IBIS

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(1) MODEL AND VERSION:

Full model name: IBIS 2.1 (The Integrated Biosphere Simulator)

Host institution: University of Wisconsin-Madison

Key references:

Foley, J.A., I.C. Prentice, N. Ramunkutty, S. Levis, D. Pollard, S. Sitch, and A. Haxeltine, An integrated biosphere model of land surface processes, terrestrial carbon balance and vegetation dynamics, Global Biogeochemical Cycles, 10, 603-628, 1996.

Kucharik, C.J., J.A. Foley, C. Delire, V.A. Fisher, M.T. Coe, S.T. Gower, J. Lenters, C.Molling, J.M. Norman, and N. Ramankutty (2000). Testing the performance of a dynamic global ecosystem model: Water balance, carbon balance and vegetation structure. Global Biogeochemical Cycles (in press)

Delire, C. and J.A. Foley (1999). Evaluating the performance of a land surface/ecosystem model with biophysical measurements from contrasting environments. Journal of Geophysical Research (Atmospheres) 104(D14), 16,895-16,909.\

(2) MODEL TYPE (E.G. ECOSYSTEM, BIOGEOGRAPHY, DGVM):

DGVM

(3) PRIMARY MODEL PURPOSE:

Perform integrated assessments of water balance, carbon balance, and vegetation structure on both global and regional scales based on an integrated modeling approach that explicitly represents competition between plant functional types (competition for light and water) - and characterize their responses to global change drivers (land use changes, climate variability and change, atmospheric CO2)

(4) MODELING APPROACH:

IBIS represents a wide range of ecosystem and land surface processes in a single, physically-consistent framework. In this way, IBIS can simulate the dynamic behavior of land surface and ecosystem processes, and their consequences for vegetation composition and structure.

IBIS is designed around a hierarchical conceptual framework, and includes several sub-models (or "modules") which are organized with respect to their characteristic temporal scale: Land surface processes (energy, water, carbon and momentum balance), soil biogoechemistry (carbon and nitrogen cycling from plant through soil), vegetation dynamics (plant competition for light, water and eventually nutrients), vegetation phenology (based on a growing degree day approach), and atmospheric coupling (IBIS is now directly coupled to GENESIS and CCM3 GCMs).

(5) RESOLUTION (SPACIAL, TEMPORAL):

IBIS can be run at varying spatial resolutions (0.5, 1.0, 2.0, 4.0 degree and UKMO) and can be set up to function at sub-daily timesteps as small as 30 minutes.

(6) SPATIAL AND TEMPORAL SCALE(S) AT WHICH THE MODEL RESULTS SHOULD BE CONSIDERED:

Spatial: 0.5, 1.0, 2.0 and 4.0 degrees

Temporal: hourly

(7) PROCESSES AND PROCESS COMPONENTS SIMULATED (E.G. CARBON: GPP, NPP, NEP):

Carbon:

GPP, above and belowground NPP, NEP, fine root and heterotrophic respiration

Water:

a) Soils:

IBIS uses a multi-layer formulation of soil to simulate the diurnal and seasonal variations of heat and moisture in the top 12 meters of the soil. The eight soil layers in IBIS have top-to-bottom thicknesses of 0.10, 0.15, 0.25, 0.50, 1.0, 2.0, 4.0 and 4.0 m, respectively. At any timestep, each layer is described in terms of soil temperature, volumetric water content and ice content [Pollard and Thompson, 1995; Foley et al., 1996]. The IBIS soil physics module uses Richard’s equation to calculate the time rate of change of liquid soil moisture, and the vertical flux of water is modeled according to Darcy’s Law [Campbell and Norman, 1998]. The water budget of soil is controlled by the rate of infiltration, evaporation of water from the soil surface, the transpiration stream originating from plants, and redistribution of water in the profile.

b) Energy balance: latent, sensible heat, aet, evaporation, transpiration

The IBIS land surface module, which is based on the LSX land surface package of Thompson and Pollard [1995a,b], simulates the energy, water, carbon, and momentum balance of the soil-vegetation-atmosphere system. The model represents two vegetation canopies (i.e., trees versus shrubs and grasses), eight soil layers, and three layers of snow (when required) (see Figure 2). Following the logic of most land surface packages, IBIS explicitly represents the temperature of the soil (or snow) surface and the vegetation canopies, as well as the temperature and humidity within the canopy air spaces. The radiation balance of the vegetation and the ground, and the diffusive and turbulent fluxes of sensible heat and water vapor, drive changes in temperature and humidity. In order to resolve the diurnal cycle, the IBIS land surface module uses a relatively short timestep (60 minutes in this study).

IBIS simulates the exchange of both solar and infrared radiation between the atmosphere, the vegetation canopies, and the surface. Solar radiation transfer is simulated following the two-stream approximation, with separate calculations for direct and diffuse radiation in both visible (0.4 to 0.7 µm) and near-infrared (0.7 to 4.0 µm) wavelengths.

c) Snow: 3 layer physically-based model of snow temperature, extension and depth

d) 'Order' of water balance: transpiration, evaporation, infiltration, runoff

The total amount of evapotranspiration from the land surface is treated as the sum of three water vapor fluxes: evaporation from the soil surface, evaporation of water intercepted by vegetation canopies, and canopy transpiration. Rates of transpiration depend on canopy conductance (see below) and are calculated independently for each plant type within the canopy. To account for evaporation from intercepted rain, the model describes the interception and cascade of precipitation (both rain and snow) through the canopies.

 

Nitrogen: nitrogen mineralization

(8) SIMULATED RESERVOIRS

Carbon:

a) vegetation: fine roots, leaves, and wood for upper canopy (trees) and fine roots and leaves for lower canopy (shrubs and grasses)

b) litter: above and belowground (fine root) separated in 3 distinct pools (decomposable, structural and resistant)

c) SOC: microbial biomass, protected and unprotected "slow" C pools, and passive C pool

Nitrogen:

a) vegetation: assumed C:N ratios for fine roots, leaves, wood

b) litter: above and belowground (fine root) separated into 3 pools (decomposable, structural, and resistant)

c) SON: microbial biomass, protected and unprotected "slow" pools, and passive pool

Soil water:

Physically based model of soil temperature, soil ice and water content in 8 soil layers (0-10, 10-25, 25-50, 50-100, 100-200, 200-400, 400-800, 800-1200 cm)

(9) CALIBRATION VARIABLE(S) AND METHOD:

NPP, soil carbon, soil CO2 flux, microbial biomass, litter, fine root biomass, LAI (See Kucharik et al., in press) on a biome by biome average basis

Energy and carbon balance (components) at selected tower flux sites (See Delire and Foley, 1999)

(10) SCALING OF THE PROCESSES TO THE GRID CELL:

Radiative Transfer :

The solar radiative transfer scheme of IBIS-2 has been simplified; sunlit and shaded fractions of the canopies are no longer treated separately. The model now uses the solution of the two-stream approximation following the approach of Sellers et al. [1986] and Bonan [1995]. Infrared radiation is simulated as if each vegetation layer is a semi-transparent plane; canopy emissivity depends on foliage density.

In IBIS-1, we scaled photosynthesis and transpiration from the leaf level to the canopy level by calculating those rates separately for sunlit and shaded leaves and then averaging them weighted by the fraction of sunlit and shaded leaves occupying the canopy. In IBIS-2, we use a new and simpler approach for canopy scaling. Our new approach assumes that the net photosynthesis within the canopy is proportional to the absorbed PAR (APAR) within it. This assumption is supported by several field studies [Hirose and Werger, 1987; Field, 1983, 1991; Evans, 1989; Kull and Jarvis, 1995], and has a theoretical basis [Field, 1983; Sands, 1995; Haxeltine and Prentice, 1996].

We use our coupled photosynthesis-stomatal conductance model to calculate net photosynthesis of a leaf at the top of the canopy. The net photosynthesis within the canopy is calculated by scaling it proportional to the APAR within it. The vertical profile of APAR through the canopy, calculated using the two-stream approximation, can be simplified to a simple exponential function of LAI. The analytical integral of this function, multiplied by photosynthesis of the top leaf gives us the canopy-integrated photosynthesis. We then diagnose a canopy-average value of CO2 concentration at the leaf surface (Cs) by using the "big leaf" approach [Amthor et al., 1994a, b; Lloyd et al., 1995a, b], and applying a diffusion equation for the leaf level CO2 concentration (not shown) to the canopy level.

(11) DISTURBANCE: (FIRE, GRAZING, HARVEST, TREE REMOVAL, ETC.):

Not explicitly simulated, but IBIS allows for set fractions of total biomass in each grid cell to be lost each due to fire (0.005 fraction of biomass in grid cell) and other types of natural disturbance (an additional 0.005 fraction of biomass in grid cells).

(12) VEGETATION I/O: (E.G. POTENTIAL, ACTUAL):

Initialized with adapted global vegetation (actual) classification suited to IBIS biomes (based on the work at Univ. of Maryland); model subsequently outputs "potential vegetation".

(13) INPUT DRIVERS (CLIMATIC, SITE, VEG, SOILS) AND RESOLUTION (E.G. DAILY, MONTHLY) REQUIRED FOR MODEL INITIALIZATION:

Base Climate Fields

a) Variables:

Name
Description
Units
cld
monthly mean cloudiness
%
deltat
min temp - avg temp of coldest month
C
prec
monthly mean precipitation rate
mm/day
rh
monthly mean relative humidity
%
rhum
monthly mean rel. humidity at sig=0.995
%
soil
soil texture
-
surta
surface type
1=land,0=ocean
temp
monthly mean temperature
C
topo
topography
m
trange
monthly mean temperature range
C
C
x monthly mean wind speed at sig=0.991
m/s
wetd
mean "wet" days per month
days
wspd
monthly mean wind speed at sig=0.995
m/s

b) Source:

Name
Source
cld
Cramer
deltat
-
prec
Cramer
rh
World Weather Disc
rhum
NCEP/NCAR reanalyses
soil
IGBP-DIS Soil Data Task
surta
adapted from ETOPO5
temp
Cramer
topo
ETOPO5
trange
Cramer
vmag
Genesis GCM runs
wetd
Cramer
wspd
NCEP/NCAR reanalyses

Cramer - new version of Lehman and Cramer (1991) 0.5x0.5 analysis of station data for 1931-60; also known as CLIMATE data set (v2.1).

World Weather Disc - adaptation of information of World Weather Disc data

ETOPO5 - 5'x5' global topography data set

NCEP/NCAR - long-term mean for 1958-1997

(14) ADDITIONAL COMMENTS: