems, and carbon cycling. The integrating factor is the movement of water and carbon dioxide through the stomates of leaves. This molecular gas transfer is a major control of coupling between land and the atmosphere, as required for modeling climate variability and change, land storage of carbon, and the dynamics of ecosystems. Recent work by the author on parameterizing these fluxes is reviewed. These parameterizations lead to a framework for calculating leaf areas as a model prognostic variable. It is necessary to also include the processes responsible for cycling of nitrogen between leaves and soil for a physically complete description. Land surface processes are a major element of current comprehensive climate/earth system models. From the viewpoint of climate variability, evapotranspiration (ET) is perhaps the most important process that couples land to atmosphere. Much of the water in ET is transpired through leaves (Dickinson, 1983). Hence, parameterization of the land vegetation addresses the most important process for moving water from land into the atmosphere. Given precipitation, evapotranspiration is closely related to runoff and the supplies of water in general since averaged over time, a change in one requires an equal and opposite change in the other. The issue of greenhouse warming requires concentrations of carbon dioxide in the atmosphere. Because the fluxes of carbon dioxide from fossil fuel burning are partioned between the atmosphere and land in nearly equal amounts, another requirement for the parameterization of land processes is to determine the fluxes of carbon dioxide between land and atmosphere. Land carbon is stored either in vegetation or in soils; the soil reservoir is almost entirely supplied by dead vegetation. All such carbon is moved from the atmosphere to land through the stomates of leaves. It is these same stomates that move water from the soil to the atmosphere. Indeed, the primary biological reason that plants transpire is that water necessarily leaks out when carbon dioxide is taken in. Hence, in most current land parameterizations, water and carbon dioxide fluxes are determined together from the same leaf model (e.g. Sellers et al., 1997). The leaf assimilation of carbon and its relationship to ET is the primary input for modeling the growth