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