Soil-Vegetation-Atmosphere Transfer (SVAT) Models

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 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 Qh Qe 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 = gsDcw 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. CO2 Atmosphere gs E Leaf Yl Rp Stem C Rs1 Rr1 Roots Ys1 Rs2 Rr2 Ys2 Rsn Rrn Ysn Soil Plant Liquid Phase Water Flow 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)