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Perspectives on water cycling in ecosystem models. Sarah Davis June 12, 2012 Water in Bioenergy Agroecosystems Workshop. Outline. What are ecosystem models? How do ecosystem models predict water balances? What is the value of ecosystem model predictions?. - PowerPoint PPT Presentation
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Perspectives on water cycling in ecosystem models
Sarah DavisJune 12, 2012
Water in Bioenergy Agroecosystems Workshop
Outline
• What are ecosystem models?
• How do ecosystem models predict water balances?
• What is the value of ecosystem model predictions?
Water cycling mediated by terrestrial ecosystems
from USGS
from Nightingale et al. 2004
Ecosystem models
(Maximum conductance)
Leaf nitrogen content
Net
pho
tosy
nthe
sis
(Realized conductance)
Temperature
RadiationVPD
Maximum Photosynthetic Potential
Physiology constrains water balance(e.g. PnET)
Gross Primary Production
Water cycling in ecosystem modelsFrom a survey of 13 ecosystem models by Hanson et al. 2004
Component balance # of models that include
Energy 8
Carbon 11
Nutrients 6
Water 13
Water Variables # of models that include
Leaf conductance 12
Canopy interception 11
Litter evaporation 3
Soil evaporation 6
Hydraulic lift 2
Stem capacitance 3
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
Davis et al. 2009
± 30-33%
Water balances mediate other ecosystem services
• Biogeochemical models: CENTURY, DNDC
• Nitrification, denitrification & mineralization: f(water-filled pore space)
• Nitrogen leaching:f(water flow)
Summary1. 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 = 0or
λ ET = Rn - G - H
Rn = net radiationG = 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 temperatureγ = psychrometric constantrs = surface resistancera = aerodynamic resistance
Allen et al. 1998
Model accuracy is lower in dry conditions
ETtoo low
Soil watertoo high
from Hanson et al. 2004
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