RHESSys in grasslands

Scott W. Mitchell, University of Toronto

• Motivation

• information / data model / uncertainty relationships in environmental modelling

• Grasslands National Park

• Earlier work (CENTURY)

• Problems encountered using RHESSys

• Interim solutions

Grasslands National ParkVal Marie, SK (49°N, 107°W)

ArchaeologyVisitor loads / servicesLocal residentsFireGrazingWildlife Native / InvasiveClimate Change

Grass Productivity

• Current status - inventory, diversity, native versus introduced, carbon budget

• Effects of grazing

• Fuel load - standing dead

• Potential response to climate change

• Feedbacks between biogeochemistry and biogeography

First experiment - CENTURY

•What can a non-spatial, monthly time step provide ?•Uncertainty in ANPP•UNCERTAINTY in climate change scenarios

RHESSys - why ?

• Daily, spatial (implicit)• Attractive data model (worldfile hierarchy,

snapshots)• Links with GRASS (GIS)• Active “local” development• Use of BGC - some reports of prior use

(BUT: untested, questions re: applicability of submodels, computer stability issues)

What was missing ? (Round 1)

• Grass morphology (no woody bits)

• Standing dead

• Seed bank ?

• Differentiating C3 & C4 photosynthesis

• Parameterization

• Numerical sanity ?!

How did it do ?

• “That doesn’t look semi-arid !”

• Very high productivity, driven by sunlight, not precipitation

Where is the water ?

ZsatUnsaturated Zone

Saturated Zone

Moisture

Solution (aka workaround)•moisture control on photosynthesis: stomatal control•Farquhar model control through conductance term•conductance from Jarvis multiplicative model•modify leaf water potential multiplier

€

gs = m final * gsMAX * LAI

€

m final = mAPAR * mtavg * mLWP

* mCO2* mt min * mVPD

€

mLWP =LWPpredawn − LWPstomclosure

LWPmin stomopen − LWPstomclosure

mLWP = mLWP × sed •(1−root 2sat )

ed

⎛

⎝ ⎜

⎞

⎠ ⎟

root2sat =zroot

zsat

Phenology

• “fixed” phenology model not good for semi-arid grasslands, especially leaf-on

• phenology data relatively rare, let alone models - main source of help White et al. (1997) using degree days + precipitation

• implemented minimum degree days for earliest possible leaf allocation, then adjusted daily rate of carbon allocation according to soil moisture

1998

0 100 200 300

Day

Summary• Modifications:

– C4 photosynthesis (update psn from BGC)

– “shallower” moisture response (kludge)

– phenology model

• Outstanding issues:– more work needed on hydrology; probably need

another layer, probably need to stop using TOPMODEL (get more data!)

– test and improve phenology

– verify C4 predictions