Perspectives on water cycling in ecosystem models Sarah Davis June 12, 2012 Water in BioenergyAgroecosystems Workshop
Outline • What are ecosystem models? • How do ecosystem models predict water balances? • What is the value of ecosystem model predictions?
Ecosystem models from Nightingale et al. 2004
Net photosynthesis Leaf nitrogen content Physiology constrains water balance (e.g. PnET) Maximum Photosynthetic Potential (Maximum conductance) Radiation VPD Temperature Gross Primary Production (Realized conductance)
Modeling stomatal conductance Most models use a Ball-Berry equation = f(vapor pressure) see Collatz et al. 1991
Modeling Evapotranspiration • Penman-Monteith • Energy balance • Leaf and litter mass • Leaf and soil water potential • Big leaf
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
Value of Ecosystem Models • Predict dynamics over time • Synthesize water dynamics with other ecosystem processes
Water Balances:Accuracy of predictions from ecosystem process models vary with management ± 30-33% Davis et al. 2009
Water balances mediate other ecosystem services • Biogeochemical models: CENTURY, DNDC • Nitrification, denitrification & mineralization:f(water-filled pore space) • Nitrogen leaching:f(water flow)
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.
Energy balance Rn - G - λ ET - H = 0 or λ ET = Rn - G - H Rn = net radiation G = soil heat flux H = sensible heat
Penman-Monteith Rn = net radiation G = soil heat flux (es - ea) = vapour pressure deficit of the air ρa = mean air density at constant pressure cp = specific heat of the air Δ = slope of relationship between saturation vapour pressure and temperature γ = psychrometric constant rs = surface resistance ra = aerodynamic resistance Allen et al. 1998
Model accuracy is lower in dry conditions ET too low Soil water too high from Hanson et al. 2004