1. Soil-Vegetation-Atmosphere Transfer (SVAT) Models
Dr. Mathew Williams

2. What are SVAT models?
Simulators of energy and matter exchange between land surface and atmosphere
Based on mechanistic understanding of the component systems
Used by meteorologists, climatologists, ecologists and biogeochemists.

3. Why do we need SVAT models?
To assist understanding of observations
To allow hypothesis testing
To extend understanding across space and time
To provide a basis for prediction

4. Model Jargon State variables Parameters Driving variables Calibration
Corroboration/validation/testing
Sensitivity analysis

5. What is the structure of a typical SVAT model?
Radiative transfer
Energy balance
Turbulent and diffusive transfer
Stomatal function
Photosynthesis and respiration
Liquid phase water flow

6. Small Group Task For a SVAT component, define the sub-model structure
What are the driving variables, the parameters and state variables?
What are the key connections to other SVAT sub-models?
How would you calibrate your sub-model?

7. Radiative Transfer Direct and diffuse NIR vs PAR Solar geometry
reflectance
Direct and diffuse
NIR vs PAR
Solar geometry
Foliar geometry
Sunlit and shaded
Absorptance
transmittance
Beer’s Law: I=Io exp(-kL)

8. Energy Balance First law of thermodynamics: Energy is always conserved
Qlin Qe Qh
Qlout Qs
Qs + Qe + Qh + Qlin + Qlout + Qc = 0 Qc

9. Turbulent and Diffusive Transfer
J = g dc/dz
Wind within
Crops and forests
Turbulent zone
Laminar zone
Boundary layer thickness
- leaf size
- wind speed
- temperature
Wind speed

10. Stomatal Function E = gs Dcw gs is responsive to: CO2 Light Leaf water
Humidity
Empirical vs. mechanistic approaches

11. Penman-Monteith Equation
g = psychrometer constant
racp = volumetric heat capacity of dry air
s = slope of saturation vapour pressure curve
l = latent heat of vapourisation
Rn = net radiation
de = vapour pressure deficit
ga = leaf boundary layer conductance
gl = leaf stomatal conductance
gH = heat conductance

12. Photosynthesis and Respiration
light
CO2 + 2H2O  CO2 + 4H + O2  (CH2O) + H2O + O2
LIGHT REACTIONS DARK REACTIONS
Metabolic model = Diffusion model
Vc(1-G*/Cc)–Rd = gt(Ca-Cc)

13. Liquid Phase Water Flow
Rs2 Rp
Rsn
Rs1 C
Ys1
Ysn
Ys2 E
Rr1
Rr2
Rrn
Plant
Soil
Atmosphere
CO2 gs
Leaf
Stem
Roots Yl
What determines:
Root resistance (Rr)?
Plant resistance (Rp)?
Soil resistance (Rs)?
Soil water potential (Yl)?

14. The Soil-Plant-Atmosphere Model
Multi-layer canopy and soils
30 minute time-step
Fully coupled liquid and vapour phase water fluxes
Biochemical model of photosynthesis

17. Harvard Forest

22. Tropical rain forest

26. Arctic tundra – northern Alaska

29. What you should have learned
Structure of typical SVAT models
Diagnostic uses (working with eddy flux data)
Prognostic uses (scaling up)
Key research areas in developing SVAT models (applicability to global change research)