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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?.

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perspectives on water cycling in ecosystem models

Perspectives on water cycling in ecosystem models

Sarah Davis

June 12, 2012

Water in BioenergyAgroecosystems Workshop

outline
Outline
  • What are ecosystem models?
  • How do ecosystem models predict water balances?
  • What is the value of ecosystem model predictions?
slide4

Ecosystem models

from Nightingale et al. 2004

slide5

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
Modeling stomatal conductance

Most models use a Ball-Berry equation

= f(vapor pressure)

see Collatz et al. 1991

modeling evapotranspiration
Modeling Evapotranspiration
  • Penman-Monteith
  • Energy balance
  • Leaf and litter mass
  • Leaf and soil water potential
  • Big leaf
soil constrains water balance
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
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
Water Balances:Accuracy of predictions from ecosystem process models vary with management

± 30-33%

Davis et al. 2009

water balances mediate other ecosystem services
Water balances mediate other ecosystem services
  • Biogeochemical models: CENTURY, DNDC
  • Nitrification, denitrification & mineralization:f(water-filled pore space)
  • Nitrogen leaching:f(water flow)
summary
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
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
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
Model accuracy is lower in dry conditions

ET

too low

Soil water

too high

from Hanson et al. 2004