18.3 Models of Ecosystem Function
1993). RHESSys addresses the dependency of spatial patterns of forest processes and hydrological
fluxes on topography through the variation topographic relief causes to solar radiation distribution,
precipitation, temperature, and soil water drainage
(Band et aI., 1993). By including hillslope hydrology with representations of biogeochemical processes, RHESSys represents seasonal plant productivity and evapotranspiration trends very
differently than previous ecosystem models, which
were restricted to homogeneous biogeochemical
fluxes and vertical soil water transport equations.
RHESSys integrates FOREST-BGC, described
previously, with TOPMODEL (Beven and Kirkby,
1979) to investigate the distributed feedbacks between ecological and hydrological processes at the
watershed scale. TOPMODEL provides for lateral
subsurface drainage of soil water from hillslopes
and production of saturation runoff from partial
contributing areas of the watershed. It also distributes soil moisture on the basis of hillslope position,
introducing important topographic controls on
canopy processes and forest growth through the
zonation of soil moisture deficits. Daily weather is
similarly distributed across the landscape with
MT -CUM (Running et aI., 1987). This subroutine
extrapolates from a base meteorological station to
adjust and distribute temperature, relative humidity, precipitation, and solar radiation across the
landscape by incorporation of elevational lapse
rates, atmospheric optical depth, and illumination
angles using site elevation, gradient, and exposure.
The spatial resolution of processes and surface
representation is given two ways: hydrologically,
where hillslopes and stream reaches are explicitly
located within a watershed (Band, 1989), and by
surface properties, derived from digital terrain data
and remotely sensed imagery, that are statistically
summarized by hillslope. Input data include vegetation type and LAI; digital elevation models for
calculation of surface slope, aspect, elevation, and
the contributing drainage area for each pixel (Band
and Wood, 1988; Band, 1989); and soil maps for
calculation of soil depth, texture, and water transmissivity. Meteorological data include daily maximum and minimum temperature, precipitation, relative humidity or dewpoint, and solar radiation, if
available (Running et aI., 1987).
There are several advantages to coupling hydrological and ecological models, in addition to that
of more realistic heterogeneous representation of
forest or ecosystem processes: the model can be
employed to examine spatial fluxes of materials
and to integrate terrestrial and aquatic processes. In
addition, the digital terrain data allow the user to
265
determine the desired level of spatial heterogeneity, thus allowing for expansion from local to regional scale simulations (Band, 1989, 1991). Use
of the system may be limited by the availability of
high-quality soils data (Nemani et aI., 1993). Topographically based corrections to coarse-resolution
soils maps can be implemented in RHESSys, but
the effectiveness of such corrections depends on
the accuracy of the digital elevation model used.
RHESSys has been used to address the effects
of spatial scale on representation of ecological and
hydrologic processes (Lammers et al., 1997), to determine whether the flushing of nitrogen from
forested catchments into streams is controlled by
topographic properties (Creed et aI., 1996), to examine the effects of land-use change on ecological
and hydrologic processes (Baron et aI., 1998a), and
to explore watershed responses to climate change
(Baron et aI., 1998b).
18.3.3 CENTURY
The CENTURY terrestrial ecosystem model simulates the major pathways for carbon and nitrogen exchange for all major vegetation types of the world
(VEMAP Members, 1995; Schimel et al., 1996). initially developed to simulate fluxes of carbon, nitrogen, and sulfur for grasslands, it has been expanded
to address management questions related to agricultural cropping and grazing practices (e.g., Paustian
et aI., 1990; Holland et aI., 1992; Cole et aI., 1993;
Rasmussen and Parton, 1994; Lyon et al., 1995;
Metherell et aI., 1995), forest responses to disturbance (Sanford et aI., 1991), ecosystem responses to
nitrogen deposition (Baron et al., 1994; Wedin and
Tilman, 1996), and land-use and climate change scenarios (Burke et aI., 1991; Schimel et al., 1991,
1994; VEMAP Members, 1995). CENTURY simulates primary productivity, soil nutrient dynamics,
soil water, trace gas emissions, and hydrologic losses
of nitrogen over long periods of time (50 to 2000
years) (Parton et aI., 1987, 1988, 1994; Schimel et
aI., 1990, 1991, 1994, 1996; Ojima et aI., 1994).
The key feature of CENTURY is a soil organic
matter model that integrates changes in production
and decomposition over time. Organic matter is
central to the cycling of plant nutrients, influences
soil water relations and erosion, and is important
to soil structure. In CENTURY, soil organic matter is made up of three fractions: an active fraction
of soil carbon and nitrogen consisting of live microbes and microbial products (1- to 5-year
turnover time), a slow fraction that is more resistant to decomposition (20- to 40-year turnover
time), and a passive fraction that is highly resistant
1993). RHESSys addresses the dependency of spatial patterns of forest processes and hydrological
fluxes on topography through the variation topographic relief causes to solar radiation distribution,
precipitation, temperature, and soil water drainage
(Band et aI., 1993). By including hillslope hydrology with representations of biogeochemical processes, RHESSys represents seasonal plant productivity and evapotranspiration trends very
differently than previous ecosystem models, which
were restricted to homogeneous biogeochemical
fluxes and vertical soil water transport equations.
RHESSys integrates FOREST-BGC, described
previously, with TOPMODEL (Beven and Kirkby,
1979) to investigate the distributed feedbacks between ecological and hydrological processes at the
watershed scale. TOPMODEL provides for lateral
subsurface drainage of soil water from hillslopes
and production of saturation runoff from partial
contributing areas of the watershed. It also distributes soil moisture on the basis of hillslope position,
introducing important topographic controls on
canopy processes and forest growth through the
zonation of soil moisture deficits. Daily weather is
similarly distributed across the landscape with
MT -CUM (Running et aI., 1987). This subroutine
extrapolates from a base meteorological station to
adjust and distribute temperature, relative humidity, precipitation, and solar radiation across the
landscape by incorporation of elevational lapse
rates, atmospheric optical depth, and illumination
angles using site elevation, gradient, and exposure.
The spatial resolution of processes and surface
representation is given two ways: hydrologically,
where hillslopes and stream reaches are explicitly
located within a watershed (Band, 1989), and by
surface properties, derived from digital terrain data
and remotely sensed imagery, that are statistically
summarized by hillslope. Input data include vegetation type and LAI; digital elevation models for
calculation of surface slope, aspect, elevation, and
the contributing drainage area for each pixel (Band
and Wood, 1988; Band, 1989); and soil maps for
calculation of soil depth, texture, and water transmissivity. Meteorological data include daily maximum and minimum temperature, precipitation, relative humidity or dewpoint, and solar radiation, if
available (Running et aI., 1987).
There are several advantages to coupling hydrological and ecological models, in addition to that
of more realistic heterogeneous representation of
forest or ecosystem processes: the model can be
employed to examine spatial fluxes of materials
and to integrate terrestrial and aquatic processes. In
addition, the digital terrain data allow the user to
265
determine the desired level of spatial heterogeneity, thus allowing for expansion from local to regional scale simulations (Band, 1989, 1991). Use
of the system may be limited by the availability of
high-quality soils data (Nemani et aI., 1993). Topographically based corrections to coarse-resolution
soils maps can be implemented in RHESSys, but
the effectiveness of such corrections depends on
the accuracy of the digital elevation model used.
RHESSys has been used to address the effects
of spatial scale on representation of ecological and
hydrologic processes (Lammers et al., 1997), to determine whether the flushing of nitrogen from
forested catchments into streams is controlled by
topographic properties (Creed et aI., 1996), to examine the effects of land-use change on ecological
and hydrologic processes (Baron et aI., 1998a), and
to explore watershed responses to climate change
(Baron et aI., 1998b).
18.3.3 CENTURY
The CENTURY terrestrial ecosystem model simulates the major pathways for carbon and nitrogen exchange for all major vegetation types of the world
(VEMAP Members, 1995; Schimel et al., 1996). initially developed to simulate fluxes of carbon, nitrogen, and sulfur for grasslands, it has been expanded
to address management questions related to agricultural cropping and grazing practices (e.g., Paustian
et aI., 1990; Holland et aI., 1992; Cole et aI., 1993;
Rasmussen and Parton, 1994; Lyon et al., 1995;
Metherell et aI., 1995), forest responses to disturbance (Sanford et aI., 1991), ecosystem responses to
nitrogen deposition (Baron et al., 1994; Wedin and
Tilman, 1996), and land-use and climate change scenarios (Burke et aI., 1991; Schimel et al., 1991,
1994; VEMAP Members, 1995). CENTURY simulates primary productivity, soil nutrient dynamics,
soil water, trace gas emissions, and hydrologic losses
of nitrogen over long periods of time (50 to 2000
years) (Parton et aI., 1987, 1988, 1994; Schimel et
aI., 1990, 1991, 1994, 1996; Ojima et aI., 1994).
The key feature of CENTURY is a soil organic
matter model that integrates changes in production
and decomposition over time. Organic matter is
central to the cycling of plant nutrients, influences
soil water relations and erosion, and is important
to soil structure. In CENTURY, soil organic matter is made up of three fractions: an active fraction
of soil carbon and nitrogen consisting of live microbes and microbial products (1- to 5-year
turnover time), a slow fraction that is more resistant to decomposition (20- to 40-year turnover
time), and a passive fraction that is highly resistant