1. Perspectives on water cycling in ecosystem models Sarah Davis June 12, 2012 Water in Bioenergy Agroecosystems Workshop

2. Outline What are ecosystem models? How do ecosystem models predict water balances? What is the value of ecosystem model predictions?

3. Water cycling mediated by terrestrial ecosystems from USGS

4. from Nightingale et al. 2004 Ecosystem models

5. (Maximum conductance) Leaf nitrogen content Net photosynthesis (Realized conductance) Temperature Radiation VPD Maximum Photosynthetic Potential Physiology constrains water balance (e.g. PnET) Gross Primary Production

6. Water cycling in ecosystem models From a survey of 13 ecosystem models by Hanson et al. 2004 Component balance# of models that include Energy8 Carbon11 Nutrients6 Water13 Water Variables# of models that include Leaf conductance12 Canopy interception11 Litter evaporation3 Soil evaporation6 Hydraulic lift2 Stem capacitance3

7. Modeling stomatal conductance Most models use a Ball-Berry equation = f(vapor pressure) see Collatz et al. 1991

8. Modeling Evapotranspiration Penman-Monteith Energy balance Leaf and litter mass Leaf and soil water potential Big leaf

9. Soil constrains water balance Single layer bucket model – (see Esser et al. 1994) Multiple layer bucket model Darcy’s law – (see Freeze and Cherry 1979) – Surface runoff as a predictor of drainage

10. Value of Ecosystem Models Predict dynamics over time Synthesize water dynamics with other ecosystem processes

11. Water Balances: Accuracy of predictions from ecosystem process models vary with management Davis et al. 2009 ± 30-33%

12. Water balances mediate other ecosystem services Biogeochemical models: CENTURY, DNDC Nitrification, denitrification & mineralization: f(water-filled pore space) Nitrogen leaching: f(water flow)

14. Summary 1. Ecosystem models are synthetic tools that simulate water balances dynamically over time. 2. Water processes are essential to all ecosystem models and are often the most empirically informed. 3. Uncertainty increases with the duration of projection, complexity of management, and extremity of climate conditions. 4. Many ecosystem process descriptions depend on accurate water balance.

16. Energy balance R n - G - λ ET - H = 0 or λ ET = R n - G - H R n = net radiation G = soil heat flux H = sensible heat

17. Penman-Monteith R n = net radiation G = soil heat flux (e s - e a ) = vapour pressure deficit of the air ρ a = mean air density at constant pressure c p = specific heat of the air Δ = slope of relationship between saturation vapour pressure and temperature γ = psychrometric constant r s = surface resistance r a = aerodynamic resistance Allen et al. 1998

18. Model accuracy is lower in dry conditions ET too low Soil water too high from Hanson et al. 2004