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

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.

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

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

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

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?

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)

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

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

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

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

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)

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)?

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

Harvard Forest

Tropical rain forest

Arctic tundra – northern Alaska

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)